applications of silicon photomultipliers in personal radiation detection and nuclear imaging

APPLICATIONS OF SILICON PHOTOMULTIPLIERS IN

PERSONAL RADIATION DETECTION AND NUCLEAR IMAGING

A Thesis

Submitted to the Faculty of Graduate Studies and Research

In Partial Fulfillment of the Requirements

For the Degree of

Master of Science

in

Physics

University of Regina

By

Jamie Sanchez-Fortún Stoker

Regina, Saskatchewan

November, 2015

© Jamie Sanchez-Fortun Stoker, 2015

UNIVERSITY OF REGINA

FACULTY OF GRADUATE STUDIES AND RESEARCH

SUPERVISORY AND EXAMINING COMMITTEE

Jamie Sanchez-Fortun Stoker, candidate for the degree of Master of Science in Physics, has presented a thesis titled, Applications of Silicon Photomultipliers in Personal Radiation Detection and Nuclear Imaging, in an oral examination held on September 15, 2015. The following committee members have found the thesis acceptable in form and content, and that the candidate demonstrated satisfactory knowledge of the subject material. External Examiner: Dr. Xue-Dong Yang, Department of Computer Science

Co-Supervisor: Dr. Zisis Papandreou, Department of Physics

Co-Supervisor: Dr. Andrei Semenov, Department of Physics

Committee Member: Dr. George J. Lolos, Department of Physics

Chair of Defense: Dr. Stephen Bend, Department of Geology

Abstract

Originally developed as the readout for calorimeters in high-energy physics experiments, the

silicon photomultiplier (SiPM) has found use in awide range of fields requiring the detection

of low-intensity light. This thesis discusses work on two such applications: in the develop-

ment of a prototype personal radiation detector (PRD), and in the imaging of a radioactive

source.

The ability to detect above-background levels of radiation has received increased atten-

tion in recent years, not least from the perspective of national security agencies in, for ex-

ample, tracking the movement of illicit radioactive materials, or in dealing with potential

fall-out from nuclear accidents. In such potentially hazardous environments, there has been

an increased demand for improved PRDs for use by emergency workers and first-responders.

Motivated by an NSERC-funded partnership with Environmental Instruments Canada Inc.

(EIC), the first part of this thesis focuses on a project to develop a small, low-cost, easy-to-use

prototype gross-counting gamma-radiation detector based upon SiPM technology. Limited

by size and cost requirements, measurements suggested the Hamamatsu S12572-050C model

(3× 3mm2) as the preferred choice of SiPM, and subsequent work tested several SiPM/light-

producing medium combinations. A PTFE (teflon)-wrapped common plastic scintillator

i

(BC-416), coupled to the SiPM, was identified as the medium providing the most signifi-

cant sensitivity enhancement with respect to EIC’s existing detection device (incorporating

a Geiger-Müller tube as its photosensor), while minimising cost and maintaining the over-

all physical size of EIC’s detector. An additional project positively assessed the suitability of

SiPMs for thermal neutron detection using scintillating fibres coated in successive layers of

zinc sulphide and boron carbide.

The second part of this thesis (funded through the Sylvia Fedoruk Centre for Nuclear

Innovation) presents a preliminary characterisation of two SensL MatrixSM-9 SiPM array

modules, in preparation for their use in a “PhytoPET”plant-imaging system to be built at the

University of Regina. Such arrays of SiPMs are now available commercially as part of modu-

lar, turnkey readout systems designed specifically for use in high-resolution, state-of-the-art

medical-imaging applications. However, PET systems designed to image plants – such as

the (operational) PhytoPET system based at Duke University – typically utilise multi-anode

photomultiplier tubes. Singles measurements are reported for each SiPM array, coupled in

turn to a PTFE-wrapped BC-416 plastic scintillator in several configurations. Pixelated im-

ages of noise and various laboratory sources are reconstructed from output data files using

dedicated codes, and the array-trigger error of each module is mapped over a wide-range of

adjustable array and pixel thresholds.

ii

Acknowledgments

I am immensely grateful to my co-supervisors, Dr. Zisis Papandreou and Dr. Andrei

Yu. Semenov, not only for providing me with the opportunity to come to the University

of Regina, but also for all of their subsequent help, guidance and advice over the past two

years. Many thanks go to Mr. Tegan Beattie for all of his efforts – and patience – in helping

me to become more familiar with the equipment found in the SPARRO Group’s Detector

Development Laboratory. I would also like to acknowledge both the Natural Sciences and

Engineering Research Council of Canada and the Sylvia Fedoruk Canadian Centre for Nu-

clear Innovation for the financial support I received.

The expertise and contribution ofMr. Kai Kaletsch, President of Environmental Instru-

ments Canada Inc., is greatly appreciated in the personal radiation detection work. I would

also like to thankDr. AkselHallin andDr. CarstenKrauss, both of theUniversity ofAlberta,

for supplying the linear alkyl benzene scintillator used in this work.

I amvery thankful tomembers of the SPARROGroup for their discussions and advice, as

well asMr. Derek Gervais for his help in preparing the plastic and PEN scintillators, andMr.

KeithWolbaum for his electronics expertise. Special thanks go to the summer students who,

iii

as members of the SPARRO Group in 2014 and 2015, contributed to the various projects

by tirelessly collecting some of the data presented in this work: to Mr. Marcello La Posta

and Mr. Mandeep Singh in 2014, and most recently Ms. Colleen Henschel and Mr. Shayne

Gryba. A big thank you goes to the faculty, staff and students in the Department of Physics

for creating such a friendly and enjoyable atmosphere in which to work every day.

Finally, I’d like to thankmy family: my wife, Katrina, for her continued love and encour-

agement throughout this endeavour, as well as our parents – Shirley and Mike Gibson, and

Gerrie and Ken Johnson – for all their prayers, love and support.

iv

For God so loved the world that he gave his

one and only Son, that whoever believes in

him shall not perish but have eternal life.John 3:16

For my darling wife, Katrina Louise.

v

Contents

Abstract i

Acknowledgments iii

Dedication v

1 Introduction 1

2 Theory & Operation of the Silicon Photomultiplier 6

2.1 The Semiconductor Diode . . . . . . . . . . . . . . . . . . . . . . . . . 10

2.2 The Avalanche Photodiode, Breakdown & The Geiger-mode Mode of Op-

eration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

2.3 The Silicon Photomultiplier . . . . . . . . . . . . . . . . . . . . . . . . . 15

2.3.1 The Structure of the SiPM . . . . . . . . . . . . . . . . . . . . . 19

2.3.2 Properties of the SiPM . . . . . . . . . . . . . . . . . . . . . . . 20

2.3.3 Noise in Silicon Photomultipliers . . . . . . . . . . . . . . . . . . 24

2.3.3.1 Thermally Generated (Dark) Noise . . . . . . . . . . . 25

2.3.3.2 Inter-microcell Crosstalk & Afterpulsing . . . . . . . . . 27

vi

3 Application: Personal Radiation Detector 29

3.1 Project Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

3.2 Photosensor Selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

3.3 Experimental Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

3.4 Light-ProducingMedia: Evaluation of the BC-416 Scintillator with Various

Wrappings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

3.4.1 Paper/Al-Foil Wrapping . . . . . . . . . . . . . . . . . . . . . . 41

3.4.2 BC-620 Reflective Paint Wrapping . . . . . . . . . . . . . . . . . 48

3.4.3 Polytetrafluoroethylene (PTFE) Thread Seal Tape Wrapping . . . . 49

3.5 Light-Producing Media: Evaluation of the LAB Scintillator . . . . . . . . . 55

3.6 Light-Producing Media: The Lead Mini-calorimeter . . . . . . . . . . . . 56

3.7 Application of EIC Electronics . . . . . . . . . . . . . . . . . . . . . . . 60

3.7.1 Light-Producing Media: BC-416 Plastic Scintillator . . . . . . . . . 60

3.7.2 Light-Producing Media: LAB Scintillator . . . . . . . . . . . . . . 64

3.7.3 Light-Producing Media: PEN Scintillator . . . . . . . . . . . . . . 66

3.8 Neutron Detection with SiPMs . . . . . . . . . . . . . . . . . . . . . . . 69

4 Application: Nuclear Imaging 74

4.1 A Brief Introduction to PET Physics . . . . . . . . . . . . . . . . . . . . 78

4.2 The SensL MatrixSM-9 System . . . . . . . . . . . . . . . . . . . . . . . 79

4.3 Multiplexing: The Scrambled Crosswire Readout . . . . . . . . . . . . . . 88

4.4 Measurements Using the SensL MatrixSM-9 Modules . . . . . . . . . . . 89

5 Conclusion 101

vii

References 105

viii

Listing of Figures

2.1.1 Schematic and I-V curve of a typical p-n junction. . . . . . . . . . . . . . . 11

2.3.2 Structure of a typical individual silicon photomultiplier unit. . . . . . . . . 16

2.3.3 Typical output pulse shapes from a SiPM and APD. . . . . . . . . . . . . 18

2.3.4 Schematics of an idealised and realistic SiPM microcell. . . . . . . . . . . . 21

2.3.5 Noise mechanisms in a silicon photomultiplier. . . . . . . . . . . . . . . . 26

3.2.1 Readout electronics constructed for the 1 × 1-mm2 and 3 × 3-mm2 SiPMs. . 34

3.3.2 The experimental set-up used in singles measurements. . . . . . . . . . . . 38

3.3.3 The experimental set-up used in coincidence measurements. . . . . . . . . 39

3.4.4 Frequency-threshold graphs for 90Sr (BC-416 scintillator, paper/Al-foilwrap-

ping, SPARRO electronics). . . . . . . . . . . . . . . . . . . . . . . . . . 45

3.4.5 Frequency-threshold graphs for 60Co (BC-416 scintillator, Al-foil & BC-620

wrappings, SPARRO electronics). . . . . . . . . . . . . . . . . . . . . . . 47

3.4.6 Frequency-threshold graphs for 60Co (BC-416 scintillator, PTFE wrapping,

SPARRO electronics). . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

3.4.7 BC-620:Al-foil & PTFE:Al-foil frequency ratios for 60Co. . . . . . . . . . . 52

ix

3.4.8 Frequency-threshold graphs for 241Am, 133Ba& 60Co (BC-416&LAB scintil-

lators, PTFE & paper/Al-foil wrappings, SPARRO electronics). . . . . . . 53

3.4.9 Frequency-threshold graphs for 60Co (BC-416 scintillator, PTFE wrapping,

SPARRO electronics). . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54

3.6.10Overview of the lead mini-calorimeter and LAB scintillator. . . . . . . . . . 58

3.7.11 Overview of EIC’s TM372A unit. . . . . . . . . . . . . . . . . . . . . . . 61

3.7.12 Frequency-threshold graphs for 241Am, 133Ba& 60Co (BC-416 scintillator, PTFE

wrapping, EIC’s electronics). . . . . . . . . . . . . . . . . . . . . . . . . . 65

3.7.13 Frequency-threshold graphs for 133Ba, 60Co & 90Sr (BC-416, LAB & PEN

scintillators, PTFE wrapping, EIC electronics). . . . . . . . . . . . . . . . 68

3.8.14 Overview of the neutron-detection experiment. . . . . . . . . . . . . . . . 72

4.1.1 Overview of positron emission tomography (PET). . . . . . . . . . . . . . 80

4.2.2 The SensL MatrixSM-9 system. . . . . . . . . . . . . . . . . . . . . . . . 81

4.2.3 The MatrixSM-9 system: Sensor Head & data transfers. . . . . . . . . . . . 85

4.2.4 Screenshots of the MatrixSM-9 GUI software. . . . . . . . . . . . . . . . . 87

4.3.5 The Scrambled Crosswire Readout and SiPM array/pixel-labelling scheme. . 90

4.4.6 Approximate positions of the scintillator end-face relative to the Sensor Head. 92

4.4.7 Frequency histograms for the PTFE-wrapped BC-416 scintillator astride ar-

rays 6 and 7. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94

4.4.8 Frequency histograms for the PTFE-wrapped BC-416 scintillator astride ar-

rays 4, 5, 7 & 8. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95

4.4.9 Array-trigger-error surfaces for array 4. . . . . . . . . . . . . . . . . . . . 97

4.4.10Frequency-difference histograms testing misalignment in array 4. . . . . . . 100

x

Listing of Tables

2.1 Physical properties of some inorganic scintillators of interest. . . . . . . . . 8

2.2 Physical properties of some organic scintillators of interest. . . . . . . . . . 9

3.1 Properties of the individual Hamamatsu 1 × 1-mm2 and 3 × 3-mm2 MPPCs. 33

3.2 List of the radioisotopes used in the PRD tests. . . . . . . . . . . . . . . . 42

4.1 Properties of the SensL MatrixSM-9-30035 SiPM. . . . . . . . . . . . . . . 84

xi

Chapter 1

Introduction

Over the past decade, the silicon photomultiplier (SiPM1) has emerged as a competitive al-

ternative to traditional radiation detectors such as the photomultiplier tube (PMT) in the

detection of low-level light. While possessing many of the properties common to the PMT,

such as a high gain (∼ 105–107) [1] and short dead time, the SiPM provides several technical

advantageswhichmake it preferable to the PMT in a number of fields. For example, aswell as

having a low power consumption, low operational voltage (∼ 20-60 V [2]) and a virtual im-

munity to tesla-level magnetic fields up to∼ 5 T [2], the SiPM is a relatively compact device,

its small dimensions allowing for a robust design and high granularity.

Initially developed for use in the calorimeters of high-energy physics experiments [3–7],

the SiPM has evolved as the photosensor of choice in many ongoing experimental tests of1The SiPM (or SPM) is also referred to in the literature by a variety of acronyms, often to provide for distinc-

tion between variousmanufacturers. They are known alternatively as the avalanche photodiode (APD),Geiger-mode avalanche photodiode (GAPD, G-APD, GMAP or GM-APD), Multipixel Photon Counter (MPPC,synonymous with Hamamatsu’s device), micro-channel APD (MAPD), Dead-space-modified avalanche pho-todiode (DAPD), Metal-resistor-semiconductor (MRS), Pixelated Geiger-mode avalanche photodiode (PPD),Solid-state photomultiplier (SSPM), Single-photon avalanche photodiode (SPAD), AMPP and Hybridavalanche photodetector (HAPD).

1

CHAPTER 1. INTRODUCTION 2

fundamental physics. There is considerable interest in the cryogenic properties of SiPMs im-

mersed in liquid N2 [8–11], He [10, 12], Ar [13–18] and Xe [19]. At such temperatures the

single-photon resolution required in, for example, dark-matter searches [19–21] and double-

beta decay experiments searching for the majorana neutrino [21, 22], can be achieved.

TheSiPMhas also foundapplications in awide variety of disciplines, ranging fromnuclear-

waste management [23] and safety in fission reactors (in the detection of thermal neutrons

around the reactor core [24]), to geophysics (in the muon radiography of Mount Vesuvius

[25–27]), the telecommunication industry [28], military2 and food safety. As a result of the

2011 nuclear disaster at Fukushima Daiichi, the LANFOS3 radiation-detection system [29]

(employing a SiPM) has been developed, a prototype device intended for commercial use,

prompted by mounting concern in the Japanese islands over the level of 137Cs in foodstuffs.

Lidar is a remote-sensing technology which measures the distance to a target by illumi-

nating it with a laser beam and analysing the reflected light. SiPMs have been employed in

studies of the terrestrial atmosphere [30, 31], as well as in the tracking of space debris [32, 33],

satellites [34] and the Moon [32], and in the automotive industry [35].

In astrophysics, SiPMs have been used in high-speed photometry [36]. Significant atten-

tion has been given to several terrestrial- [37–39] and space-based [40, 41] astroparticle exper-

iments designed to detect photons from galactic and extragalactic gamma-ray sources in deep

space. For example, the FACT telescope4 is the first Čerenkov telescope [42–45] equipped

with a camera incorporating SiPMs, and is used in themonitoring of, for example, highly en-2In the implementation of, for example, night vision. See the Philips application note Potential Applica-

tions, retrieved from http://www.research.philips.com/initiatives/digitalphotoncounting/potential-applications.html.

3The so-called Large Food Non-Destructive Area Sampler.4The First G-APD Čerenkov telescope, built on the mount of the HEGRA CT3 telescope, located at an

altitude of 2396 m at the Observatorio del Roque de los Muchachos, La Palma, Canary Islands, Spain.

http://www.research.philips.com/initiatives/digitalphotoncounting/potential-applications.html

http://www.research.philips.com/initiatives/digitalphotoncounting/potential-applications.html


ergetic (TeV) blazars (very compact quasars) associated with supermassive black holes at the

centre of giant elliptical galaxies.

Recent developments in imaging systems used in nuclear medicine, such as PET, MRI

and SPECT,5 as well as time-of-flight PET [50–54] and hybrid multi-modal systems such as

PET/MRI [55–65], are dominatedby advances in solid-state photodetectors such as the SiPM

[66]. In addition to systems for use in human and small-animal patients, PET scanners using

position-sensitive PMTs have been developed to image plants[67–71]. Applications of the

“next generation” of SiPMs – so-called digital silicon photomultipliers [72–81] – to medi-

cal imaging has been the subject of intense investigation by several groups (see, for example,

references [52, 82–101].

Technological advances including the SiPM are of interest in biophotonics, with increas-

inglyminiaturised biodiagnostic devices becoming available for use in both clinical [102] and

extra-clinical “point-of-care” settings [103]. For example, Mester et al. [102] have utilised

SiPMs in a handheld probe for β+-emitting radiotracer detection in operating theatres, while

Renna et al. [103]have evaluated a SiPMinadevice for implementing integrated fluorescence-

based microarrays, which are of use in DNA sequencing and as protein biomarkers in fields

such as oncology and pathogen detection.

Over the past decade, the silicon photomultiplier has found commercial use in areas of

homeland security such as threat detection (for example, in neutron and gamma detection in

cargo scanners [104, 105]) andnuclear hazards [106]. In a 2013 request for information (RFI),6

5Positron emission tomography (PET) [46, 47], Magnetic Resonance Imaging (MRI) [48] and Single-photon emission computed tomography (SPECT) [49] are all well-known imaging modalities used in modernnuclear medicine. A brief overview of PET is given in Section 4.1.

6Low Cost, High Performance Radiation Detection Request for Information (RFI), Solicitation Number:DARPA-SN-13-47. Retrieved from https://www.fbo.gov.

https://www.fbo.gov


and a subsequent Broad Agency Announcement through its SIGMAProgram,7 the Defense

AdvancedResearch Projects Agency (DARPA) – an agency of theUnited StatesDepartment

of Defense – publicly solicited for innovative approaches to the development of inexpensive,

mass-producible, high-efficiency, packaged radiation detectors for identifying hidden nuclear

and radiological threats. Such an initiative clearly demonstrates the requirement for new per-

sonal radiation detector (PRD) technologies [107]. Portable, hand-held radiation detectors –

for use by first responders in potentially hazardous environments – have been available com-

mercially for many years. However, the SiPM is beginning to supersede existing radiation

sensors used in such devices.

In this thesis, applications of SiPMs are considered in the development of PRDs and in

nuclear imaging. Both projects advertise positive links between the SPARRO Group at the

University of Regina and other organisations within Saskatchewan.

An NSERC-funded partnership8 with Environmental Instruments Canada Inc. (EIC),9

provided an opportunity to advance the development of a low-cost, rugged, small, unob-

trusive, easy-to-use prototype PRD based on SiPMs, incorporating a wireless BluetoothTM

connection.

A second project, funded10 through the Sylvia Fedoruk Canadian Centre for Nuclear In-

novation,11 uses arrays of SiPMs, available as part of modular, turnkey readout systems de-7SIGMA Solicitation Number: DARPA-BAA-14-11. Retrieved from https://www.fbo.gov.8Z. Papandreou et al., Investigation of Portable, Low Cost, Radiation Detection Technolo For Use With

Wireless Personal Radiation Detection Equipment, NSERC Engage Grant (2014-2015).9Environmental Instruments Canada, Inc., 202-135 Robin Crescent, Saskatoon, SK S7L 6M3, Canada

(http://www.eic.nu).10Z. Papandreou et al., Investigation of Silicon Photo-Multiplier Arrays for NuclearMedicine and Industrial

Instrumentation, Fedoruk Grant no. J2012-107.11Sylvia Fedoruk Canadian Centre forNuclear Innovation, 111-54 Innovation Boulevard, Saskatoon, SK S7N

2V3, Canada (http://www.fedorukcentre.ca).

https://www.fbo.gov

http://www.eic.nu

http://www.fedorukcentre.ca


signed for use in high-resolution, state-of-the-art medical imaging systems. As a precursor to

the development of a plant-imaging PET program (using SiPMs as the photodetector) at the

University of Regina, results of singles measurements are presented in the imaging of a 90Sr

source using two such SiPM array modules coupled to a plastic scintillator.

In Chapter 2 the theory, properties and operation of the silicon photomultiplier are dis-

cussed, before a presentation of the results of the PRDand nuclear-imaging projects in Chap-

ters 3 and 4, respectively. Conclusions of this work are given in Chapter 5.

Chapter 2

Theory & Operation of the Silicon

Photomultiplier

While the mechanisms underlying the theory of the photodetector can be traced back to the

quantum hypothesis of Planck [108, 109] and the explanation of the photoelectric effect by

Einstein [110], the first experimental photodetectors included those of Smith [111], in demon-

strating photoconductivity in selenium,Hertz [112] (in the first demonstration of the photo-

electric effect) and Thomson [113, 114] (in the discovery of the electron). The past 120 years or

so has seen a significant development of increasingly sensitive andmore efficient detectors for

use in nuclear and particle physics. All such devices operate on same basic principle: a trans-

fer of part/all of the particle’s energy to the detector, which converts it into somemeasurable

property,1 which constitutes a detection event. Here, the focus is on scintillating detectors,

in which the excitation/ionisation of a scintillatingmaterial by the incident particle results in

the emission of a photon, which is directed into the detector.1For example, the electrical resistance of Se in the case of Smith’s experiment, and luminosity in Thomson’s.

6

CHAPTER 2. THEORY & OPERATION OF THE SIPM 7

Modern photodetectors are electronic devices which, upon the detection of an incident

photon, generate an electrical output which is then passed to a data acquisition (DAQ) sys-

tem. Naturally, the development of photosensors – and their associated electronics andDAQ

systems – as well as scintillators have benefited greatly from the significant technological im-

provements of the past several decades. Advances in the hardware and software capabilities of

modern computer systems have had a profound impact on DAQ systems and data analysis,

while a new generation of crystal scintillators with good energy-deposition, time-resolution

and relatively large light outputs have become available. For example, cerium-doped, silicate-

based inorganic scintillators such asLutetiumoxyorthosilicate (LSO) [115, 116] andLutetium-

yttrium oxyorthosilicate (LYSO) [117] are now in widespread use in themedical industry, of-

fering desirable properties such as high stopping power, fast decay time and high light yield

(see Table 2.1).

Since its inception in 1935 [118], the PMT was almost unrivalled as the photodetector

of choice in most applications. However, significant advances in semiconductor design and

manufactureover thepast forty years have resulted in solid-statedetectors such as the avalanche

photodiode, and more recently the silicon photomultiplier. Based upon thin silicon wafers,

solid-state technology has enabledphotosensors to be developedwhich, while retainingmany

of the desirable properties of the PMT, offer additional advantages such as much smaller di-

mensions and lower cost.

Here, the semiconductor diode – which forms the basis of the SiPM – is considered ini-

tially, before discussing the operation and properties of the silicon photomultiplier itself.


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http://www.crystals.saint-gobain.com/uploadedFiles/SG-Crystals/Documents/SGC%20BC400-404-408-412-416%20Data%20Sheet.pdf


2.1 The Semiconductor Diode

It is well known that the electrical conductivity of an intrinsic semiconductor can be mod-

ified by the introduction of dopant materials, enabling n-type (that is, electron-rich, hole-

deficient) and p-type (hole-rich, electron-deficient) semiconductors and compound devices,

such as diodes and transistors, to be produced. The initial formation of a p-n junction, in the

region between the p- and n-typematerials, is accompanied by a rapid diffusion of oppositely

charged carriers towards the opposing region: conduction-band electrons (holes) from the n-

doped (p-doped) region diffuse into the p-doped (n-doped) region adjacent to the boundary,

leaving behind immobile positive (negative) ions. In both regions there is a recombination of

electrons and holes, and a steady-state is quickly reached in which further diffusion of charge

carriers is inhibited by the intrinsic electric field, E, created by the oppositely charged ions.

The net effect is an insulating region in the vicinity of the junction, known as the depletion

layer, in which there is an absence (or depletion) of mobile charge carriers (Figure 2.1.1a).

The behaviour of a semiconductor diode in a circuit is characterised by its current-voltage

(I-V) dependence, the shapeof the corresponding I-V curvebeingdeterminedby themobility

of charge carriers through the depletion region. While the zero bi voltage ΔV = EΔx across

the p-n junction (in which Δx is the width of the depletion layer) is intrinsic to the diode, an

externally applied forward bi or reverse bi may be used to control the current output

(Figure 2.1.1b). In the former, the applied bias ΔV > 0 V generates a field Ef opposing E:

as ΔV is increased, the depletion layer is reduced and the electrical resistance of the diode

decreases, allowing a (forward) current of majority charge carriers (electrons and holes) to

flow freely.

Semiconductor diodes are more commonly operated using reverse bias (ΔV < 0 V), in


(a) Schema c of a p-n junc on

(b) Full I-V curve for a p-n junc on

Figure 2.1.1: A typical p-n junc on. (a) The opera on of the junc on in thermal equilibrium with zero applied biasvoltage. Upper: schema c of the p-n junc on, showing how the carrier concentra ons vary with distance across thediode. The blue and red lines indicate the electron and hole concentra ons, respec vely, whereas the grey regionsare neutrally charged. Within the space-charge (or deple on) region in the vicinity of the p-n boundary, the light-redand light-blue areas indicate regions of posi ve and nega ve charge, respec vely. Lower: sketch graphs of the charge,electric field and voltage across the diode. (b) the “full” I-V curve of a p-n junc on. Regions of the graph in whichV > 0 V and Vbr < V < 0V are referred to as “forward bias” and “reverse bias”, respec vely, while the diodebreaks down when V ≤ Vbr (see text). IS is the reverse-bias satura on current. In the SiPM, the I-V behaviour ofthe diode opera ng in reverse-bias and breakdown modes is of singular interest. Both figures fromWikipedia: TheFree Encyclopedia.


which E is enhanced by the applied electric field Er. As |ΔV| is increased, electrons (holes)

in the n-doped (p-doped) region are pulled away from the junction, widening the depletion

layer and leading to an increasingly large resistance. Under such conditions, the only cur-

rent flow is a residual “leakage” current (Figure 2.1.1b) due to minority carriers: conduction

electrons (holes) in the p-side (n-side) are dragged towards the cathode (anode). However, at

very large reverse bias (that is, in a very strong electric field), both conduction electrons and

holes may acquire enough kinetic energy to trigger a cascade effect in which electron-hole

multiplication generates a large current. Beyond a critical voltage the diode suffers reverse

breakdown, usually due to either avalanche or Zener breakdown.2 In both cases the deple-

tion zone breaks down: in the avalanche effect (Section 2.2), beyond the breakdown or peak

inverse voltage, Vbr > 4Eg/q (in which Eg is the band gap and q the charge of the carrier)

[123], the diode suffers reverse breakdown in which the minority carriers suddenly generate

large numbers of electrons and holes whichmove away from the p-n junction, and a large cur-

rent begins to flow. In the Zener effect, when the applied reverse bias is Vbr < 4Eg/q,3 the

strong electric field allows electrons to tunnel from the valence band of the p-type material to

the conduction band of the n-type material, increasing the number of free minority carriers

and hence the reverse current.4 The so-called avalanche photodiode (APD) – which forms

the basis of the SiPM – is deliberately designed to exploit the avalanche mechanism.5

2A third mechanism inducing breakdown, known as thermal instability, is a major influence in semicon-ductors with relatively small band gaps (see, for example, Sze and Ng [123]).

3In the intermediate region 4Eg/q < Vbr < 6Eg/q, the breakdown is due to a combination of both theavalanche and Zener effects [123].

4In heavily doped p-n junctions – in which the depletion layer is extremely thin – the latter phenomenonforms the basis of the Zener diode.

5For the remainder of this report, unless otherwise specified the terms “bias” and “current” are used to referto “reverse bias” and “reverse current”, respectively.


2.2 The Avalanche Photodiode, Breakdown & The

Geiger-mode Mode of Operation

Silicon APDswere first developed byMcIntyre [124] andHaitz [125] in the 1960s. Avalanche

multiplication in such devices is a multi-stage cascade process in which a photoelectron, pro-

duced via the photoelectric effect from an initial photon incident upon a Si atom, is in a suffi-

ciently strong electric field (generated by the external reverse-bias voltageV) that it is energetic

enough to subsequently induce a chain reaction in the substrate. Due to so-called impact ion-

isation, successive atom-free electron and/or hole collisions result in the avalanche – that is,

an exponential increase in the number of free charge carriers – in a process analogous to the

Townsend discharge (or Townsend avalanche) [126] in gases.

Impact ionisation occurs when a charge carrier collides with a bound valence electron,

exciting the electron into the conduction band while leaving a hole in the valence band, thus

generating a mobile electron-hole pair. For a typical collision, the probability of ionisation

ultimately depends upon the strength of the electric field. The ionisation rate for the (pre-

collisional) primary electron (hole) may be quantified by the ionisation coefficient, α, that

is, the number of (post-collisional) secondary electrons (holes) produced via ionisation by

the primary charge carrier per unit path length x. Mathematically, for electrons (holes), the

ionisation coefficients is αe = ne dne/dx (αh = nh dnh/dx), in which ne (nh) is the electron

(hole) density. It is well-known [127] that αe and αh depend upon the electric field strength,

E, and that in Si, for any value of E, αe > αh. That is, the probability of impact ionisation

due to an electron is greater than that of a hole.

Consider the depletion layer x1 ≤ x ≤ x2 in the semiconductor, in which the applied


electric field is assumed to be constant. The gainGe of the diode can be expressed empirically

as a function of V and the breakdown voltage Vbr by [128]

Ge =

[1 −

∣∣∣ VVbr

∣∣∣]−n

(2 < n < 6) , (2.2.1)

and so if E = dV/dx, a dependence ofGe on the field strength is obtained. In the weak-field

regime (the “no-gain” region), Ge ≈ 1, i.e., the applied field is too weak to initiate impact

ionisation. The resulting current collected at the electrodes is then I = Ipe (in which Ipe is the

initial photoelectron current).

In the strong-field regime (the so-called “gain region”),Ge ≫ 1, and the resulting current

I = GeIpe ≫ Ipe. While the electrons generate an avalanche, the operational mode of the

diode depends upon the probability of ionisation of the holes (recall that αh < αe): if E ∼

150 kV/cm, then the impact ionisation ratio k = αe/αh ∼ 100: there is a low probability

of ionisation – the holes are not energetic enough to trigger a self-sustaining avalanche. The

photodiode operates in so-called proportional or linear mode (see for example, Knoll [129]),

and Ge ∼ 102 − 104 [130]. Meanwhile, as V → Vbr such that E ∼ 500 kV/cm, k ∼ 2.5,

and the holes are energetic enough to generate an avalanche: Ge increases strongly until at a

critical electric field – corresponding toVbr – the avalanche diverges rapidly (Ge ∼ 105− 106),

and the resulting APD operates in Geiger-mode mode. The multiplication regions due to

various initial photoelectrons begin to merge together to form a single avalanche, forming a

large output pulse from a single incident photon.


2.3 The Silicon Photomultiplier

The silicon photomultiplier (Figure 2.3.2) is a highly miniaturised and integrated semicon-

ductor device, comprised of a densely packed (≳ 103 mm2 [131]) rectangular array/matrix of

essentially identical microcells (subject to high quality control in all the silicon and deposi-

tion stations of the foundry) connected in parallel. Each microcell is a Geiger-mode APD

connected in series with a relatively large (∼ 105 Ω) quenching resistor RQ (Figure 2.3.2d).

At a large applied voltage V – supplied by an external power source and typically a few volts

above breakdown (referred to as the overbi Vob = V − Vbr) – any photons incident upon

one or more microcells will cause the microcell to “fire” (that is, generate a corresponding

avalanche). In this way each microcell operates as an independent photodetector.

In each microcell, a self-sustaining avalanche can, in principle, propagate indefinitely.

However, in practice the avalanche current increases to a maximum value Imax = (V −

Vbr)/(RQ + RS), in which RS is the resistance of the undepleted regions of the APD [127],

before being subsequently quenched by the accompanying resistor of valueRQ (Figure 2.3.3).

As the current I, given by

I = −[1 − exp

(− tRSC

)], (2.3.2)

increases (where C ∼ 0.1 pF [131] is the capacitance of the depletion region of the APD), the

voltage VR = IRQ across RQ increases while that across the APD, VAPD = IRS, is reduced

below Vbr, inhibiting subsequent avalanches. The current then decreases as

I = − exp(− tRQC

), (2.3.3)


(a) Hamamatsu S12571-050C MPPC series. (b) Hamamatsu S12572-050C MPPC series.

(c) Photograph of a 1 × 1 mm2 SiPM. (d) Schema c of the microcells.

Figure 2.3.2: The structure of a silicon photomul plier. (a) The Hamamatsu S12571-050C MPPC series (photosensi-ve area 1 × 1 mm2). (b) The Hamamatsu S12572-050C MPPC series (photosensi ve area 3 × 3 mm2). (c)Magnified

view of a 1 × 1-mm2 SiPM (obtained from the Na onal Research Nuclear University MEPhI (Moscow EngineeringPhysics Ins tute), showing the 16 × 16 arrangement of individual microcells. (d) Schema c of a circuit showing APDmicrocells. Each microcell consists of a light-sensi ve APD opera ng in Geiger mode connected in series with aquenching resistor. An external power source reverse-biases the array to a voltage that is up to a few volts above thebreakdown voltage of an APD. Based on Figure 1b of Piatek [127]. Figures (a) and (b) are the SiPM units used in thePRD tests discussed in Chapter 3. Both units are most sensi ve to photons in the spectral range 320 nm-900 nm,with a peak spectral response at 450 nm. From the Hamamatsu data sheets for the respec ve units (see text).


discharging through C. When the output current is completely quenched, the APD relaxes

to its quiescent state (I = 0 A) with VAPD > Vbr, ready for the next photon. The operation

of the SiPM in this way is known as limited Geiger-mode.

Since the output signal is independent of the number of incident photons – the current

maximum always occurs after approximately the same number of avalanches (regardless of

their photon or photoelectron of origin) – the resulting output pulse from a fired microcell

is always of the same amplitude. Each microcell will provide the same fixed gain Ge = Q|e|

when fired (in which Q is the total charge accumulated in the microcell), with a charge Q =

C(V− Vbr) ∼ 10 fC collected at the anode. In this way each microcell effectively operates as

a binary device, its “on” (“off”) state indicating the presence (absence) of an avalanche. It can

be thought of as a counter of the initial photons – and hence particle detections – incident

upon it. Although the output of eachmicrocell is a binary digit of information, summing the

charges from all the microcells in the array yields an analogue signal at the common anode.

For low-level light, it is assumed that an avalanche in each of the microcells has been trig-

gered by a single incident photon. Under such conditions, the output current of the SiPM is

proportional to the number of incident photons, and the SiPM is said to exhibit a linear re-

sponse. However, increasing the intensity of the light results in a greater probability of more

than one photon triggering a microcell “simultaneously” (that is, within the fall/relaxation

time of the output pulse). The response of the SiPM becomes increasingly non-linear, and

the current output is disproportionate to the number of incident photons.


(a)Oscilloscope trace of a real pulse from a SiPM array.

(b) Sketch of an idealised APD pulse.

Figure 2.3.3: Typical output pulse shapes from a SiPM and APD. (a) Example of a typical oscilloscope trace of a realsignal from a Hamamatsu SiPM array due to laser light. From Tahani [132]. (b) Sketch of a typical pulse from an APD,as described in the text. Note the characteris c shape: the asymmetry of the pulse (exaggerated) is due to the rela-ve sizes RSC ≪ RQC of me constants in the rise (RSC) and fall (RQC) regions. Typical rise and fall mes of the

pulse are< 100 ps and∼ 10-100 ns [133], respec vely. Based on Figure 8 of Piatek [127].


2.3.1 The Structure of the SiPM

A SiPM microcell is fabricated by initially growing a thin crystalline overlayer (a so-called

“epitaxial layer”) on a low-resistance, silicon-wafer substrate,6 followed by depositing succes-

sive layers of p- or n-type dopant of varying concentration in order to create a p-n junction.

Figure 2.3.4a shows a simple schematic of a microcell: a p−-type7 epitaxial layer (of typical

thickness ∼ 2 − 4μm) is deposited upon a p+ wafer/substrate (of thickness ∼ 300μm),

with a shallow diffusion of subsequent p+ layer, as shown, ∼ 0.5,μm below the surface of

the microcell. Finally, a very thin n+ layer is created on the surface. Applying a bias voltage

V ≳ Vbr (that is, operating in Geiger-mode), avalanche breakdown resulting from photon-

induced an electron-hole pair creation occurs within the photosensitive depletion region of

the resulting p-n junction. As shown in Figure 2.3.4a, in order to generate the electric field

necessary for avalanche (E = dV/dx ∼ 300 − 500 kV/cm), this depletion layer is neces-

sarily thin (dx ∼ 0.7 − 0.8μm). Such a configuration – known as a n-on-p structure – is

particularly sensitive to red light (i.e., long-wavelength, low-energy photons), which will be

absorbed within a few microns of the n+ layer. In contrast, short-wavelength (blue) pho-

tons must penetrate into the p-layer in order to generate photoelectrons which can initiate

avalanches [1], and so are less likely to be detected.8 Electrons generated in the avalanches

drift towards the electrode connected to the n+ layer via the quenching resistor, while holes6For most applications, the epitaxial layer has the same crystalline structure as the substrate, although prop-

erties such as the doping concentration and conductivity of the layer are typically different.7The superscript notation is adopted by some authors to classify the level of dopant concentration in im-

purity atoms. For example, Joshi [134] uses the following classification: light doping (n or p), 1014 − 1015 dopantatoms/cm3;medium doping (n+ or p+), 1016− 1017 dopant atoms/cm3; heavy/degenerate doping (n++ or p++),≥ 1018 dopant atoms/cm3.

8AtV ≫ Vbr, there is a much higher probability of detecting blue photons: under such conditions they arecapable of initiating avalanches in the n+-type material [1]. Note that in a p-on-n structure – in which a p-typematerial is deposited upon an n-type epitaxial layer/substrate – the opposite situation occurs. That is, the deviceis particularly sensitive to blue light, and less so to longer wavelengths.


drift through the low-field substrate and are collected at the other electrode. Guard rings

(n−-type) surrounding each microcell ensure a uniform electric field within the microcell,

preventing lateral breakdown, while each microcell is connected to the rest via a common

aluminium strip, allowing the SiPM signal to be read out. An anti-reflective coating (ARC),

made ofmaterials such as SiO2, increases the spectral response of themicrocell in the blue and

near-UV regions of the spectrum.

Due to efforts to minimise the so-called dark current in each microcell (see Section2.3.3),

commercially available devices are significantly more advanced than the simple structure dis-

cussed above. For instance, Figure 2.3.4b shows the schematic of a device fabricated by STMi-

croelectronics9 and analysed by Pagano et al. [135]. Additional features include, for example,

a p/p+ epitaxial layer and n−− substrate, the p+-n−− junction preventing photogenerated

carriers in the latter from reaching the former [136]. The implanted p+ material on the epi-

taxial layer enriches the p-n junction, while the deep optical trenches surrounding the active

region serve to minimise cross-talk (Section 2.3.3) between adjacent microcells.10

2.3.2 Properties of the SiPM

While preserving many of the attributes of the PMT, SiPMs also offer several important ad-

vantages in addition to those noted in Section 1. For example, not only are SiPMs relatively

inexpensive compared to the PMT– and easier to manufacture – but their small dimensions

allow for an extremely compact, light and robust mechanical design (while still maintaining

a high photosensitivity), thus making them easily adaptable to different applications.9STMicroelectronics N. V., Headquarters: 39, Chemin du Champ des Filles, Plan-Les-Ouates, Geneva CH

1228, Switzerland (http://www.st.com).10Not allmanufacturers of SiPMs implement optical trenches: for example,Hamamatsu donot include such

components in many of their devices.

http://www.st.com


(a) An idealised SiPM microcell.

(b) A realis c SiPM microcell.

Figure 2.3.4: Schema cs of an idealised and realis c SiPM microcell. (a) Le : A sketch of a simple SiPM microcell (notdrawn to scale), consis ng of an n-on-p structure. Right: The varia on of the electric field with depth, x, in the epi-taxial layer. From Buzhan et al. [137]. (b) Le : The components of a SiPM microcell fabricated by STMicroelectronics.The do ed line is the boundary of the deple on region, L is the ac ve area length and LP is the perimeter-lengthextension. The various layers and current components (1)-(5) are discussed in Pagano et al. [135]. Right: Front-sideview of the SiPM microcell: AB andAP are, respec vely, the ac ve and perimeter areas of the microcell. Adaptedfrom Pagano et al. [135].


The sensitivity of a photodetector may be characterised by its photon detection efficiency

(PDE), a property measuring the capacity of the device to detect radiation incident upon

it. Not every photon entering the SiPM fires a corresponding microcell: indeed, depending

upon the manufacturer, a typical PDE – defined by the ratioNdetect/Nphotons of the number

of detected to incident particles – is ∼ 20 − 40%, compared to a value of ∼ 20% at 420

nm for typical PMTs.11 In a SiPM of Ntotal microcells, the number Nfired firing is related to

Nphotons by [139]

Nfired = Ntotal

[1 − exp

(−Nphotons

Ntotal· PDE

)], (2.3.4)

and the number of avalanche photoelectrons is then N = Ge · Ndetect, yielding an output

current dQ/dt given by

I = e · dNdt = e ·dNphotons

dt · PDE · Ge . (2.3.5)

in which e is the charge of an electron.

Equation (2.3.5) assumes that the response of the SiPM is linear. That is, for low-intensity

light, in whichNphotons ≪ Ntotal, a Geiger-mode microcell may be fired by a single incident

photon before relaxing back into its quiescent state (recall Figure 2.3.3). However, at high

intensities (Nphotons ≥ Ntotal) the SiPM becomes saturated: the photon flux is so high that

multiple photons may be incident upon a microcell, either generating separate avalanches or11Cinti et al. [138] state a typical range of 20− 30% for a Hamamatsu MPPC. That of the PMT is from the

SensL technical note Introduction to the SPM – retrieved from http://www.sensl.com/downloads/ds/TN%20-%20Intro%20to%20SPM%20Tech.pdf –who also report a PDE of∼ 25% at a peak wavelength of 520nm for a SiPMwith 100μmmicrocell size. The PDEs of theHamamatsu S12571-050C and S12572-050CMPPCsused in Chapter 3 are 35% (see Table 3.1), while that of the SensL “M” series of SiPM used in Chapter 4 is 20%(see Table 4.1).

http://www.sensl.com/downloads/ds/TN%20-%20Intro%20to%20SPM%20Tech.pdf



occurring during the deadtime of the detector. In both cases only one photon is detected: in

the former, the avalanches are merged and quenched, while in the latter only the initial, trig-

gering photon is counted . AsNphotons −→ N−total, the SiPM thus operates in an increasingly

non-linear manner, and Equation (2.3.5) is no longer valid.

The PDE can be expressed as

PDE = εgeo · εtrigger (λ,V,T) · εQE (λ) , (2.3.6)

with the variables are described as follows:

• Geometric efficiency, εgeo: also known as the geometric fill factor, this is the ratio of the

active/photo-sensitive area of the SiPM to its total physical area.

• Tri ering probability, εtrigger: also known as the Geiger discharge probability, this de-

scribes the likelihood that electrons and holes created in the photon interaction initi-

ate an electrical breakdown, generating an avalanche. The triggering probability de-

pends upon the SiPM structure, and in particular the photon absorption length in

silicon (which varies from∼ 0.01μm to fewmicrometres for wavelengths in the range

300 nm < λ < 700 nm [140]).

• Quantum efficiency (QE), εQE: may be quantified by the ratio εQE(λ) = Ne−h/Nphotons

of the number of photogenerated electron-hole pairs, Ne−h, resulting in the output

current to the number of photons incident upon the photosensitive SiPM surface.

That is, the QE represents the probability that an incident photon will enter the de-

tector and generate a photoelectron, and so depends upon the photon wavelength λ.

Quantum efficiencies of ∼ 55% have been reported for Hamamatsu MPPCs [138],


while SiPMs manufactured by Ketek12 have values of ∼ 80%. The latter report the

QE of a PMT to be 25 − 40%.

2.3.3 Noise in Silicon Photomultipliers

It is well known that for low-intensity light the SiPM is capable of single-photon resolution

at room temperature (see, for example, the discussion of photopeak spectra in Buzhan et al.

[131]). However, although it is desirable that the avalanche current generated by a microcell

originates only from an initial photoelectron in the silicon, spurious avalanches can be trig-

gered by other mechanisms within the depletion layer of the semiconductor. The thermal,

tunnelling and trapping processes outlined below can all generate free charge carriers which

subsequently induce avalanches in the substrate, contributing to an overall noise component

superimposed upon the photon-triggered signal. This noise is recognised as an inherent part

of the output current, and impairs the sensitivity of the detector [141].13

Themain source of noise in a PMT is due to the so-called thermionic emission of electrons

emitted by the photocathode (and, to a lesser extent, the dynodes and glass wall of the PMT

[142]).14 As a random process, the resulting dark current exiting the dynode chain adds a sta-

tistically fluctuating component to the overall photoelectron current measured by the PMT.

By comparison, as well as an inherent component due to thermally generated electron-hole

pairs, the SiPM contains “parasitic” contributions to the noise – known as crosstalk and af-

terpulsing – which arise from the primary, photoelectron-induced avalanche. Here, each of12Ketek GmbH, Hofer Straße 3, 81737 München, Germany (http://www.ketek.net).13Note that due to its relatively high gain, the level of electronics noise of a SiPM is negligible, corresponding

to less than 10% of the signal from one photoelectron [129].14For example, a PMT with a 50-mm-diameter photocathode may generate∼ 105 such electrons per second

at room temperature [142].

http://www.ketek.net


these components is considered in turn.

2.3.3.1 Thermally Generated (Dark) Noise

Thermally generated conduction-band electrons in the semiconductor substrate are recog-

nised as being themain source of noise which limits the ability to resolve single-photon peaks

in the SiPM’s photopeak spectrum. Since a microcell is equally responsive to a thermal elec-

tron as it is to a photoelectron, then upon firing, both charge carriers will produce the same

avalanche current, adding a random-noise component to the scintillation signal.

Such noise in a SiPM can be reduced in two ways. Using a discriminator in the read-out

electronics, it is found that the dark rate decreases significantly with an increase in threshold

level. Setting a threshold level corresponding to the simultaneous firing of successive num-

bers of microcells gives rise to a distinctive staircase function (Figure 2.3.5a). It is seen that

an increase in the threshold voltage by an amount corresponding to the one-photoelectron

amplitude reduces the dark rate by almost an order ofmagnitude, while setting the threshold

to higher than the four-photoelectron amplitude results in a dark rate below 1 kHz [1].

A second approach is to cool the SiPM: it is well known that the dark rate in a SiPM

is temperature dependent. For instance, while a factor of ∼ 2 reduction in dark rate has

been reported for every 8 K drop in temperature [1],15 decreasing for example from∼ 1 − 2

MHz/mm2 at room temperature to∼ 200 Hz/mm2 at 100 K [140].15Both SensL and Ketek report a 50% reduction in dark rate for every 10 K decrease for their respective

SiPM devices. See, for example, the SensL technical note Introduction to the SPM (http://www.sensl.com/downloads/ds/TN%20-%20Intro%20to%20SPM%20Tech.pdf) and Ketek documentation at http://www.ketek.net/products/sipm-technology/device-parameters, respectively.



http://www.ketek.net/products/sipm-technology/device-parameters

http://www.ketek.net/products/sipm-technology/device-parameters


(a) Dark-rate-versus-discriminator-threshold behaviour in a SiPM.

(b) Primary mechanism: op cal crosstalk and “trapped” a erpulsing.

(c) Secondary mechanism: delayed crosstalk and “diffused” a erpulsing.

Figure 2.3.5: Noise mechanisms in a SiPM: (a) The dis nc ve “staircase func on” of the dark-rate-versus-discriminator-threshold for a typical 1 × 1 mm2 SiPM (the Hamamatsu S10362-11-050C) operated at a gain of6.5 × 105. From Renker and Lorenz [1]. (b) The primary noise mechanism in a SiPM: a erpulsing (AP-traps) is dueto carriers trapped in crystal defects within a microcell, while (almost instantaneous) op cal crosstalk (CT-opt) is dueto secondary photons reaching neighbouring microcells (see text). (c) The secondary noise mechanism in a SiPM: thediffused a erpulsing (AP-diff) and crosstalk (CT-diff) effects due to the ac on of secondary photons in the siliconsubstrate (see text). Figures (b) and (c) are from Rosado et al. [143].


2.3.3.2 Inter-microcell Crosstalk & Afterpulsing

It is well known that a reverse-biased p-n junction undergoing breakdown fluoresces [144],

and Lacaita et al. [145] have shown that these “secondary” photons (of energy ≳ 1.14 eV)

are created at a rate of about 3 × 10−5 per electron crossing the junction. Such photons are

strongly correlated to a primary avalanche and, if created in amicrocell, are capable of almost

instantaneously triggering Geiger discharge(s) in adjacent microcell(s) in a process known

as optical crosstalk (Figure 2.3.5b). For a SiPM gain of 105 − 107, it is therefore expected that

∼ 3−300 suchphotonswould be generated by amicrocell during breakdown. In a secondary

mechanism, those secondary photons which are photoabsorbed in the substrate can generate

minority carriers, which either diffuse to the initially fired microcell or to a neighbouring

microcell, with the latter effect knownasdelayed crosstalk (Figure 2.3.5c). Such charge carriers

are accelerated in the electric field and energetically capable of inducing avalanches in these

locations. Indeed, the triggering of adjacentmicrocell(s) in this way can even spread in a chain

reaction across multiple microcells, contributing appreciably to the SiPM noise.

In afterpulsing, defects and/or impurities in the silicon substrate of the microcell act as

a charge trap. During an avalanche, a charge carrier may be trapped in the defect and then

released after themicrocell has relaxed to its quiescent state, only to trigger subsequentdelayed

avalanches (∼ 100 ns − 2μs) within the same microcell (Figure 2.3.5b). Due to the various

types of trapping mechanisms in a semiconductor, there may be several components to the

afterpulsing effect. A second mechanism may occur (Figure 2.3.5c) in which the minority

carriers triggered by the secondary photons are themselves trapped in the defect, leading to

so-called diffused afterpulsing (see, for example, Rosado et al. [143]). Afterpulsing results in

an increased count rate for an incident photoelectron.


The effects of crosstalk and afterpulsing are critical factors in the design and operation

of SiPMs. For example, introducing blocking/isolation trenches – comprised of an optically

absorbingmaterial – between adjacent microcells (see, for example, Figures 2.3.4b, 2.3.5b and

2.3.5c) is a common strategy in reducing optical crosstalk. However, increasing the width of

such trenches reduces the geometric fill factor of the SiPM, thus decreasing the PDE.

Theoretical techniques to model noise are based upon upon approaches such as Monte

Carlo simulations and statistical models, with several physical models dealing with crosstalk

and afterpulsing having appeared in the literature (see, for example, references [146–159]).

The theory and operation of the SiPM outlined in this chapter forms the basis of the

photosensors used in both applications discussed in this thesis. By applying a reverse bias to

the SiPMs of interest, such that it operates in limited Geiger-mode, the detection of an inci-

dent photon occurs due to the large gain of the photosensor. However, the resulting signal

output is necessarily affected by the inherent, undesirable noise mechanisms outlined in this

chapter. In both applications, it is important to find the optimal configuration in which the

desired signal frequency can be detected at a level sufficiently far above the noise (or indeed

any additional components due to extraneous sources in the surrounding environment) to

be attributed (with confidence) solely to the radioactive source alone. Since cooling of the

SiPM device is impractical in both applications, an important part of both projects was to

investigate the discrimination threshold – or range of thresholds – above/within which the

source signal was maximised while that of the noise suppressed. Experimental details of such

techniques, and the subsequent analysis, are discussed in the following two chapters.

Chapter 3

Application: Personal Radiation Detector

Recently, several groups have reported on the use of SiPMs in radiation detectors, intended

for use by emergency workers and first responders. The first commercially available PRD

was the “SENTIRAD” system of Osovizky et al. [160–165], subsequently renamed “PDS-

GO” [107], a devicewhich the authors claim [107] satisfies theDARPAand SIGMAprogram

requirements discussed in Chapter 1. Their PRDuses a 3× 3 mm2 SensL1 SiPM (the SPM303

model) coupled to an 8×8×30mm3 CsI(Tl) crystal scintillator. Meanwhile, the “MiniSpec”

systemdue toBecker and Farsoni [166] – also employing a SiPMmanufactured by SensL (the

MicroSL60035model) coupled to aCsI(Tl) crystal –hasbeendesigned as a low-cost, compact

radioisotope identifier, capable of interfacing wirelessly with any cell phone. Ahmadov and

co-workers are currently developing [167] a compact γ-ray detector using a 3× 3× 0.5 mm3

SiPM (developed by Sadygov et al. [168]) optically coupled to a teflon-wrapped Lutetium

Fine Silicate (LFS) crystal scintillator (Table 2.1).2

1SensL Technologies, Ltd., 6800 Airport Business Park, Cork, Ireland (http://sensl.com/).2These authors also present results of α-particle measurements with LFS, as well as fast-neutron measure-

ments using a teflon-wrapped Stilbene crystal scintillator (Table 2.2).

29

http://sensl.com/

CHAPTER 3. APPLICATION: PERSONAL RADIATION DETECTOR 30

This chapter is organised as follows: initially, an overview of the project is provided (Sec-

tion 3.1), followed by a discussion of the SiPMunits that aremost promising to be adapted to

EIC’s requirements. Details of the experimental set-up (Section 3.3) – and subsequent results

(Sections 3.4 and 3.5) – in evaluating various SiPM-wrapped scintillator configurations are

then presented. In Section 3.6, measurements pertaining to the use of a leadmini-calorimeter

as a possible light-producingmedium are given. Details of correspondingmeasurements, ob-

tained when the nuclear electronics housed in the SPARROGroup’s Detector Development

Laboratory (DDL) were replaced by their modified EIC counterparts, are discussed in Sec-

tion 3.7.

A further investigation considers the use of SiPMs in neutron detection (Section 3.8).

Neutron radiation can present a serious nuclear and biological hazard,3 and the ability to

detect its presence remains of critical importance in, for example, homeland security.4 It

is hoped that this preliminary study may form the basis of further collaboration between

SPARRO and EIC, with the possibility of the latter incorporating a neutron-detection capa-

bility into a PRD.3Over time, neutron beams can also inflict appreciable radiation damage at a microscopic level in exposed

materials, leading to, for example, their embrittlement or swelling. Degradation in solids via theWigner effect[169] is of concern in the neutron moderators of fission reactors.

4Although the ability to detect neutrons is a DARPA requirement, the PDS-GO PRD [107] does not cur-rently have that capability.


3.1 Project Background

EIC’s existing PRD platform, the CT007 Gamma Detector,5 employs a Geiger-Müller (G-

M) tube (the LND 713 Thin wall beta-gamma detector6) as its photodetector. The CT007

device has no buttons or screens: instead, it communicates wirelessly with a smart-phone

(or other mobile computing platform) via a wireless BluetoothTM (BT) connection, allowing

radiation-exposure data to be displayed easily to the user and/or transmitted to a remote data

repository. Although compact in its dimensions (with an effective length of 27.9mm and di-

ameter 7.77 mm), the LND device lacks the durability of the SiPM-based detector, as well as

requiring a larger operating voltage (∼ 450-650 V), and possessing a longer dead time (45μs)

and lower sensitivity to γ-ray photons (7.5 cps/mr/hr for 60Co) than a typical SiPM-based

detector. Furthermore, G-M tubes are incapable of detecting neutrons, and in operating es-

sentially as a gross gamma counter do not provide spectral information (and therefore cannot

be used in, for example, radionuclide identification).

The target of theEIC/SPARROcollaborationwas todevelop aprototype gross-counting

gammacounter basedon theSiPM(and coupled to an appropriate light-producingmedium),

which offers a significant enhancement in light sensitivity over that of the CT007 unit, while

still minimising cost and retaining the overall physical dimensions (50 × 90 × 15 mm3) of

the latter. It is hoped that such a device could subsequently be used by EIC as the basis for a

commercially available detector, replacing the existing CT007 Gamma Detector.5See, for example, http://www.eic.nu/CT007.html, with technical specifications available at http:

//www.gammawatch.com/pdf/PromoCT007withScreenAndButton.pdf.6LND Inc., 3230 Lawson Boulevard, Oceanside, New York 11572, United States (http://www.lndinc.

com/).

http://www.eic.nu/CT007.html

http://www.gammawatch.com/pdf/PromoCT007withScreenAndButton.pdf

http://www.gammawatch.com/pdf/PromoCT007withScreenAndButton.pdf

http://www.lndinc.com/

http://www.lndinc.com/


3.2 Photosensor Selection

The SiPMs of interest in this project, listed in Table 3.1, are from the Hamamatsu7 S12571-

050C and S12572-050C series (Figure 2.3.2). Units of the former contain a 20 × 20 pixelated

array, with microcell dimensions 50 × 50μm2, giving an overall effective photosensitive area

of 1 × 1 mm2. Those of the S12572-050C series provide a larger effective photosensitive area

of 3 × 3 mm2,8 with 3600 microcells arranged in a 60 × 60 array with pitch of 50μm.9 The

individual units are identified by the corresponding serial numbers given in Table 3.1.

In a previous technical report [170] the SPARROGroup presented results of characteri-

sation tests performed on the S12571-050C and S12572-050C MPPCs, coupled to a paper/Al-

foil wrapped, 13× 13× 50-mm3 BC-416 plastic scintillator, in order to assess the photosensor

requirements of this project.10 As shown in Figure 2.3.2, units of both SiPM series did not

have any accompanying electronics, and so readout circuits were constructed “in-house” in

the DDL at the University of Regina, with those given in Figure 3.2.1 being found to provide

SiPM output pulse shapes of sufficient amplitude and duration.11 Using the group’s existing7Hamamatsu Photonics K. K., 360 Foothill Road, Box 6910 Bridgewater, New Jersey 08807, United States

(http://www.sales.hamamatsu.com/).8For the remainder of this chapter, the S12571-050C/P and S12572-050C series units are often referred to in

the text by their sizes – 1 × 1 mm2 and 3 × 3 mm2, respectively.9Technical details of the S12571-050C and S12572-050C series are available at http://www.hamamatsu.

com/jp/en/S12571-050C.html and http://www.hamamatsu.com/jp/en/S12572-050C.html, respec-tively.

10Although Tables 2.1 and 2.2 list numerous scintillators with superior light-yields compared to BC-416, thecommercial viability of PRDsnecessarily requires the use of low-costmaterials (a stipulation also outlined in theDARPARFI discussed in Chapter 1). The cost of several of the scintillators listed in the respective tables makesthemprohibitively expensive for use in the current project. Also, whileNaI(Tl) (Table 2.1) would be expected tobe the preferred choice of scintillator for this project, due to its considerable light yield and relatively low-cost,its strong hygroscopicity (requiring it to be housed in an air-tight enclosure) and relatively long decay timemakeit unsuitable.

11The electronics of an existing 1 × 1-mm2 unit (SiPM #1853) produces a noise signal of ∼ 5.5 mV (width∼ 50 ns) at Vob = 1.5 V, while that of the 3 × 3-mm2 SiPM (#269) is∼ 8 mV at Vob = 0.0 V, with a durationof∼ 150 ns.

http://www.sales.hamamatsu.com/

http://www.hamamatsu.com/jp/en/S12571-050C.html




Serie

sSe

rial

Num

ber

Ope

ratin

gvo

ltage

(V)

Gain

G(10

5 )Darkco

unt

(k/(0.5t

hr))

Refractive

inde

x

Spectra

lresp

onse

(nm

)

Peak

resp

onse

(nm

)PD

E(%

)

S125

71-0

50C

184

66.85

12.6

91.7

1.41

320-

900

450

35188

66.91

12.5

88.5

S125

72-0

50C

266

66.91

12.5

1000

1.59

320-

900

450

3526

766

.91

12.4

1100

269

66.84

12.5

950

Table3.1:Proper

esoftheindividualHam

amatsu

S12571-050C(1×

1mm2 )andS12572-050C(3×

3mm2 )MPPCunits

used

intheEIC/SPARR

Ocollab-

oraon.The

“Serialnum

ber”ofeach

device

isoftheform

3L000X

XX,inwhich

XXXreferstothethree-digitnum

bershown.“Dark”referstothedark-count

measurement.Inbothseries,units

have

thestated

operang

voltages,recommendedtobe

2.6Vabovebreakdow

n(V

br=

65±

10V)bytheHam

amatsu

datasheets.The

PDEvalues

areatpeak

wavelengths,and

donotinclude

crosstalkandaerpulsing.A

llproper

esarestated

fora

temperatureof25

◦ C.The

seriesdatashow

nisfrom

thecorrespondingHam

amatsu

datasheets(see

text),whilethatoftheindividualunits

isfrom

theiraccom

panyingpackaging.


(a) 1 × 1-mm2 readout electronics.

(b) 3 × 3-mm2 readout electronics.

Figure 3.2.1: Readout electronics constructed in the DDL at the University of Regina for the 1× 1-mm2 and 3×3-mm2

series of SiPMs (both series have no accompanying electronics). The RC circuit was used to change the shape of theoutput pulse.


familiarity and experience with older (2010) Hamamatsu units, devices such as the S12045X

SiPM array (3× 3 mm2) were employed as “control” units with which to evaluate the newer

units. While external-trigger tests (using a PMT) showed that 1 × 1-mm2 units could suc-

cessfully reproduce the characteristic ADC spectra of 241Am and 90Sr, it was found that they

could not produce a signal sufficient enough for use in a self-triggering configuration (the

preferred set-up).12 Self-triggering tests on the 3× 3-mm2 SiPMs successfully reproduced the

characteristic energy spectra of 90Sr (β-spectrum) and 60Co (γ-spectrum), respectively, over a

range of overbias voltages. The results of the report suggested that for the 3 × 3-mm2 units,

a clear distinction between signal and background could be obtained, tentatively identify-

ing the S12572-050C series – with the simple readout circuit shown in Figure 3.2.1b – as the

preferred choice of photosensor for use in this project.

3.3 Experimental Method

While the cost and space requirements of the PRD would naturally favour a singles mode

of operation, there remains the possibility that a design involving two 3 × 3-mm2 units –

operating in coincidence – would be the part of the preferred set-up. Therefore, tests for

both singles and coincidence configurations – with schematics shown in Figures 3.3.2 and

3.3.3, respectively – were considered for SiPM units (discussed in Section 3.2), coupled to a

light-producing medium, for several wrapping materials chosen for their high reflectivity.

Recall that in addition to a contribution from the source, the SiPM output signal neces-

sarily includes a background component which, in turn, contains an inherent contribution12External trigger tests on the 3× 3 mm2 units were not performed, since in having the capacity of nine times

the light collection of their smaller counterparts, they would also be expected to reproduce the 90Sr spectrumcomfortably for EIC’s requirements.


due to noise. The background originates mainly from cosmic rays,13 as well as natural and

artificial sources in the surrounding environment, while the noise contribution arises from

that associated with the SiPM (see Section 2.3.3) and, to a lesser extent, the accompanying

electronics [129]. In the tests reported here, the size of the source signal relative to that of the

background and noise components was obtained by measuring the count rates of the noise

(measured in the absence of the scintillator and source), background (measured in the pres-

ence of the scintillator, but no source), and “total” (i.e., combined source, background and

noise) signals over a range of discriminator threshold voltages.

In coincidence measurements, each SiPM unit was positioned opposite to each other in

physical contact at either end of the wrapped scintillator. When required, the source was

placed in contact with the wrapping approximately half-way along the scintillator. Each

SiPMhad a bias voltage,V, at the operating voltage listed in Table 3.1, supplied to it by a sepa-

rate Keithley 6487 Picoammeter,14 with the resulting SiPM output signal being processed by

readout electronics. Two discriminator models were employed during this project: a Phillips

Scientific Model 705 Octal Discriminator15 and a CAEN N840 8 Channel Leading Edge Dis-

criminator,16 while a Joerger Model VS Dual-channel 100 MHz Visual Scaler17 recorded the

number of counts over an arbitrary time interval. While this arrangement allowed the coinci-

dencemeasurement to be performed by a Phillips ScientificModel 755Quad Four-fold Logic13For horizontal detectors, the experimentally accepted value for the vertical flux of cosmic-ray muons of

energy Eμ > 1 GeV is∼ 1/cm2/min at sea level [171].14Keithley Instruments, Inc., 28775 Aurora Road, Cleveland, OH 44139, United States (http://www.

keithley.com/).15Phillips Scientific, 31 Industrial Avenue, Suite 1, Mahwah, NJ 07430, United States (http://www.

phillipsscientific.com/).16CAENTechnologies, Inc., 1140Bay Street, Suite 2C, Staten Island,NY 10305,United States (http://www.

caentechnologies.com/).17Joerger Enterprises Inc., 51 Idle Day Drive, Centerport, NY 11721, United States (http://www.

joergerinc.com/).

http://www.keithley.com/

http://www.keithley.com/

http://www.phillipsscientific.com/

http://www.phillipsscientific.com/

http://www.caentechnologies.com/

http://www.caentechnologies.com/

http://www.joergerinc.com/

http://www.joergerinc.com/


Unit, singlesmeasurements for each SiPMcould be conveniently performed by subsequently

re-routing the corresponding input to the logic unit directly to the output of the discrimina-

tor. A similar arrangement was used in obtaining the background and noise measurements.

For a given threshold voltage, the count ratewas determined bymeasuring the total num-

ber of counts (read from the Joerger Scaler) recorded over a given time interval, the lattermea-

sured using a stopwatch. The length of time used in each trial was decided on an ad hoc basis:

for example, at lower thresholds (inwhich a larger number of counts could be expected), time

intervals typically yielding greater than 100 counts were used. For moderate-to-high thresh-

olds, if the resulting count ratewas low enough that the 100-countminimumwas impractical,

longer time intervals were often chosen to ensure sufficient statistics were obtained to more

fairly represent the count rate.

Preliminary tests showed that in some instances, themeasurednoise signalwas larger than

the corresponding background and 90Sr source signals.18 To help minimise the noise con-

tribution, the detectors and accompanying readout electronics were moved to a grounded,

∼ 35× 23× 12 cm3 copper box, constructed at the University of Regina, which it was hoped

would provide the components with electromagnetic shielding from external radiofrequen-

cies. Further tests showed that by shielding the detectors and power cables in such a manner,

an appreciable reduction in noise (in some cases a factor of∼ 5 at∼ 50 mV threshold) could

be achieved. This shielding configuration was retained throughout the experimental trials.

As always, care was taken to ensure that the SiPM and scintillator operated in an envi-

ronment in which external sources of light (in particular, the ambient light of the laboratory)

were kept to a minimum. All photosensitive devices were housed and operated in a dark18This phenomenon was also observed when replacing the BC-416 plastic scintillator with the linear alkyl-

benzene (LAB) scintillator.


Figure 3.3.2: The experimental set-up used in singles measurements of the ac vity of a source (shown for a SiPMcoupled to a wrapped BC-416 plas c scin llator). The dashed-outlined boxes are included for clarity: the upper boxencloses the power supplies, while the lower indicates the electronics and measuring devices. Although the CAENdiscriminator is shown, the Phillips model was also used in some instances (see text).


Figure 3.3.3: The experimental set-up used in coincidence measurements of the ac vity of a source, for two SiPMs(shown for two S12572-050C units coupled to a BC-416 plas c scin llator). The dashed-outlined boxes are includedfor clarity: the upper box encloses the power supplies, while the lower indicates the electronics and measuring de-vices. Although the Phillips discriminator is shown, the CAEN model was also used in some instances (see text).


enclosure: initially in a 4.5-m long, dark box, housed in the DDL, which was later replaced

by the grounded copper box placed within the dark box. The various components forming

the detector were connected to power supplies, current, voltage and signal-processing devices

on the outside by carefully controlled openings in the walls of both enclosures. Additional

precautions were employed to minimise extraneous light, such as draping a thick black cloth

over the box, and switching off the laboratory lights.

3.4 Light-Producing Media: Evaluation of the BC-416

Scintillator with Various Wrappings

Attention is now paid to tests on the 13 × 13 × 50-mm3 block of BC-416 plastic scintillator,

wrapped with various reflective materials, which – when coupled to the preferred choice of

SiPM–maximises the number of photons originating fromwithin the scintillator which are

subsequently detected by the photosensor, producing a SiPM output signal with an appre-

ciable signal-to-noise ratio. Recalling the 50 × 90 × 15-mm3 dimensions of the CT007 unit,

the challenge is to limit the detection apparatus to a volume of∼ 60× 13× 13 mm3, with the

remaining space reserved for EIC’s electronic circuitry and BluetoothTM technology.

Benchmark tests on commercial PRDs are often performed using 60Co (emitting γ-rays

of energy ∼ 1.3 MeV), with some industry standards also requiring the capability to detect

gammas of energies as low as∼ 60 keV (provided, for example, by 241Am). In the current re-

port, radiation levels at the lower limit of those typically found in potentially hazardous envi-

ronments were replicated experimentally using the commercially available laboratory sources

listed in Table 3.2.


It is well known that scintillators emit photons over a broad range of wavelengths [172],

with many such materials having a peak emission in the general range 375 nm-480 nm [115,

172–177] (see, for example, Tables 2.1 and 2.2), providing a favourable optical coupling to

commercially available SiPMs including, for example, those used in the current tests (see Ta-

ble 3.1). To maximise the amount of light incident upon a SiPM, it is common practice to

wrap a scintillator in a highly reflective material (providing a high reflection coefficient at the

scintillator’s emission wavelength), in order to minimise the photon flux generated within

the scintillator from being refracted through its surface. Additional techniques that are of-

ten employed to improve light collection include finely polishing the scintillator surface, or

introducing a light guide to focus the scintillation light onto the photosensitive surface of the

SiPM. A “dry” scintillator-detector coupling necessarily includes a thin air-gap between the

two media, resulting in some light being lost due to reflection from the scintillator-air and

air-SiPM boundaries. Application of an intermediate layer of a substance – such as optical

epoxy (referred to above), optical grease or a silicon “cookie” – can noticeably increase light

collection: such materials have refractive indices similar to those of the scintillator and SiPM

surface, providing an optically favourable path for the transport of light to the detector.

3.4.1 Paper/Al-Foil Wrapping

Commonwhite printer paper and aluminium foil were initially chosen as reflective materials

to wrap the BC-416 plastic scintillator. The former has a typical reflectivity of ∼ 0.90 (for

undyed paper at a wavelength of 450 nm) [179], while Janecek andMoses [180] have reported

a reflection coefficient of ∼ 0.78 (at 440 nm) for commercially available aluminium foil of

0.025 mm thickness. Paper and aluminium-foil sheaths were constructed such that a layer of


Isotope Cat. T1/2 Source E A0 ANo. (MeV) (105 Bq) (105 Bq)

24195 Am ST162 432.2 y

α

5.485

3.70(01/03/04) 3.65

5.4435.3885.5465.512

γ0.05950.02630.0332

13356Ba SS258 10.51 y

β 0.080

0.395(01/05/90) 0.091

0.133

γ

0.3560.0810.3030.3840.2760.0800.0530.1610.223

6027Co SS098 5.271 y

β 0.318 0.375(01/11/89) 0.019γ 1.333

1.1739038 Sr

SS389

28.79 y β 0.546

36.9(15/11/08) 33.8

9039Y 64.00 h

β2.28010.51940.0938

γ 2.18621.7607

Table 3.2: Summary of the radioisotopes used in the PRD tests. “Cat. No.” is the catalogue number in the Radioac veMaterials Inventory at the University of Regina. A0 is the approximate original ac vity of the sample, calculated usingthe date of purchase (included in parentheses, in dd/mm/yy format) and the approximate ac vity,A (listed in theinventory and assumed to have been measured on the date of its publica on, July 10, 2012). T1/2 is the half-lifeof the isotope (“y” and “h” indica ng years and hours, respec vely), while “Source” and E refer to, respec vely, theradia on type (α-, β- and γ-decay) and corresponding (approximate) energy, emi ed by the isotope and/or its decayproduct(s). In some instances, only a selec on of energies are listed. Note that 90Sr β−-decays via 90Sr →90 Y →90

Zr (with the 90Zr being stable); the 1.7607 MeV and 2.1862 MeV γs – and various∼ 1.8− 18 keV X-rays (not shown)– emi ed by 90Y have such low frequencies that 90Sr/90Y is almost a pure β− emi er (with the 2.2801 MeV electrondominant). Data was obtained from reference [178].


aluminium-foil tape, manufactured by Venture Tape,19 was glued to one side of the white pa-

per, with the foil attached to the inside surface of the sheath. This arrangement was carefully

folded to enclose the four 13× 50-mm2 and one 13× 13-mm2 surfaces of the scintillator, with

the remaining end-face being in contact with the SiPM.

In the following figures, comparisons are made between the frequency obtained in the

presence of a source and those measured for the background and noise (using either the

Phillips Scientific Model or CAEN discriminators). Note that while each of the graphs in-

cludes the frequency-threshold behaviour of the “source”, it is recalled that the overall SiPM

signal due to scintillating photons originating from the source necessarily includes that of

the background which, in turn, contains a noise component. Therefore, it is by observing

the frequency curve of the overall signal relative to that of the background which provides

an indication of how the frequency of the actual source varies with threshold, and hence the

quality of the light-producing medium in distinguishing the source component of the SiPM

signal at a given threshold.

Characteristic frequency-threshold plots were expected for both singles and coincidence

measurements. At low thresholds, even thermal electrons in the Si substrate can trigger a

sufficiently large output voltage signal in the SiPM to contribute to the total count. Con-

sequently, the noise component contributes appreciably at the lowest thresholds, before de-

creasing by typically 2-4 orders of magnitude over a small range of threshold voltages as the

threshold is increased above the voltage generated by most of the low-level SiPM and elec-

tronic noise.

Figure 3.4.4 shows plots using the Phillips Scientific Model discriminator with the out-19VentureTapeCorporation, 30CommerceRoad, Rockland,MA02370-1053, United States (http://www.

venturetape.com).

http://www.venturetape.com

http://www.venturetape.com


put signal amplified×10, the latter explaining the considerably higher frequencies measured

over a larger threshold range with respect to those tests employing the CAEN device. The

singles curves for units #266 and #267 (Figures 3.4.4a and 3.4.4b, respectively) are more-or-

less identical, with the “total” frequency decreasing steadily from∼ 6 kHz to∼ 3 kHz over

the 40− 100mV threshold range. Meanwhile, the frequency due to the 90Sr source increases

quite substantiallywith respect to thebackground/noise over the same range,with the former

comprising almost all of the measured frequency (∼ 1 kHz) for ≳ 80 − 100 mV compared

to the latter (≲ 10 Hz).

With respect to the coincidence plot (Figure 3.4.4c), there is an appreciable count rate due

to the source even at lower thresholds, while the background and noise fall-off considerably

(the former levelling-off somewhat to lie between 0.4 − 1.0 Hz). Beyond ∼ 25 mV, almost

all of the frequency over the measured range is due to the source.

It is evident in Figures 3.4.4a-3.4.4c that the background decreases significantly with in-

creasing discriminator threshold as the output signal resulting from scintillation photons

due to secondary radiation sources in the surrounding environment of the DDL falls below

threshold. The fact that all three 90Sr signal curves are significantly larger than their respective

backgrounds over mid-to-high thresholds suggests that the background is not due to cosmic

rays, which would be expected to deposit a similar energy (∼ 2 MeV/(g/cm2)) in the 13-mm

thick scintillator as the 90Sr β−-particles (2.2801 MeV), resulting in similar frequency magni-

tude thresholds.20

20In calculating the energy deposition per unit distance travelled,−dE/dx, of particles in amaterial using theBethe formula [181], it is found that at low incidentmomenta, the energy loss−dE/dx decreases to aminimum,before increasing steadilywith increasingmomentum. Particleswithmomenta corresponding to thisminimum– such as cosmic-ray muons – are known as minimum ionising particl (MIPs), and the loss −dE/dx withina material is relatively constant, with the stated value of 2 MeV/(g/cm2) being the commonly accepted result[171].


(a) Singles (SiPM #266): 90Sr, BC-416 scin llator,Al-foil wrapping.

(b) Singles (SiPM #267): 90Sr, BC-416 scin llator,Al-foil wrapping.

(c) Coincidence (SiPMs #266 and #267): 90Sr, BC-416scin llator, Al-foil wrapping.

Figure 3.4.4: Frequency-threshold graphs, in the presence of the 90Sr source, for SiPMs #266 and #267, dry coupledindividually (in singles mode) and collec vely (in coincidence mode) to a paper/Al-foil-wrapped, BC-416 plas c scin-llator. Note that it is the behaviour of the “source” (i.e., dark blue) curve rela ve to the background (red) curve which

provides an indica on of how the frequency of the 90Sr signal varies with threshold (see text). Measurements wereperformed with the SiPM signal being amplified (×10) prior to entering the Phillips Scien fic Model discriminator.


In Figure 3.4.4, as with all frequency-threshold graphs in this chapter, the error, σf, in

frequency f = N/Δt, in which N is the total number of counts (with error σN =√N), is

given via

σf =

√(∂f∂N

)2

σ2N +

(∂f∂Δt

)2

σ2Δt (3.4.1)

as

σf =√

N(Δt)2 [1 +Nσ2Δt] , (3.4.2)

in which σΔt is the error in the run-time Δt.

Figure 3.4.5 involves the 60Co source with the CAEN discriminator and an unamplified

signal. Over the much smaller range of thresholds, the graphs exhibit the characteristic be-

haviour discussed above. For example, in the singles curves, for thresholds ≲ 4 mV, the

background/noise frequency dominates the overall signal (even comprising approximately

one-third of it at 4 mV), before the noise falls off by a factor of 104 over the range 4 − 8

mV. The background does not decrease as dramatically, being reduced to∼ 0.1 − 1 Hz over

the range 6 − 25 mV. While the overall signal approaches that of the background at higher

thresholds, in the threshold “window” of 5 − 15 mV the signal due to the 60Co is dominant:

for example, at 10mV the source frequencymeasured by SiPM #266 is a factor of∼ 50 larger

than the background. That is, the PRDoperating in this configuration within this threshold

window would easily detect 60Co.

As in Figure 3.4.4c, the frequency contribution from the source dominates the coinci-

dence curve (Figure 3.4.5c) at low threshold, being 3− 4 times larger than background/noise


(a) Singles (SiPM #266): 60Co, BC-416 scin llator,Al-foil & BC-620 wrappings.

(b) Singles (SiPM #267): 60Co, BC-416 scin llator,Al-foil & BC-620 wrappings.

(c) Coincidence (SiPMs #266 and #267): 60Co,BC-416 scin llator, Al-foil & BC-620 wrappings.

Figure 3.4.5: Frequency-threshold graphs similar to those in Figure 3.4.4, except for the 60Co source, coupled to aBC-416 plas c scin llator wrapped alternately in Al foil and BC-620 reflec ve paint. Measurements were performedwith the CAEN model discriminator and no amplifica on.


even at the lowest recorded measurement. The noise is reduced by a factor of ∼ 1000 over

a range of 2 mV, providing a negligible contribution at thresholds beyond 3 mV. A similar

threshold window of 2 − 13 mV exists over which the vast majority of the counts arise from

the source rather than background.

3.4.2 BC-620 Reflective Paint Wrapping

BicronBC-620, a diffuse, reflective paintmanufacturedbySaint-GobainCrystals,21 was tested

as a scintillator wrapping. This is a highly efficient reflector (with a reflectivity of∼ 0.96 over

a wavelength range of∼ 440− 550 nm22) employing a special grade of titanium dioxide in a

water-soluble binder. Each 13× 50-mm2 side received three coats of paint.

Singles and coincidence measurements for the 60Co source are presented alongside those

for the paper/Al-foil wrapping in Figure 3.4.5. In comparison with the corresponding mea-

surements of the latter, it is evident that above the smaller thresholds, replacing the paper/Al-

foil with reflective paint leads to a steady increase in light production over the mid-range

(∼ 6 − 17 mV) of thresholds, before the two signal curves reach a maximum difference,

and approach each other at the higher threshold voltages. In both singles measurements, the

frequency curve for the reflective paint is approximately four-times greater than that of the

paper/Al-foil by∼ 15 mV, and almost an order of magnitude larger by∼ 17 mV.Meanwhile,

in coincidencemode, the frequency curves of the background are of similar shape andmagni-

tude as those observed in the paper/Al-foil measurements, while the total curve for the paint

is a factor of∼ 10 times larger than the corresponding paper-foil result in the range 15−17mV.21Saint-Gobain Crystals, 17900 Great Lakes Pkwy, Hiram, OH 44234, United States (http://www.

crystals.saint-gobain.com/).22See the BC-620 data sheet (http://www.crystals.saint-gobain.com/uploadedFiles/

SG-Crystals/Documents/SGC%20BC620%20data%20sheet.pdf).

http://www.crystals.saint-gobain.com/

http://www.crystals.saint-gobain.com/

http://www.crystals.saint-gobain.com/uploadedFiles/SG-Crystals/Documents/SGC%20BC620%20data%20sheet.pdf

http://www.crystals.saint-gobain.com/uploadedFiles/SG-Crystals/Documents/SGC%20BC620%20data%20sheet.pdf


The performance of bothwrappings over themid-range of thresholds suggests that, given the

relative inexpense of both materials, the BC-620 reflective paint would be more suitable for

the needs of the current project.

3.4.3 Polytetrafluoroethylene (PTFE) Thread Seal Tape Wrapping

Due to its very high reflectivity (∼ 0.99 at 440 nm [172, 182–184]), common, store-bought

Polytetrafluoroethylene (PTFE) thread seal tapewas considered as awrapping.23 Figures 3.4.6

and 3.4.8a show frequency-threshold data for 241Am, 133Ba and 60Co sources. It is clear from

the singles measurements in Figure 3.4.6 that in the case of 60Co, beyond the initial sharp de-

cline below 5mV, the total frequency remains in the range∼ 1−40Hz over a wide threshold

window, only falling below 1 Hz at approximately 23 mV. While the background curves are

generally in agreement with those found using the previous wrappings (∼ 0.1 Hz beyond the

initial decrease at low thresholds) – suggesting that any systematic errors in themeasurements

are at a minimum – it is evident that the signal due solely to the 60Co is significantly larger

than the background, comprising≳ 95%of the total count rate over the 10−20-mVwindow.

In Figure 3.4.7, the frequency ratios of the BC-620 to paper/Al-foil, and PTFE to paper/Al-

foil signals, as a function of the threshold, are presented for the source data given in Figure

3.4.6. For example, at a threshold of 17 mV, the PTFE frequency is ∼ 2 times greater than

that of the BC-620 for the singles measurements, with this factor increasing to∼ 3 for the co-

incidence results. At 20mV, a direct comparison between the corresponding data shows that

the PTFE frequency is larger than that of the BC-620 by factors of ∼ 3.5, ∼ 4.3 and ∼ 7.1,

for the #266, #267 and coincidence measurements, respectively. The superior response of23PTFE is a synthetic fluoropolymer of tetrafluoroethylene, often known as “Teflon”, the brand name origi-

nally due to the DuPont Company (E. I. du Pont de Nemours and Company), who discovered the compound.


the PTFE wrapping over such a wide threshold window (e.g., 15-20 mV), with respect to the

other candidates, suggests a preference for it as the wrapping of choice when coupled to the

BC-416 scintillator.

Motivated by the superior performance of the PTFE wrapping, a further test looked at

the possibility of enhancing the light collection of the SiPM by optically coupling its sur-

face to the bare end-face of the wrapped scintillator with optical epoxy. Recall that in dry

contact, since an air-gap necessarily exists between the two surfaces, there is the possibility of

light exiting the scintillator being reflected off the epoxy surface of the SiPM, and hence not

be collected. However, the optical glue has a refractive index similar to that of the plastic scin-

tillator, enabling the photons to travel from the scintillator to the SiPM without reflection,

thus improving light collection by the SiPM.

Figure 3.4.8a shows the resulting singles data for three sources and background. Com-

paring the background and 60Co curves with those in Figure 3.4.6b, the effect of the glue

can be seen to increase the frequencies by factors of ∼ 3 − 10 over the range 5 − 20 mV.

However, the 241Am source performs poorly, with most of the α-particles being stopped by

the PTFE wrapping: the few scintillation photons detected by the SiPM are not recorded at

moderate-to-high thresholds, and so the resulting frequency curve is essentially that of the

background. Although producing a slightly higher frequency than 60Co between 3 − 7 mV,

the 133Ba curve does notmaintain this count rate, the lower-energy γ-rays being unable to gen-

erate large enoughSiPMvoltage signals at higher thresholds, the frequencyquickly decreasing

to background level at∼ 16 mV.

The effect of polishing the scintillator before wrapping can be understood by comparing

the above measurements with those of a polished scintillator. Figure 3.4.9 shows the results


(a) Singles (SiPM #266): 60Co, BC-416 scin llator,Al-foil, BC-620 & PTFE wrappings.

(b) Singles (SiPM #267): 60Co, BC-416 scin llator,Al-foil, BC-620 & PTFE wrappings.

(c) Coincidence (SiPMs #266 & #267): 60Co, BC-416scin llator, Al-foil, BC-620 & PTFE wrappings.

Figure 3.4.6: Frequency-threshold graphs for the PTFE-wrapped, unpolished BC-416 plas c scin llator using the60Co source. The data has been overlaid onto that of Figure 3.4.5, to allow for comparison with both the paper/Al-foil and BC-620 results. Measurements were performed with the CAEN model discriminator, no amplifica on, and adry SiPM/scin llator contact.


(a) Singles (SiPM #266): 60Co, BC-416 scin llator;BC-620:Al-foil & PTFE:Al-foil ra os.

(b) Singles (SiPM #267): 60Co, BC-416 scin llator;BC-620:Al-foil & PTFE:Al-foil ra os.

(c) Coincidence (SiPMs #266 & #267): 60Co, BC-416scin llator; BC-620:Al-foil & PTFE:Al-foil ra os.

Figure 3.4.7: BC-620:Al-foil and PTFE:Al-foil frequency ra os for the BC-416 plas c scin llator using the 60Cosource, based upon the corresponding data in Figure 3.4.6, to allow for a comparison between the BC-620 and PTFEresults.


(a) Singles (SiPM #266): 241Am, 133Ba & 60Co, BC-416scin llator, PTFE wrapping.

(b) Singles (SiPM #269): 90Sr, LAB scin llator,paper/Al-foil wrapping.

Figure 3.4.8: Frequency-threshold graphs for PTFE-wrapped scin llators. (a) The 241Am, 133Ba & 60Co sources, usingSiPM #267 in glued contact (singles mode) with a PTFE-wrapped BC-416 plas c scin llator. Measurements wereperformed with the CAEN discriminator. (b) The 90Sr source, using SiPM #269 in in greased contact (singles mode)with a paper/Al-foil-wrapped vial of LAB (liquid) scin llator. The background was recorded using the Phillips Sci-en fic discriminator while the frequencies due to the source and noise were measured using a Tektronix TDS2022Oscilloscope.


(a) Singles (SiPM #266): 60Co, BC-416 scin llator,Al-foil, BC-620 & PTFE wrappings.

(b) Singles (SiPM #267): 60Co, BC-416 scin llator,Al-foil, BC-620 & PTFE wrappings.

(c) Coincidence (SiPMs #266 & #267): 60Co, BC-416scin llator, Al-foil, BC-620 & PTFE wrappings.

Figure 3.4.9: Frequency-threshold graphs for the PTFE-wrapped, polished (†) BC-416 plas c scin llator for the 60Cosource. The corresponding data in Figure 3.4.6 – for paper/Al-foil-, BC-620- and PTFE-wrapped unpolished (§) BC-416 scin llator – has been overlaid for comparison with the paper/Al-foil and BC-620 results. Measurements wereperformed with the CAEN discriminator and no amplifica on.


of Figure 3.4.6 with the background and 60Co curves for the corresponding polished scin-

tillator overlaid. Both the source and background curves for the polished scintillator fail to

maintain the frequency level of their unpolished counterpart over a wide threshold range.

In singles and coincidence modes, while the unpolished scintillator yields significantly larger

source frequencies at 5 mV compared to the polished results (a factor of ∼ 50 in all three

instances), there is a marked decrease in the source curves of the unpolished scintillator with

increasing threshold. For example, at 20 mV the polished and unpolished signal curves are

of similar size for the singles measurements, while in coincidencemode the unpolished signal

frequency is∼ 2 − 3 times smaller than the corresponding polished result.

3.5 Light-Producing Media: Evaluation of the LAB

Scintillator

Liquid organic scintillators such as linear alkylbenzene (LAB)24 are of importance in several

fields (see, for example, references [186–189]), particularly in low-energy neutrino detection

[185, 190, 191].25 Such materials hold several advantages over plastic and crystalline scintilla-

tors: for example, they are of low cost, possess a transparency to self-radiation [185] (∼ 20 m

in the case of LAB), and often produce a higher light yield.

Although a full immersion of a SiPM in the LAB would maximise the light-collection

solid angle of the detector, an uncertainty over its behaviour in chemical contact with the de-24LAB (C6H5CnH2n+1, inwhichn = 10−13) is a solventmixture of severalmonoalkyl-derivatives of benzene

[185]. Physical properties of typical LAB-based liquid scintillators are listed in Table 2.2.25LAB is the preferred scintillator of theDEAP-3600 (DarkMatter Experiment usingArgon Pulse-shape dis-

crimination) experiment based at the SNOLAB facility (the SudburyNeutrinoObservatory, Ontario, Canada),the SNO+ neutrino detector (also housed at SNOLAB) [192], and in other neutrino detectors such as theRENO experiment (South Korea), LENA (Finland) and Daya Bay Reactor Neutrino Experiment (China).


tector led to the submersion plan not being pursued. While LAB is of low toxicity, compat-

ible chemically with acrylic polymers [185], and possesses a relatively high flash temperature

(130◦C [193]), no data was available to assess any potential chemical corrosion on the SiPM

components (for example, epoxy surface and mount) over time. Consequently, a 3× 3-mm2

SiPM (#269) was attached (with a greased contact) to the surface of a cylindrical vial contain-

ing LAB, with the whole ensemble wrapped in aluminium foil followed by white paper (Fig-

ure 3.6.10a). Measurements were performed with the Phillips Scientific discriminator (back-

ground) and Tektronix TDS2000C Digital Storage Oscilloscope26 (90Sr source and noise).

Singles measurements are shown in Figure 3.4.8b for a 90Sr source. Compared to the

singlesmeasurements inFigure 3.4.4a or 3.4.4b,whichused thepaper/Al-foil-wrappedplastic

scintillator, it can be seen that the frequency curve for the total signal is at least an order

of magnitude smaller over the range of thresholds considered (taking into account the ×10

amplification of the previous test). The total frequency shows an appreciable decrease over

the threshold range, more so than the corresponding decrease in, for example, Figure 3.4.4a.

This behaviour, together with concerns over its chemical reactivity, led to the paper/Al-foil-

wrapped liquid scintillator being eliminated as a candidate light-producing medium.

3.6 Light-Producing Media: The Lead Mini-calorimeter

As an alternative to the wrapped plastic and liquid scintillators discussed above, a minia-

ture lead calorimeter (Figure 3.6.10c) was tested as a light-producing medium. This was a

26 × 13 × 13-mm3 block, machined out of an excess piece of lead/scintillating-fibre matrix26Tektronix Inc., 14150 SW Karl Braun Drive, P.O. Box 500, Beaverton, OR 97077, United States (http:

//www.tek.com/).

http://www.tek.com/

http://www.tek.com/


(Figure 3.6.10b) obtained during the construction of fifty-two, ∼ 228 cm2 × 390 cm GlueX

electromagnetic calorimeter modules at the University of Regina in 2011. Forty-eight such

modules were used in the barrel calorimeter (BCAL) of the GlueX experiment, housed in

Hall D at the Thomas Jefferson National Accelerator Facility (Jefferson Lab27) [194–196].28

In high-energy physics experiments, calorimeters measure the energies and spatial loca-

tions of both charged and neutral particles incident upon the detector. The output signal

from a calorimeter is proportional to the total energy deposited (with the proportionality

factor determined through calibration of the detector). For an incident particle depositing

its energy in the absorbingmaterial, characteristic energy-dependent interactions initiate par-

ticle showers/cascades which are subsequently detected. In an electromagnetic calorimeter,

incident electrons and photons initiate electromagnetic showers of secondary photons, elec-

trons and positrons. As the shower proceeds through the depth of the calorimeter, the over-

all number of secondary particles in themedium increases exponentially, with each successive

generation having a decreased average energy. Secondary electrons can dissipate their energy

through ionisation,29 whereas the dominant interactions for secondary photons in lead at

low energies are photoelectric absorption (Ephoton ≲ 0.5MeV [197]) and Compton scattering

(Ephoton ≈ 0.5− 5 MeV [197]). These particles produced in the electromagnetic showers can

impact the embedded scintillating fibres throughout the depth of the calorimeter, generating

scintillation photons which travel the length of the fibre, via total internal reflection, to the27Thomas Jefferson National Accelerator Facility, 12000 Jefferson Ave, Newport News, VA 23606, United

States (https://www.jlab.org/).28The scintillating fibreswereKuraray SCSF-78MJ, double-clad, blue-green fibres,manufacturedbyKuraray

(Kuraray America Inc., Headquarters 2625 Bay Area Boulevard, Suite 600 Houston, TX 77058, United States(http://www.kuraray.us.com/)).

29For electron energies greater than the critical energy Ec, radiative loss through bremsstrahlung andČerenkov radiation dominates the energy loss from ionisation.

https://www.jlab.org/

http://www.kuraray.us.com/


(a) Photograph of the∼ 48-mm-high,∼ 14-mm-diameter PTFE-wrapped vial.

(b) Schema c of the lead-scin lla ng-fibre (Pb-SciFi) matrix.

(c) Photograph of the 26 × 13 × 13 mm3 lead mini-calorimeter.

Figure 3.6.10: Photographs and/or schema c of the lead mini-calorimeter and LAB scin llator. (a) Photograph of thevial containing the scin llator, with the 3 × 3-mm2 SiPM #269 a ached to the outer, curved surface of the vial (itstwo pins being clearly visible). The vial was wrapped in PTFE tape (see Sec on 3.7). (b) The schema c of the Pb-SciFimatrix shows the placement of the 1-mm fibres (from Leverington [196]). (c) Photograph of the lead mini-calorimeter.This lead/scin lla ng-fibre matrix was machined out of an excess piece obtained during construc on of the GlueXelectromagne c barrel calorimeter modules at the University of Regina (see text).


photodetector.

Empirically, it is found that the probabilities of Compton scattering and photoelectric

absorption vary as Z/Ephoton and Z4−5/Ephoton, respectively, for an atomic number Z of the

absorbing medium.30 In lead (Z = 82, Ec = 7.42 MeV [197]), it is found [197] that at the

photon energies listed in Table 3.2, Rayleigh scattering, Compton scattering and photoelec-

tric absorption are the important processes attenuating the shower particles.

Placing aHamamatsu S10943-0258(X) series SiPM(3×3mm2) in dry contactwith an end-

face of the miniature lead calorimeter-scintillating fibre block, the 60Co source was placed

on one side. Various electronics set-ups were used in the tests in order to identify the best

configuration. For example, the SiPM was self-triggered, discriminating on the split-output

signal (using the Phillips Scientific Model device at 60 mV threshold) after having amplified

the signal (using the ×10 LeCroy amplifier). A PMT was employed as an external trigger

(using a light-guide between themini-calorimeter and PMT), with a discriminator threshold

of∼ 17 mV.

However, it was found that the mini-calorimeter yielded a count rate of∼ 3-times lower

than in the case of the BC-416 block, indicating a considerably lower efficiency of light de-

tection in comparison with the plastic scintillator. This can be attributed to the significant

absorption of incident radiation by the lead. For Ephoton = 1.333 MeV photons in lead, the

total mass attenuation coefficient is μm = 0.0564 cm2/g, with the dominant contribution

from Compton scattering (μComptonm = 0.0433 cm2/g), followed by photoelectric absorp-

tion (μphoto.abs.m ∼ 0.0104 cm2/g), with that due to (nuclear field) pair production being

μppm = 0.0007 cm2/g [198]. Calculations show that the incident photon intensity is atten-30At higher energies (Ephoton ≳ 5 MeV for photons in lead [197]), pair-production is the dominant photon

process in matter, with a cross section∼ Z2 loge(2Ephoton).


uated by ∼ 47.3% after travelling 1 cm in lead, with a ∼ 27.7% energy absorption. For

β-radiation, since Eelectron = 0.318MeV ≪ Ec, the energy loss will be largely due to colli-

sions/ionisation in the lead: it is found that electrons and positrons have a mass stopping

power of 1.238 MeV cm2/g [199]. These values suggest that the relatively low efficiency of

the mini-calorimeter can be attributed to a significant absorption of the incident radiation

from 60Co by the lead: only a few incident particles manage to deposit their energy in the

scintillating fibres.

3.7 Application of EIC Electronics

Various frequency-thresholdmeasurements were repeated with the nuclear electronics in the

DDL replaced by those associated with a Tri-Met TM372A sample counter provided by EIC

(Figure 3.7.11). Designed as a completely self-contained, grossα-counter, theTM372A (model

#4) originally contained a sample tray, with a ZnS scintillator coupled to a PMT.31 The elec-

tronics consisted of a discriminator and×10 amplifier, implemented using the “Gain” switch

(Figure 3.7.11a); a “Pre-setTime” can be used to establish a user-defined time range overwhich

a scaler counts the number of detected events in real time, output via an LED display.

3.7.1 Light-Producing Media: BC-416 Plastic Scintillator

Recall that using theDDLelectronics, itwas established that of the candidate light-producing

media, a 3 × 3-mm2 SiPM glued to the PTFE-wrapped BC-416 plastic scintillator provided31TheTM372 series of devices have been used for radon progenymeasurements in Canadian uraniummines

for several decades. Initially developed by Tri-Met Instruments Ltd. in 1972, all assets relating to the TM372serieswere acquiredbyEIC in 2000. TheTM372Amodel has been supersededby theTM372B (see, for example,http://www.eic.nu/TM372B.html).

http://www.eic.nu/TM372B.html


(a) TM372A: Front view.

(b) TM372A: circuit.

Figure 3.7.11: Photograph of the TM372A unit, provided by EIC. (a) Front view of the device, showing the variouscontrols referred to in the text. (b) View of the circuit.


the more favourable light-collection combination. An almost identical configuration was

adopted for measurements with the TM372A device, with black electrician’s tape wound

around the SiPM-scintillator in order to reduce light leakage from, and extraneous light to,

the detector (a layer of white paper was introduced between the PTFE and black tape).

Using the readout electronics shown in Figure 3.2.1b, the 3× 3-mm2 SiPMwas connected

to the preamplifying circuit of the TM372A device. Preamplifiers typically employ an RC

circuit in order to shape the SiPM pulse – increasing its amplitude and decay time – prior to

it being processed by subsequent electronics. The amplitude of the output signal determines

the charge deposited in the SiPM (which is proportional to the energy), whereas a long decay

time (∼ 100μs) ensures complete charge collection from the SiPM (the rise time is controlled

by the characteristics of the detector itself). In TM372A’s existing preamplifying circuit, the

pulse shape is dominated by a 1 MΩ resistor: the effect of this original RC circuit – as well as

two slight modifications – on the frequency of the resulting SiPM output signal were inves-

tigated. An accompanying 100 kΩ resistor, placed in parallel with the 1 MΩ component (to

increase the amplitude of the pulse), and an additional 1 μ F capacitor (to increase the decay

time, through the time constant RC) were both attached, in turn, to the TM372A board.

Frequency-thresholdbehaviour for source, backgroundandnoisemeasurementswas anal-

ysed for the above configurations, with the discriminator threshold adjusted using the rotary

button shown in Figure 3.7.11a, and the number of counts recorded in the pre-set time estab-

lishing the frequency.32 The TM372A also supplied the bias to the detector (adjustable via a

potentiometer), with a small connection added to the electronics board (Figure 3.7.11b) to al-

lowmonitoring of the bias using amultimeter. As in the trials using theDDL electronics, the32Note that the scale on the discriminator of the TM372A could not be established relative to that of the

CAEN and Phillips devices.


ambient RF signal was minimised by housing the SiPM/scintillator in the sealed, grounded

copper box.

For the purposes of discussing the results, let the 1 MΩ circuit be referred to as “circuit

A”, that incorporating both the parallel 1 MΩ and 100 kΩ resistors be “circuit B”, and that

involving the 1 μF capacitor alongside the 1 MΩ and 100 kΩ resistors be “circuit C”. Fig-

ure 3.7.12 shows results for 241Am, 133Ba and 60Co, along with their corresponding noise and

background spectra, for circuits A, B and C.

Over the range of thresholds, it can be seen that circuits B and C result in the larger noise

frequencies: for example, in the case of circuit B the noise is a factor of∼ 40 bigger than that

of circuit A at a ∼ 0.5-unit threshold, rising to ∼1700 times larger than that of circuit A at

a 1.3 unit threshold. Meanwhile, the noise curve for circuit C (i.e., the addition of the 1 μF

capacitor) does not offer any appreciable increase with respect to circuit B. That is, it is the

addition of the 100 kΩ resistor to the RC circuit which results in a higher noise frequency.

Whereas the noise of circuit A falls away quite rapidly with increasing threshold, those of

circuits B and C provide significant contributions to the background and “total” signal over

a large range of thresholds.

For 241Am (Figure 3.7.12a), over a narrow threshold window of ∼ 0.5 − 0.9 units the

source signal due to circuit A is slightly larger than its corresponding background, while the

same appears to be the case for circuits B and C over the range ∼ 1.0 − 1.5 units. However,

in all three cases, the signals quickly approach the background, becoming more-or-less indis-

tinguishable at high thresholds.

In contrast, for 133Ba (Figure 3.7.12b), while the backgrounds remain at ∼ 1 − 2 Hz be-

yond 1.2 threshold units, the total frequencies are clearly distinguishable over the same range.


A similar pattern is true in the case of 60Co (Figure 3.7.12c): once the noise falls below approx-

imately 1Hz, the total signals in all three circuits are about 50 times larger than their respective

backgrounds. Therefore, a comparison of the source signals suggests that while the current

set-up struggles to identify the 241Am source, it is quite capable of detecting both 133Ba and

60Co.

Although previous work had suggested a preference for the 3× 3-mm2 SiPMunits, it was

nevertheless decided to perform a series of trials incorporating the 1×1-mm2 units using EIC’s

electronics. Since Figure 3.7.12 established that the preamp circuit with only the additional

100 kΩ consistently led to higher frequencies, the frequency-threshold behaviour of 133Ba and

60Co using one of the 1 × 1-mm2 SiPMs was investigated. The noise, background and total

frequencies (shown in Figure 3.7.13a) appear to be of approximately one order of magnitude

less than their 3 × 3-mm2 counterparts.33

3.7.2 Light-Producing Media: LAB Scintillator

Frequency-threshold measurements were performed with the set-up shown in Figure 3.6.10a

– that is, with a 3× 3-mm2 SiPM (unit #269) in a greased coupling to the curved surface of a

cylindrical vial containing LAB scintillator, the ensemble wrapped in PTFE tape –was tested

with the EIC electronics (with unit #266 used, for convenience, to measure the noise). Each

source of interest was placed, in turn, against the PTFEwrapping, diametrically opposite the

SiPM. Since the correspondingBC-416measurements discussed above demonstrated that the

RC circuit with the additional 100 kΩ resistor consistently led to the highest frequencies, this33As shown inFigure 3.7.12, units #184 and#188werebothused inmeasurements (seeTable 3.1). Whereas unit

#188 was glued to the PTFE-wrapped scintillator for background and source measurements, it was convenientto use unit #184 to measure the noise.


(a) 241Am: BC-416 scin llator & PTFE wrapping.

(b) 133Ba: BC-416 scin llator & PTFE wrapping.

(c) 60Co: BC-416 scin llator & PTFE wrapping.

Figure 3.7.12: Frequency-threshold graphs for the PTFE-wrapped BC-416 plas c scin llator. (a) The 241Am source.(b) The 133Ba source. (c) The 60Co source. In each case, the noise measurements were obtained using SiPM #266,while the background and source used SiPM #267. The specific RC circuits are indicated as follows;A: 1 MΩ re-sistor, B: 1 MΩ and 100 kΩ resistors in parallel, and C: 1 μF capacitor alongside the parallel 1 MΩ and 100 kΩresistors.


configuration was implemented.

Measurements of the background, and in the presence of 133Ba and 60Co sources, for two

“degrees” of wrapping were performed. Initially, a straightforward (“normal”) PTFE wrap-

ping of the curved and bottom surfaces of the vial was considered. However, since poten-

tial loss of light through the screw-cap end of the vial was of concern, an additional, more

“complete” PTFE wrapping over the vial screw thread and opening was performed (before

replacing the black screw cap).

Figure 3.7.13b shows the results of the various trials.34 It is seen that the “complete” PTFE

wrapping does indeed give rise to slightly higher frequencies, relative to the “normal” wrap-

ping, in all three cases (background and sources). Apart from the case of the “complete”

wrapping within the very narrow window of 1-2 threshold units, the results show that for

133Ba the signal is not sufficiently above background over the threshold range to suggest that

γ-rays from 133Ba could be detected with confidence using this set-up. In contrast, above∼ 1

threshold unit, the total signal involving 60Co consistently remains more than one order of

magnitude above the background.

3.7.3 Light-Producing Media: PEN Scintillator

PolyethyleneNaphthalate (PEN) is a thermoplastic polyester synthesised by the polyconden-

sation of dimethyl-2,6-naphthalenedicarboxylate and ethylene glycol [122]. Of great impor-

tance in the textile industry, PEN is readily available due to the significant worldwide com-

mercial market surrounding its use in, for example, common household objects such as din-34Note that as in Figure 3.7.13a, the measurements shown in Figure 3.7.13b used twoMPPC units as a matter

of convenience, with #267 being in greased contact with the scintillator (for source and background measure-ments), and #266 used for noise.


ner sets and plastic containers. Using a spectrometer, Nakamura et al. [122] have measured

a light yield of∼ 10500 photons/MeV and peak emission wavelength of 425 nm, suggesting

that PEN possesses scintillation properties comparable to well-known commercial organic

scintillators such as BC-408, while being superior to BC-416 (Table 2.2).

A 210×290×1-mm3 sheet of PEN, obtained fromTeijinChemicals,35 was cut into thirteen

13 × 1 × 50-mm3 strips. Gluing the 13 × 50-mm2 sides together (using epoxy glue36) formed

a 13 × 13 × 50-mm3 prism of similar dimensions as the BC-416 scintillator block discussed

previously. A 3× 3-mm2 SIPM (#267) was glued to one of the 13× 13-mm2 end-faces, and the

resulting combination wrapped completely in successive layers of PTFE tape, white paper

and black tape.37 This scintillator-detector arrangement was placed in the copper dark box,

as previously, with the 100 kΩ resistor still employed as part of the RC circuit.

From Figure 3.7.13, it is apparent that the 90Sr, 60Co and background curves obtained

using PEN are of relatively low frequency. For example, comparing the 60Co curve with that

obtainedwith theBC-416 plastic scintillator (Figure 3.7.12c), it is quite clear that after its initial

sharp decrease at very low threshold values, the frequencies obtained using PEN are at least

twoorders ofmagnitude smaller over awide rangeof thresholds. Given the light-yield ofPEN

reported byNakamura and co-workers, it is difficult to understand the rather large deviation

betweenPENandBC-416 in the present results. Apossible source of discrepancymay involve35Teijin Chemicals K. K., Teijin Building 6-7, Minami-hommachi 1-chome, Chuo-ku, Osaka 541-8587, Japan

(http://www.teijin.com).36The optical adhesive employed was BC-600 Optical Cement, a clear epoxy resin formulated specif-

ically for creating optical joints with plastic scintillators such as BC-416. The epoxy consists ofa two-part mixture of resin and hardener (manufactured by Saint-Gobain Crystals) – in the ratioof 100:28 – producing a low-viscosity adhesive which cures at room temperature. See, for exam-ple, the data sheet at http://www.crystals.saint-gobain.com/uploadedFiles/SG-Crystals/Documents/SGC%20BC600%20Data%20Sheet.pdf.

37As in Figure 3.7.13b, it was convenient to use MPPC unit #266 to measure the noise.

http://www.teijin.com

http://www.crystals.saint-gobain.com/uploadedFiles/SG-Crystals/Documents/SGC%20BC600%20Data%20Sheet.pdf

http://www.crystals.saint-gobain.com/uploadedFiles/SG-Crystals/Documents/SGC%20BC600%20Data%20Sheet.pdf


(a) SiPMs #184 & #188: 133Ba & 60Co, PTFE-wrappedBC-416 plas c scin llator.

(b) SiPMs #266 & #269: 133Ba & 60Co, PTFE-wrapped LABscin llator.

(c) SiPMs #266 & #267: 60Co & 90Sr,PTFE-/paper-/black-tape-wrapped PEN scin llator.

Figure 3.7.13: Frequency-threshold graphs of singles data, measurements performed with EIC electronics. No am-plifica on was applied. (a) The 133Ba and 60Co sources, using SiPMs #184 and #188, glued to PTFE-wrapped, BC-416 plas c scin llator. (b) The 133Ba and 60Co sources, using SiPMs #266 and #269 with a greased coupling to a vialof LAB liquid scin llator: † indicates a “normal” amount of PTFE wrapping, ‡ indicates a “complete” amount PTFEwrapping (see text). (c) The 60Co and 90Sr sources, using SiPMs #266 and #267 glued to a PTFE/paper/black-tapewrapped PEN scin llator. Note that in each figure, the use of different units from the same MPPC series was purelyfor convenience (see text).


the PEN scintillator used in the current tests being an amalgamation of strips of PEN, rather

than as a single block.

3.8 Neutron Detection with SiPMs

Traditionally, 3He has been amongst the most effective isotopes used in neutron detection.

In a 3He-filled detector, thermal neutrons (∼ 0.025 eV) are absorbed by 3He, the process

3He+10 n →3

1 H+11 H+ 0.764MeV having a relatively large absorption cross section (5330 b

[107]). The charged products are subsequently detected by a gas proportional counter (see,

for example, Crane and Baker [200]). Helium-3 is obtained as a by-product of the β-decay

(31H →32 He+ + e− + νe) of the tritium extracted from nuclear-weapons production. How-

ever, due to the reduction in nuclear-weapon stockpiles by the United States and Russia,

and increased use by U.S. Homeland Security, the past several years have seen a world-wide

shortage of 3He [201]. This has prompted alternative methods of thermal neutron detection

[201, 202] to be sought.

One approach has been to use neutron conversion in neutron-activematerials such as 6Li

or 10B together with a ZnS scintillator (Table 2.1) in a colourless, epoxy binder (for example,

LiF-ZnS(Ag), with the silver acting as an activator).38 Thermal neutrons incident upon the

6Li initiate the process 63Li(10n,42 He)31H (with an absorption cross section∼ 910 b and detec-

tion efficiency of∼ 55% [203]), the resulting heavy α-particles exciting the ZnS(Ag) scintilla-

tor. The∼ 160000 scintillation photons produced per initial neutron [204, 205] (efficiency

∼ 9.2% at 450nm) [204] are detectedusingwavelength-shifting (WLS) fibres [202, 206] cou-38These neutron-active materials allow for an enhanced neutron-detection capability, offering a more lo-

calised and more rapid neutron-detection capability than is possible using the more traditional gas detectors[200].


pled to an appropriate photosensor. In PRDs, where space is limited, scintillator-based de-

tectors are known [107] to provide a larger neutron detection efficiency per unit volume than,

for example, 3H-tubes. Although several authors have utilised large-area single- or multi-

anodePMTs [207–211], these photodetectors are not feasible inPRDsdue to cost and volume

considerations. Osovizky, Ginzburg and co-workers have recently employed a PMT in the

development of a prototype device, offering a 100-cm2 detection area, capable of the simul-

taneous detection of α- and β-particles [212]. Low-cost, small-sized devices such as the SiPM

seem to be ideally suited for implementation in such systems. However, as outlined else-

where, the intrinsic dark rate of SiPMs operated at room temperature is significantly higher

than that of the PMT: direct replacement of PMTs by SiPMs requires a substantial reduc-

tion of the dark rate of the latter, which can only be achieved through an appreciable cooling

[140, 213] (lending additional cost and space-requirements to the PRD). Recently, Stoykov

and co-workers [214, 215] have reported the development of a (1× 1 mm2) SiPM-based 6LiF-

ZnS(6LiF) neutron detector of dimensions 2.4× 2.8× 50 mm3 to be operated at room tem-

perature.

Boron-10 has a large thermal neutron absorption cross section (∼ 3838 b [193]), based

upon the well-known neutron-capture reactions 10B(10n,42 He)7Li, namely

10n +10

5 B −→

73Li (1.015MeV) +4

2 He (1.777MeV) (6%) .

73Li∗ (1.47MeV) +4

2 He (1.47MeV) + γ (0.478MeV) (94%) .

(3.8.3)

As a result,many 10B-enrichedmaterials are better suited to neutron detection than, for exam-


ple, 6Li. For instance, semiconducting boron carbide (B4C)39 has been used for several years

in thin-film diodes for solid-state neutron detection (see, for example, references [217–220]).

After neutron capture, the 7Li and α-particles have short ranges, depositing their energies

into the scintillator. However, as the main-branch reaction in (3.8.3) suggests, a considerable

number of photons are emitted, which can be subsequently detected in a similar approach as

above – for example, via a WLS fibre coupled to a photosensor.

A prototype thermal neutron detector was constructed using a simple, but novel, ap-

proach involving B4C for neutron conversion (according to (3.8.3)), and ZnS(Ag) to convert

the resulting α-particles into scintillating photons (which could be subsequently collected by

a WLS fibre coupled to a SiPM). Figure 3.8.14 provides a schematic of the detector: a single

∼ 20-cm long piece of WLS fibre was cut and polished, and coated with a single layer of

ZnS(Ag) powder using a clear, non-yellowing and commercially available spray adhesive.40

The coated fibre was housed in a B4C micropowder41 “sheath”, with Scotch-tape enclosing

the ensemble. Fixing this set-up to a support, one end of the WLS fibre was placed in dry

contact flush against the surface of a Hamamatsu S12571-050C 1 × 1-mm2 SiPM.42

The detector was placed in close proximity to the 241Am-Be neutron “howitzer” located

inside the radiosources storage room in theDepartmentofPhysics at theUniversity ofRegina.

Positioned such that a steady streamof thermal neutronswas incident upon it, a series of tests39It is well known (see, for example, Domnich et al. [216]) that the stoichiometry of boron carbide is not

exactly 4 : 1. In practice, the material is always very slightly carbon deficient, and X-ray crystallography revealsa complex structure of C-B-C chains mixed with B12 icosohedra.

40Krylon All-Purpose Spray Adhesive, manufactured by the Krylon Products Group, 101 W. Prospect Ave.,Cleveland, OH 44115, United States (http://www.krylon.ca/).

41The B4Cmicropowder was obtained fromDelta Scientific Laboratory Products Ltd., 346Watline Avenue,Mississauga, ON L4Z 1X2, Canada (http://www.delta-sci.com/).

42The properties of the S12571-050C SiPM and its accompanying readout module (C11205-150) were alreadywell-known to researchers in the SPARRO group. For example, in spite of its large amplifier, the unit wasknown to have very little noise.

http://www.krylon.ca/

http://www.delta-sci.com/


(a) ZnS(Ag)- and B4C-coated WLS fibres. (b) Neutron-detec on schema c.

(c) Background spectrum (red) superimposed upon thesignal spectrum (blue).

(d) Noise spectrum (red) superimposed upon the signalspectrum (blue).

Figure 3.8.14: Photograph/schema c of the neutron-detec on set-up and ADC spectra. (a) The upper WLS fibre iscoated with ZnS(Ag) (white), while the lower fibre shows a ZnS(Ag)-coated fibre with an outer layer of B4C microp-owder, enclosed in a common tape sheath. (b) Schema c of the experimental set-up. Neutrons incident on the outerB4C layer ul mately result in scin lla on photons entering the WLS fibre (see text). (c) & (d) Normalised ADC spectra(signal, background and noise) from thermal neutron detec on at 34 mV threshold.


were performed to assess the quality of the detector. Figures 3.8.14c and 3.8.14d show signal,

background and noise spectra at a discriminator threshold of 34 mV. While the background

was measured several metres away from the neutron source, the noise component was ob-

tained by measuring the signal due solely to a (separate) ZnS(Ag)-coated fibre.

The pedestal peak at low energy is clearly visible in theADC spectra, with the background

and noise signals having largely disappeared by channels 350 and 750, respectively. It is pos-

sible to conclude that since the source signal forms an appreciable and clearly identifiable

portion of the overall ADC spectrum, these preliminary tests show that neutrons are indeed

being detected, and that further development of the prototype is of interest.

Chapter 4

Application: Nuclear Imaging

Scintigraphic methods in nuclear medicine have their origins in the pioneering work of Sei-

dlin et al. [221] in 1946, and in the development of the first rectilinear scanner [222] and

Anger (scintillation) camera [223] in the 1950s. Significant advances in subsequent decades

have firmly establishedmedical imaging as an important field inmodern clinicalmedicine and

medical research. Modern imaging devices are potentially capable of resolving structures at

a cellular level, and such technology is proving to be an essential tool in the molecular imag-

ing of biological processes in both humans (for example, in oncology [224] and neurology

[225–227]) and small animals (in drug development [228]).

Radioisotope use in plant physiology pre-dates medical imaging [70], with 11CO2 first

utilised as a tracer in photosynthesis studies in 1939 [229, 230]. However, specialised systems

and techniques developed for imaging and image processing in nuclearmedicine andbiomed-

ical researchwill continue to be of critical importance in, for example, plant phenomics [231],1

1Plant phenomics is the study of the structural, biochemical, physiological and performance-related charac-teristics/traits of plants in response to, for example, genetic mutation and environmental factors [231].

74

CHAPTER 4. APPLICATION: NUCLEAR IMAGING 75

in the twenty-first century [232]. For instance, fears of the effects of rapid population growth

and global climate change on the supply of food [233] have led to interest in the production

of crops that are resistant to stresses resulting from factors such as droughts and temperature

extremes, as well as attacks frommicrobial, viral and bacterial sources [233]. Although the ge-

netic modification of crops is commonplace, their yield is determined by both the genotype

of the plant and its growth environment [233].

Reviews of non-invasive imaging and image processing have been given by Dhondt et

al. [231] and Fiorani et al. [234]. Imaging modalities used to study plants have included

visible techniques [235], as well as CT [236] (in imaging the root-soil interface) and MRI

[235]. Carbon-11, and other isotopes used in the radiolabelling of molecules of biological im-

portance (such as 13N, 15O and 18F), are all positron emitters, making positron emission to-

mography (PET) a valuable imaging technique for plant studies. There have been numerous

studies reported in the literature which have developed and/or applied PET devices to the

plant and soil sciences. For example, several groups have studied processes in subsurface soils

– such as the geochemical transport (by imaging andmodelling soil hydrology [237–240]), as

well as monitoring in situ bacterial activity [241]. At the Brookhaven National Laboratory,

Pritchard and co-workers [242]used amodified clinical PET scanner to study thedistribution

of metabolites in plant roots, while Budassi et al. [243] andWang et al. [233] have developed

prototype PET-based plant imagers. Other groups have also reported modified clinical and

pre-clinical PET devices: in Japan, the “PETIS” system (“Positron Emitting Tracer Imaging

System”) [244–250] has been used to study barley [244] and rice [245, 246]. The “Plant To-

mographic Imaging System” ( “PlanTIS”) [251–254], developed at the Phytosphere Institute

in Jülich, Germany, has been used to study phloem transport in the leaves of a species of oak


tree [255]. There is also a report of a hybrid device: Garbout et al. [256] have used a hybrid

clinical PET/CT scanner to track the uptake of 11CO2 through the plant and root structure

of a radish plant.

Of relevance to the current study is the “PhytoPET” system due toWeisenberger and co-

workers [67, 69, 71, 257–259].2 Housed in the Phytotron in Department of Biology at Duke

University, PhytoPET is a collaboration between Duke, Jefferson Lab and the University of

Maryland,3 that utilises the accelerator facilities of the Triangle Universities Nuclear Labo-

ratory (TUNL) to produce 11C. Through funding by the Sylvia Fedoruk Centre, there is a

plan to introduce a similar system at theUniversity of Regina: initially, an exact replica of the

Duke device (employing position-sensitive PMTs as the photosensor) will be installed and

tested, and subsequently upgraded to include SiPMs.

PET systems utilising PMTs are typically bulky, expensive devices with a limited quan-

tum efficiency. Even replacing the PMT by APDs (the latter offering, for example, a higher

QE and more compact structure) presents difficulties with timing properties and a degraded

signal-to-noise ratio [260]. Recently, there has been considerable interest in combining ar-

rays of SiPMs into large-area,multi-channel detectormodules [72, 261–276]. Such devices of-

ten include readout electronics and are available commercially as turnkey systems optimised

for use in PET detectors (see, for example, references [272, 275]). Depending upon the ar-

rangement of electronic readout channels, two distinct detector designs have emerged [277],

resulting in so-called position-sensitive SiPMs [263, 268, 278–280] and array-pixel SiPMs [62,

281–283].4 In the former, each pixel in the detector is connected in a simple readout tech-2The same group have also developed “PhytoBeta” [68, 70], a compact β+-/β−-particle imager for 11CO2-

leaf imaging.3West Virginia University was also part of the institutional collaboration until 2012.4Here, we consider a SiPM array as a square configuration of n2 pixels (n = 1, 2, 3, · · · ). Each pixel is


nique (for example, by a built-in resistor-grid method), with only five readout channels re-

quired (four for positioning and one for timing [263, 268, 278, 279]). While this approach has

enjoyed some success, the spatial and temporal resolution of these devices is currently limited

by their large capacitance and high dark-count rate [277, 284].

The array-pixel design is based upon numerous separate SiPM pixels located on the same

silicon substrate. Although each pixel provides excellent timing [275, 285, 286], and 1:1 cou-

pling is possible (thus minimising detector decoding errors) [277], such a design results in a

relatively large number of output channels if each pixel is readout individually. Under such

circumstances, a complicated channel multiplexing architecture is typically implemented to

reduce the number of output channels leaving each module [287–294].5

This project utilises two such array-pixel SiPMdevices, with the reported results intended

to provide a preliminary step towards a complete characterisation of both detector modules,

in preparation for their proposed use in the PhytoPET system. As such, Section 4.1 provides

a brief discussion of the physics underlying standard PET scanners, while in Section 4.2 an

overview of the structure and operation of the selected SiPM array detector is presented. A

basic synopsis of the multiplexing architecture used in the SiPM array is discussed in Section

4.3 in order to facilitate an understanding of the results presented in Section 4.4.

Given the necessity for fast, high-light-yield scintillators in modern PET systems, previ-

ously reported works involving the MatrixSM-9 system [277, 295] have focused exclusively

on coupling with LYSO and/or LSO scintillators. Furthermore, in their user MatrixSM-9

manual, SensL provide a list of “recommended settings” for various system parameters, op-

considered to be a collection of∼ 103 − 104 microcells.5While multiplexing decreases the number of output channels, lowering the cost and decreasing the com-

plexity of the subsequentDAQelectronics [294], it can also lead to a degradation in detector performance [277].


timised using LYSO, as well as a 22Na energy spectrum. While it will be necessary for similar

scintillators to be eventually incorporated into the Saskatchewan PhytoPET system, the cur-

rent work presents results of important module-characterisation tests using a BC-416 plastic

scintillator as part of the initial “proof-of-principle” phase of the project, while offering expe-

rience to the group in thoroughly understanding the basic operational characteristics of the

units before using them for image reconstruction.

4.1 A Brief Introduction to PET Physics

Following the evaluation of each SiPM array detector, subsequent trials are planned which

will operate the two detectors in coincidence mode. Interest in coincidence measurements

is motivated by the physical mechanism underlying positron emission tomography. In con-

ventional PET systems, a radionuclide AZX inserted into a sample (e.g., patient) undergoes

positron (β+)-decay via AZX →A

Z−1 Y+β++νe (inwhich νe is an electron neutrino). After trav-

elling several millimetres,6 the positron annihilates with an electron (e−) through the process

e− + β+ → γγ (Figure 4.1.1a). The two 511 keV photons, emitted essentially back-to-back,7

are detected in coincidence by two scintillator/photosensor arrangements (typically a pixe-

lated crystal array coupled to a PMT) placed on diametrically opposite sides of the sample.

The detection of such events establishes a so-called line of response (LOR), that is, a volume of

the sample in which the annihilation could have occurred. Dedicated image-reconstruction6Note that the range of positrons is material- and β+-energy-dependent, decreasing with the increasingma-

terial density, as well as being proportional to the β+ energy. For example, consider the maximum/averageranges of β+ (of maximum energy Emax

positron) in water for the following radionuclides; 11C (Emaxpositron = 0.97

MeV): 3.8mm/0.85mm, 13N (Emaxpositron = 1.20MeV): 5.0mm/1.15 mm, 15O (Emax

positron = 1.74MeV): 8.0mm/1.80mm, and 18F (Emax

positron = 0.64 MeV): 2.2 mm/0.46 mm. See, for example, Saha [46].7Shibuya et al. [296] have measured the non-collinearity of the two 511 keV photons from the human body

to deviate from 180◦ by a Gaussian distribution with FWHM (0.54 ± 0.02)◦.


algorithms are used to process all such LORs from a circular ring of detectors (Figure 4.1.1b),

yielding a spatial map of the radionuclide activity in the sample. For example, cluster infor-

mation recorded in “Region of Interest” mode from MatrixSM-9 blocks operating in coin-

cidence can be used in positional algorithms such as the centre-of-gravity algorithm due to

Anger [297].

4.2 The SensL MatrixSM-9 System

As part of SensL Photonics’ series of large-area SiPM array detectors, theMatrix system8 (Fig-

ure 4.2.2) was the first modular, turnkey readout system designed specifically for imaging

applications in nuclear medicine (see, for example, the MatrixSM-9 user manual9). Such a

device provides a fully solid-state, four-side scalable photodetector optimised for coupling

to fast scintillator arrays of LYSO or BGO (Table 2.1). The high degree of modularity, rela-

tive pixel uniformity, digitised output signal, good timing resolution and reasonable energy

resolution [295], coupled with the advantages of the SiPM, make the system a suitable alter-

native to PMT-based detectors in applications which require high-resolution imaging, such

as in positron emission tomography (pre-clinical PET, positron emissionmammography and

small-animal PET), SPECT and the gamma camera [295].

There have been at least two studies characterising the energy and time properties of the

Matrix 9 system in assessing its suitability for PET. Sanjani et al. [295] coupled two detectors8SensL’s nomenclature identifies the particular series of SiPMs used in the Matrix 9 modules. For example,

use of the “L” (2010) and “M” (2012) series of SiPMs are reflected in the names of theMatrixSM-9 andMatrixSL-9 models, respectively. The past three years have seen the release of the “B” (2013) and “C” (2014) series, withthe “J” series imminent.

9The SensL MatrixSM-9 user manual is available from http://www.sensl.com/downloads/ds/UM-MatrixSM9.pdf.

http://www.sensl.com/downloads/ds/UM-MatrixSM9.pdf



(a) PET mechanism.

(b) PhytoPET detector configura on.

Figure 4.1.1: Overview of the opera on of a conven onal positron emission tomography (PET) scanner. (a) Cartoonshowing the physical mechanism underlying PET: the positron (β+) from a β+-emi ng radionuclide annihilateswith a nearby electron (e−), producing two back-to-back 511 keV photons. (b) Example of the circular scin lla-tor/photodetector arrangement used in conven onal PET systems. The photograph shows the mul -anode PMTconfigura on used in the PhytoPET system at Duke University (courtesy of the Jefferson Laboratory).


(a) Basic components of the MatrixSM-9 system.

(b) Cartoon showing the modules/board/PCconnec ons.

(c) The EVB communica ons boards showing the primaryconnec ons.

Figure 4.2.2: Overview of SensL’s MatrixSM-9 system, showing the modular nature of the detector. (a) Ba-sic components of the system. Note that the front-end electronics board shown on the right is housed perma-nently within the MatrixSM-9 Readout Module shown on the le . The 3 × 3 SiPM array is clearly visible on theSensor Head of the MatrixSM-9 Readout module (see text). The MatrixSM-9X1 or MatrixB-9X1 Sensor Headwere not used in this study, and so their details may be omi ed. From the SensL “Matrix System User Manual”(http://www.sensl.com/downloads/ds/UM-MatrixSM9.pdf). (b) Cartoon of the connec ons betweenthree Readout Modules, each with a serial connec on to the EVB communica ons/coincidence board by an 80-wayflexible IDC connector (green). Power to the board and modules is provided by+5 V and+35 V inputs, respec-vely. The board itself may be connected to a PC via a high-speed USB cable. From Sanjani et al. [295]. See also

Figure 4.2.2. (c) The EVB communica ons board, showing the three primary connec ons (USB2, IDC and powerinputs). From theMatrix System User Manual.



to a 13.1× 13.1× 20 mm3 pixelated (4× 4) LYSO crystal and, using a 22Na source, evaluated

the energy resolution and gain uniformity of both detectors, andmeasured their coincidence

time resolution. Meanwhile, motivated by the success of a small-animal PET scanner based

upon the PMT [298], Du et al. [277] coupled an 8 × 8 × 6 mm3 LSO array to an L-series

Matrix 9 module (and using a 68Ge source) to evaluate the detector as a candidate to replace

the PMT.

Figure 4.2.2 provides an overview of the modular design of the system. There are two

principal hardware subsystems: oneormoreReadoutModul (of dimensions∼ 46×48×133

mm3) – each housing a Sensor Head and integrated front-end readout board – and aMatrix-

EVB communication/coincidence board. Power to the systemwas supplied by aW-IE-NE-R10

multi-channel high- and low-voltage modules housed in an MPOD mini crate,11 with main

power provided by an LV module (4 V - 5.5 V, 1 A to the Matrix-EVB board and 0.5 A per

Readout Module) and bias (35 V - 40 V, 5 mA per Readout Module) supplied by the HV

module.

The Sensor Head (Figure 4.2.2) is a large-area 3× 3 array of 144 SiPM pixels, based upon

SensL’s SiPM technology (MatrixSM-9-30035-OEM), with each array comprised of 16 SiPMs

arranged in a 4 × 4 configuration, the surface covered by ∼ 0.5-mm thick thermoset clear

epoxy glass (of refractive index 1.54 at 589 nm). Properties of the SensLM-series are provided

in Table 4.1. Upon detection of an event by the SensorHead, the resulting signal is processed

by the front-end electronics: a specific multiplexing architecture (Section 4.3) channels an

array and pixel signal through a preamplifier and then a comparator, providing the spatial10Worldwide-Industrial Electronics-Nuclear Electronics-Resources, Plein & Baus GmbH, Linde 18, D-51399

Burscheid, Germany (http://www.wiener-d.com/).11For documentation, see the W-IE-NE-R MPOD manual (revision 2.7.1) at http://file.wiener-d.

com/documentation/MPOD/WIENER_MPOD_Manual_2.7.1.pdf.

http://www.wiener-d.com/

http://file.wiener-d.com/documentation/MPOD/WIENER_MPOD_Manual_2.7.1.pdf

http://file.wiener-d.com/documentation/MPOD/WIENER_MPOD_Manual_2.7.1.pdf


location of the event. The signal is time-stamped, shaped (yielding the energy of the signal),

and flagged (through programmable array/pixel effective threshold discriminators). A 12-bit

serial ADC digitises the signal, which is subsequently read out to the EVB communications

board through a fast serial interface, while timing information is acquired via a 16-bit TDC

with a timing resolution of 0.5 ns. Additional details of the signal processing by the ADC

and TDC devices can be found in the MatrixSM-9 user manual.

The Matrix-EVB board is capable of supporting up to sixteen individual Readout Mod-

ules operating in singles or coincidence modes. It provides a common 50 MHz clock, and

upon receiving and buffering event data from the ReadoutModule(s) (Figure 4.2.2) at a rate

of 50 Mbps, transfers spatial, temporal and energy data to the PC via a two-way high-speed

USB interface (Figure 4.2.3b). In coincidence mode, additional temporal-coincidence analy-

sis on candidate events is performed by SensL firmware installed on the board itself, reducing

the data transfer rate to the PC.

A series of software and firmware tools freely available from the SensL website12 provides

a user-friendly interface with the Readout Module and EVB board, both in the processing,

analysing and displaying of channel event data received from the EVB board, and in the pro-

gramming of various operational parameters via the Main-Dialogue screen of the GUI (Fig-

ure 4.2.4a). Of particular interest are the following settings:

• Array Threshold& Pixel Threshold: sets the effective discriminator threshold voltages

for the nine array and sixteen pixel signal channels by amounts above corresponding12The measurements discussed in this report used the following suite of software/firmware tools: Matrix9

GUI Software version 3.02, Matrix9 DLLDriver Library version 1.03, Coincidence Board FPGA Firmware ver-sion 1.03 andReadoutModule FPGAFirmware version 1.05. See the SensLMatrix9/L9 Software Release Not ,revision 3.02 (July 11, 2014) at http://www.sensl.com/downloads/sw/MATRIX9_SW_Release_Notes_v3p2.pdf.

http://www.sensl.com/downloads/sw/MATRIX9_SW_Release_Notes_v3p2.pdf

http://www.sensl.com/downloads/sw/MATRIX9_SW_Release_Notes_v3p2.pdf


Property Value Notes

Breakdownvoltage, Vbr

27.0 V - 28.0 VDefined as the reverse bias atwhich a 100 nA current flows indark conditions

Overvoltage 1.0 V - 5.0 V

Spectralrange 400 nm - 1000 nm

Peak wavelength,λmax

500 nm

PDE (at λmax) 20% “True” sensor PDE, excludingcrosstalk and afterpulsing

Gain,Ge 2.3 × 106 Anode-to-cathode readout

Dark current 3.8 μA (typical)8.0 μA (max.)

Dark currentrate

10 MHz (typical) Derived from dark-currentmeasurement asNdark = ISiPM/Ge/q (Hz)22 MHz (max.)

Rise time 0.8 ns Measured as 10%-90% of peakamplitude

Signal pulsewidth 2 ns Fast output (FWHM)

Microcellrecovery time 130 ns

Time for microcell to recharge(90%-10% of pulse peakamplitude)

Capacitance(anode cathode) 870 pF

Internal capacitance of sensorCapacitance (fastterminal to cathode) 14 pF

Temperaturedependence of Vbr

20 mV/◦C Change in Vbr from pulsed lasergain measurements

Cross talk 21%

Table 4.1: Proper es of the SensL MatrixSM-9-30035 SiPM model, of size 3 × 3 mm2. Each unit contains 4774microcells, with a microcell size of 35 × 35μm. All measurements are made at Vob = +2 V (that is, Vbr + 2 V) and21◦C. Values obtained from the SensL M-series data sheet, available from http://sensl.com/downloads/ds/DS-MicroMseries.pdf.

http://sensl.com/downloads/ds/DS-MicroMseries.pdf

http://sensl.com/downloads/ds/DS-MicroMseries.pdf


(a) Dimensions of the MatrixSM-9 Sensor Head, showingthe 12 × 12 pixel array.

(b) Data transfer between the Sensor Head, readout and communica ons boards.

Figure 4.2.3: Schema cs of the MatrixSM-9 Sensor Head and readout/communica ons boards: (a) The schema cof the Sensor Head, showing the 144 SiPM pixels arranged in a 12 × 12 configura on. The overall dimensions are46.31 × 47.80 mm2, with each pixel measuring 3.17 × 3.17 mm2. The intra-array dead space is∼ 0.2 mm, whilethe inter-array dead space is 2.22 mm and 2.75 mm in the horizontal and ver cal direc ons, respec vely. The pixelsensi ve area is 8.12mm2, and dE/E (FWHM) for LYSO at 511 keV is< 17 %. The op cal response uniformity overall pixels – defined as the ra o of the pixel response to the median pixel response – is< ±10 %. Figure from Du etal. [277] (b) Data transfer between two Sensor-Head/Readout-Electronics modules (top part of the figure) and thecoincidence/communica on board (bo om part). Figure from SensL’s “MatrixSM Family Product Brief”, revision 1.1,November 2013 (retrieved from http://www.sensl.com/downloads/ds/PB-Matrix9SM.pdf).

http://www.sensl.com/downloads/ds/PB-Matrix9SM.pdf


array/pixel offset values.

• SPM Bi : allows the bias voltage provided to all 144 SiPMpixels to be set collectively:

set to 32.5 V.

• Mode: enables the user to choose between:

– Plot pixels: event count-rate data is displayed on the GUI map in real time.

– Stream to file: FIFO event data is streamed continuously to an output file (for

post-processing).

• Format: determines the mode of operation, based upon information recorded for a

fired pixel:

– Single ener mode: displays the time stamp, pixel/array location and energy

value of the fired pixel.

– Region of interest: displays additional energy information of the cluster – that is,

the energies of the eight surrounding pixels.

– All Energi : displays additional energy information of all sixteen pixel energies

in the corresponding array.

Although adjustable via the GUI interface, the remaining parameters shown in Figure

4.2.4 were fixed at values suggested in the SensL MatrixSM-9 manual (optimised for 511 keV

gamma rays andLSO/LYSOscintillator). After running thedetector blockover auser-defined

time interval at a particular array and pixel threshold, the resulting data is save to file and anal-

ysed.


(a)MatrixSM-9 GUI: Main-Dialogue screen.

(b)MatrixSM-9 GUI: Energy/ ming-plot screen.

Figure 4.2.4: Screenshots of the MatrixSM-9 GUI so ware, showing the main and energy/ ming screens. Values oftypical se ngs are shown in both cases (see text).


4.3 Multiplexing: The Scrambled Crosswire Readout

Severalmultiplexing schemes have been reported in the literature. For instance, a simple row-

column summing method combines the output signals originating from the same row/col-

umn, producing four outputswhich are used to obtain the position and energy of an incident

scintillation photon. Initially developed for multi-anode PMTs [299], this has been applied

to SiPMs by Wang et al. [270]. A different strategy, also originally applied to PMTs [300]

and subsequently to SiPMs, is the so-calledDiscretised Proportional Counter (DPC) approach

[288, 291, 294, 301–303]. This technique feeds the SiPM signals into a charge division resistor

network to produce four outputs, again allowing spatial and energy information of the initial

photon to be calculated.

Figure 4.3.5a shows the multiplexing architecture used in the SensLMatrixSM-9 readout

module to reduce the complexity of the readout electronics. The Scrambled Crosswire Read-

out (SCR) technique enables the 144 pixels to be readout by only twenty-five data channels.

In each array, a total SiPM array signal is formed by summing the sixteen cathode pixel sig-

nals. Meanwhile, the anode signals from the same pixel in each of the nine arrays are summed

together to form a SiPMpixel signal. Therefore, the hypothetical 144 output channels are re-

duced to 9 array and 16 pixel channels.

To enable very precise time stamps to be recorded (of particular importance when oper-

ating in coincidence mode), the SCR design requires the availability of a fast trigger signal.

When a particular SiPM pixel fires, the resulting pixel signal triggers the DAQ to digitise the

16 pixel signals while the array pulse identifies the corresponding array. In this way the ar-

ray/pixel location of the fired pixel can, in theory, be found. However, as pointed out by Du

et al. [277], since it is the first array signal arriving at the DAQ which is analysed, the array


location of a fired pixel can be determined incorrectly. If the effective array threshold is suffi-

ciently low that the array is triggered by noise, the array in which the “true” event originated

will be erroneously determined. It is only at high array thresholds – above that of the noise,

but still less than the true array signal – that the correct event location will be recorded.

4.4 Measurements Using the SensL MatrixSM-9 Modules

In the presentwork, the EVB communications board is identified by its IDnumber 15B181E9,

and the two detector blocks, with ID numbers 2CCFF2 and 2CFFA4, are referred to as the

“OLD” and “NEW” modules, respectively.13 A series of noise and source singles measure-

ments were performed initially with two MatrixSM-9 detector blocks, with the goal of char-

acterising each block in preparation for a set of coincidence measurements (characterisation

involved finding the optimal operating conditions with respect to array/pixel discriminator

thresholds, minimal noise contribution, and appreciable count rate over an acceptable run

time). As in Chapter 3, noisemeasurements were performed by operating the detector blocks

in the absence of a source or scintillator. Source measurements were recorded using the same

13 × 13 × 50 mm3 PTFE-wrapped BC-416 plastic scintillator used in Chapter 3, which was

placed up against the Sensor Head (in a dry or wet connection – the latter using grease or

a Si-cookie), with a source placed approximately half-way along the scintillator. Measure-

ments were performed with the entire ensemble – Readout Modules, scintillator and source

– placed in a dark box, with a thick black cloth draped over the box tominimise light-leakage

into the box. The ambient temperature inside black box was not controlled, but monitored

using a common thermometer placed∼1-2 cm from the Sensor Head, with the temperature13This labelling scheme is purely chronological, referring to their acquirement by the University of Regina.


(a) The Scrambled Crosswire Readout mul plexing technique.

(b) The MatrixSM-9 array-labelling scheme.

(c) The MatrixSM-9 pixel-labelling scheme.

Figure 4.3.5: Schema c of the mul plexing and labelling schemes used in the MatrixSM-9 system. (a) The Scram-bled Crosswire Readout (SCR) mul plexing technique used in the Matrix9-SM module. From Du et al. [277]. (b) Thearray- and pixel-labelling scheme used by the MatrixSM-9 so ware. Note that whether or not these labels corre-spond to those on the physical Sensor Head depends upon the orienta on of the detector block (see text). From theMatrixSM-9 user manual.


of the Sensor Head itself monitored via an integrated digital temperature sensor located un-

der the detector head (granularity 0.125◦C), with output on the Main Dialogue screen.

To identify a pixel on a Sensor Head, the MatrixSM-9 software uses the representation

shown in Figures 4.3.5b and 4.3.5c, which correspond to the map on the Main-Dialogue

screen. However, there is some potential for confusion in using this labelling scheme in con-

junction with the physical Sensor Head. Although the rectangular sides differ by 1.49 mm

(Figure 4.2.3a), with respect to the eye the SensorHead arguably appears square. In all subse-

quentmeasurements, theReadoutModulewas oriented such that the 80-way IDCconnector

on its end-face is positioned horizontally, with the SensL logo upright. This introduces a ro-

tation of the array/pixel labels, whereby the location of, for example, array “0”, on the Sensor

Head is rotated+90◦ relative to array “0” on the GUI map.

Figure 4.4.6 shows several configurations of scintillator-SiPM array couplings investi-

gated as part of this work. Typical PET systems can incorporate numerous pixelated scin-

tillators, with the end-face dimensions of each matched precisely to the active area of a SiPM

array, and a careful alignment of each pixel to a corresponding SiPM. However, the current

set-up utilised a single block of the plastic scintillator, with the 13 × 13-mm2 end-face being

of slightly smaller area than the 13.48 × 13.48-mm2 area of each array in the Sensor Head.

Although pixelated scintillators would not normally be positioned astride neighbouring ar-

rays in PET systems – as in positions “A” and “B” in Figure 4.4.6a – the response of the

MatrixSM-9 to such configurations was investigated nevertheless.

Figures 4.4.8 and 4.4.7 show a series of frequency histograms at a pixel threshold voltage

of 0.07 V for the “OLD” and “NEW”modules, with the PTFE-wrapped BC-416 scintillator

(together with the 90Sr source) in dry contact astride positions “A” and “B”, respectively. It is


(a) Posi ons astride two and four arrays.

(b) Posi ons rela ve to the central array.

Figure 4.4.6: Schema c showing the approximate posi ons of the 13 × 13-mm2 end-face of the BC-416 plas cscin llator (not drawn to scale), rela ve to the Sensor Head. (a) The end-face posi oned astride mul ple arrays:posi on “A” (astride arrays 6 and 7) and posi on “B” (astride arrays 4, 5, 7 and 8). (b) The end-face posi oned overarray 4: in posi on “C” the end-face is offset from the 4 × 4 SiPM configura on by∼ 2.72 mm, while in posi on “D”the end-face lies over the 4 × 4 SiPM configura on.


evident that in comparing results of bothmodules at the same array threshold, quite different

frequency histograms can result, in spite of both devices being of the same series. In both fig-

ures, for each module, at the lowest array threshold (0.08 V) the frequency histograms show

quite an appreciable noise contribution distributed throughout the 144 pixels (especially in

the case of the “OLD” module), while at an array threshold of 0.13 V, the noise is sufficiently

reduced, revealing those arrays indeed covered by the scintillator. These results are consistent

with the observation of Du et al. [277] discussed above; that is, the undesirable triggering by

noise at low array thresholds leads to an erroneous determination of the location of the fired

pixels, resulting in a “smearing out” of the scintillator’s “image” across the whole array-pixel

structure. It is only when the array threshold is set above the noise that the correct locations

of the fired pixels are recorded.

However, it is notable that in both Figures 4.4.8 and 4.4.7, the source-induced scintilla-

tor “image” is not limited to those pixels in actual contact with the scintillator face (that is,

those pixels indicated as being fully or partially covered in Figure 4.4.6a). Rather, any of the

pixels in the corresponding arrays appear capable of being fired. As well as due to some effect

arising from the SCR architecture, a possible explanation for this phenomenon could be the

dry scintillator/Sensor-Head contact, leading to unwanted reflections between the various

boundaries. An interesting test would be to repeat these experiments with a greased contact,

in order to minimise any such reflections.

An additional feature of these low-array-threshold results (see, for example, Figures 4.4.7a

and4.4.8a) is that although triggeredby (random)noise, a remarkably regular pattern emerges

between frequency histograms of individual arrays in the samemodule. One possible reason

for this effect is that in the case of a particularly “hot” (i.e., high-frequency) pixel, for example,


(a) OLD: AT: 0.08 V, PT: 0.07 V,T = 24.375 ◦C.

(b) OLD: AT: 0.11 V, PT: 0.07 V,T = 27.0 ◦C.

(c) OLD: AT: 0.13 V, PT: 0.07 V,T = 28.25 ◦C.

(d) NEW: AT: 0.08 V, PT: 0.07 V,T = 24.375 ◦C.

(e) NEW: AT: 0.11 V, PT: 0.07 V,T = 27.0 ◦C.

(f) NEW: AT: 0.13 V, PT: 0.07 V,T = 27.75 ◦C.

Figure 4.4.7: Frequency (i.e., count-rate) histograms (with the ver cal scale in Hz) resul ng from a 90Sr source, placedon a PTFE-wrapped BC-416 plas c scin llator placed astride two con guous arrays (numbers 6 and 7 in Figure4.3.5b) – that is, posi on “A” in Figure 4.4.6a. Results are presented at a pixel threshold (PT) of 0.07 V over a range ofarray thresholds (AT): (a) AT: 0.08 V (“OLD” module). (b) AT: 0.11 V (“OLD” module). (c) AT: 0.13 V (“OLD” module). (d)AT: 0.08 V (“NEW” module). (e) AT: 0.11 V (“NEW” module). (f) AT: 0.13 V (“NEW” module). In all six cases, the FIFOrun me was 60 s, and the temperature of the Sensor Head at the beginning of the run is included. The white areascorrespond to frequencies of less than 1/60 Hz.


(a) OLD: AT: 0.08 V, PT: 0.07 V,T = 26.25 ◦C.

(b) OLD: AT: 0.11 V, PT: 0.07 V,T = 25.625 ◦C.

(c) OLD: AT: 0.13 V, PT: 0.07 V,T = 24.125 ◦C.

(d) NEW: AT: 0.08 V, PT: 0.07 V,T = 26.375 ◦C.

(e) NEW: AT: 0.11 V, PT: 0.07 V,T = 25.625 ◦C.

(f) NEW: AT: 0.13 V, PT: 0.07 V,T = 24.25 ◦C.

Figure 4.4.8: Frequency histograms similar to those shown in Figure 4.4.7, except with the scin llator placed astridefour con guous arrays (numbers 4, 5, 7 and 8 in Figure 4.3.5b) – that is, posi on “B” in Figure 4.4.6a. Results arepresented at a pixel threshold (PT) of 0.07 V over a range of array thresholds (AT): (a) AT: 0.08 V (“OLD” module). (b)AT: 0.11 V (“OLD” module). (c) AT: 0.13 V (“OLD” module). (d) AT: 0.08 V (“NEW” module). (e) AT: 0.11 V (“NEW”module). (f) AT: 0.13 V (“NEW” module). In all six cases, the FIFO run me was 60 s, and temperature of the SensorHead at the beginning of the run is included. The white areas correspond to frequencies of less than 1/60 Hz.


there is a high probability that noise will trigger the same pixel position in all the other arrays,

resulting in the same pixel having a relatively high frequency in each of the other arrays. That

is, the high number of counts in the original hot pixel will be smeared out amongst pixels of

the same location in the other arrays.

Du et al. [277] have quantified the mispositioning of events on the central array (and

hence the reduction in the “true” frequency of array 4), due to the undesirable noise-induced

triggering mechanism discussed above, by introducing the array-tri er error, ξ, given by

ξ = 100{1 − n4∑8

i=0 ni

}(i = 0, 1, · · · , 8) , (4.4.1)

as the percentage of total recorded events not detected by array 4. Here, ni is the number

of events detected by array i. Using a 68Ge source together with a LSO scintillator, optically

coupled to the MatrixSM-9 surface with optical grease, these authors have reported array-

trigger-error curves for a fixed pixel threshold (0.3 V) over a wide range (0 V - 0.75 V) of array

thresholds and at different temperatures (5 − 25 ◦C, in 5 ◦C steps). In Figure 4.4.9, this idea

has been generalised to produce an array-trigger-error surface, in which ξ has been calculated

for a range of array- and pixel-threshold voltages. Here, measurements have been performed

with the BC-416 scintillator in positionC (Figure 4.4.6b), in greased contact with themodule

surface, over the temperature range∼ 23.5−29 ◦C. Note that although the array-trigger-error

curves ofDu et al. extend out to an array threshold of 0.75 V, in Figures 4.4.9a and 4.4.9b the

maximum array threshold value is 0.2 V. It was found that at such values, obtaining sufficient

statistics required increasingly longer run times. This disparity between the two experiments

can be attributed to the superior light-producing capacity of LSO as compared to BC-416

(see, for example, Tables 2.1 and 2.2).


(a)OLD module.

(b) NEW module.

Figure 4.4.9: Array-trigger-error (defined in the text) surfaces for the “OLD” and “NEW” modules. The BC-416 plas cscin llator (with 90Sr source placed on top) was coupled to the central array (array 4) of each module in turn, in po-si on C (see Figure 4.4.6b). The measurements contribu ng to both surfaces were taken over an approximate range23.5 − 29 ◦C of Sensor-Head temperatures.


With respect to Figures 4.4.9a and 4.4.9b, it is evident that the array-trigger error can vary

appreciably with array threshold, as seen by the large change in ξ over a∼ 0.02-V range. In

contrast, apart from the feature at low pixel threshold and high array threshold in the case of

the “OLD” module, ξ does not change significantly with a change in pixel threshold. This

suggests that the array-trigger error ismainly dependentupon the array threshold,while being

largely independent of the pixel threshold. This is consistent with the implication of Section

4.3: due to the SCR architecture, although the pixel signal triggers a particular pixel/SiPM it

is both the pixel and array signals which verify this event.

Furthermore, the surfaces show that although both modules are of the samemodel, they

show distinctly different characteristics. In particular, while both figures show the same gen-

eral shape, the “OLD” module maintains a larger array-trigger error over a wider range of

array thresholds, with the sharp decrease in ξ occurring at∼ 0.12 V, compared to∼ 0.08 V

for the “NEW” module. This effect can be seen in Figures 4.4.7 and 4.4.8: for example, in

the latter, the “OLD”module still shows a significant amount of noise at array thresholds of

0.08 V and 0.11 V (that is, at positions high on the array-trigger-error “plateau”). Meanwhile,

the corresponding plots for the “NEW” module show a much lower amount of noise; the

AT=0.08V, PT=0.07 V result is in the region of the surface dominated by the steep decrease,

and the AT=0.11 V, PT=0.07 V is on the low-trigger-error plateau (i.e., in which ξ ∼ 0%).

In these measurements, scintillator-array alignments were performed by eye, and so were

susceptible to slight differences when, for example, switching between the twomodules or re-

assembling configurations in order to repeat earlier measurements. To investigate the result-

ing systematic errors that could result from such misalignments, the scintillator was placed

in positions C and D (Figure 4.4.6b) – with the former offset from the latter by∼ 2.5 mm –


with an upper portion of the scintillator face lying in the dead space between arrays 1 and 4.

Under such circumstances, it is reasonable to expect a slightly reduced amount of light hit-

ting array 4 in position C as compared to D. In position C, care was taken to ensure that the

scintillator was not astride arrays 1 and 4: preliminary measurements at high array thresholds

demonstrated that, consistent with the above results, no pixels in array 1 fired.

Figure 4.4.10 shows the histograms of the (absolute) difference in frequencies between

the two configurations for the scintillator in greased contact with bothmodules in turn. The

array and pixel thresholds were chosen somewhat arbitrarily: with the exception of Figure

4.4.10a, in order to minimise the potential effect of array-trigger errors, the selected array

thresholds all correspond to ξ ∼ 0%. These figures show that apart from, for example, pixel

2 of array 4 – which has a frequency difference of∼ 30 − 40 Hz – that of the vast majority

of pixels seems to differ by ≲ 10 Hz. Even in Figure 4.4.10a, in which Figure 4.4.9 suggests

an appreciable array-trigger error, the absolute difference in frequency between the two po-

sitions is ≲ 2 − 3 Hz. Therefore, in general, a slight misalignment of the scintillator in the

manner shown in Figure 4.4.6b leads to a relatively small difference in frequency, as expected.


(a) OLD: AT: 0.12 V, PT: 0.13 V,T = 28.375 ◦C.

(b) OLD: AT: 0.17 V, PT: 0.12 V,T = 26.875 ◦C.

(c) OLD: AT: 0.18 V, PT: 0.14 V,T = 27.5 ◦C.

(d) NEW: AT: 0.12 V, PT: 0.13 V,T = 23.375 ◦C.

(e) NEW: AT: 0.17 V, PT: 0.12 V,T = 24.5 ◦C.

(f) NEW: AT: 0.18 V, PT: 0.14 V,T = 24.625 ◦C.

Figure 4.4.10: Frequency-difference histograms mimicking a misalignment of the PTFE-wrapped BC-416 plas c scin-llator coupled to array 4 (using a 90Sr source). The scin llator was placed, in turn, at posi ons C and D of Figure

4.4.6b. The histograms show the (absolute) frequency difference between resul ng measurements. With the excep-on of (b) and (e) – in which the FIFO run me was 60 s – the run mes were 600 s. The temperatures of the Sensor

Head at the beginning of the run are included. The white areas correspond to frequencies of less than 1/60 Hz.

Chapter 5

Conclusion

Silicon photomultipliers have emerged as a rival device to traditional photosensors such as

the photomultiplier tube (PMT) in numerous fields, providingmany of the properties of the

PMT while offering several additional advantages. Two applications of SiPMs were consid-

ered in this work: in developing a prototype personal radiation detector, and in the imaging

of a radioactive source using arrays of SiPMs.

Fears over the potential use of illicit fissilematerial by terrorist organisations, aswell as the

biological threat posed by decades of use of radioactive material in, for example, military ar-

mament and nuclear-waste management, have made the need for increasingly more efficient

radiation scanners/detectors crucially important. In partnership with EIC, the first part of

this study focused on implementing SiPMs to develop a prototype gross-gamma counter for

use by first responders and emergencyworkers entering potentially hazardous environments.

EIC’s interest in producing a commercial product placed size and cost limitations on the pro-

totype: the goal was to maximise the light-collection capacity of the detector while minimis-

101

CHAPTER 5. CONCLUSION 102

ing its cost. Initialmeasurements established individual units of theHamamatsuS12572-050C

series (with a photosensitive area of 3 × 3 mm2) as the preferred choice of SiPM. Although

nine-times larger thanunits of the 1×1-mm2 S12571-050C series, initial self-triggering tests (the

preferred configuration from the point of view of size) had established that the 3 × 3 units

offered a superior light-collection, without being prohibitively more expensive. Subsequent

trials with various combinations of light-producingmedia, coupled to the 3× 3-mm2 SiPMs,

and operating in singles mode, were tested using both Regina’s and EIC’s nuclear electronics.

It was established that a 13 × 13 × 50-mm3 BC-416 plastic scintillator, wrapped in common,

highly reflective PTFE thread-seal tape, provided the largest signal-to-background measure-

ments for gammas from 60Co and 133Ba over a moderate range of discriminator thresholds,

while satisfying the spatial requirements of EIC’s existing device.

It is well known that, under identical conditions, a scintillator with a greater light yield

offers a larger light-collection capacity. Although not limited to those listed in Tables 2.1 and

2.2, there are a number of materials capable of providing a significant improvement in light

production than the BC-416 scintillator, and thus considered to be a more-desirable light-

production medium for use in the above project. In the current PRD project, incorporating

a scintillator such as LYSO or LSO in a commercially available device would not be cost-

effective. While the low-cost and high light yield of NaI(Tl) would suggest it to be the ideal

scintillator, its strong hygroscopicity and long decay time make it unsuitable for this work.

BC-416 is marketed as a general, all-purpose scintillator, and as such is considerably less ex-

pensive than an equivalent volume of, for example, LYSO or LSO. However, a future PRD

project at the DDL will seek to explore the use of GAGG scintillators as the preferred light-

producing medium. Of moderate expense, such materials offer ∼ 7 times the light yield of


BC-416.

Preliminary results of the neutron-detection project suggest that thermal neutrons can

be detected above the background using aHamamatsu SiPM, coupled to aWLS fibre coated

with successive layers of ZnS(Ag) and B4C. Further work is ongoing, with plans to develop

a detector utilising a bundle of ZnS(Ag)/B4C-coated WLS fibres. In a possible future col-

laboration with EIC, this work has the potential to lead to a marketable neutron-detection

device.

As outlined in Chapter 4, while details and results of existing plant-imaging systems have

been discussed in the literature, such devices have not as yet incorporated SiPMs as the photo-

sensor. The nuclear-imaging project sought to gain a familiarity with two SensLMatrixSM-9

SiPM arrays, in preparation for their ultimate use in the “PhytoPET” plant-imaging system

to be installed at the University of Regina. Initial trials imaged a 90Sr source, placed on top of

the PTFE-wrapped 13× 13× 50-mm3 BC-416 plastic-scintillator block, coupled to both SiPM

array modules in turn. Frequency “maps” with the scintillator in contact with the Sensor-

Head surface, but sitting astride multiple SiPM arrays, showed that at sufficiently high array

thresholds, a multitude of pixels in the corresponding arrays fired, even those pixels not di-

rectly covered by the scintillator. Important features of the Scrambled Crosswire Readout

multiplexing architecture were observed, consistent with the observations of Du et al. [277]:

in particular, the importance of the array-signal threshold voltage in providing an accurate

array location of a fired pixel. The effect of a slight misaligning the scintillator block on the

Sensor-Head surface was investigated, with the resulting difference in frequency typically be-

ing of a few Hertz in magnitude, as expected. Each module was characterised by producing

its array-trigger-error surface, providing a quantitative basis from which effective array and


pixel discriminator thresholds can be chosen.

For the purposes of PET imaging, the light-yield of plastic scintillators is unacceptably

low, while the superior performance of materials such as LYSO/LSO have resulted in their

widespread use in such systems. Indeed, with respect to the MatrixSM-9 system, several of

the recommended operational settings used in the currentmeasurements – involving BC-416

– were optimised by SensL using a LYSO scintillator. Hence, although frequency measure-

ments presented in the second part of this report provide a preliminary step in characterising

the modules, they are necessarily at the lower end of what would be expected from a scintil-

lator with superior light-yield. Work on evaluating both modules is ongoing, with further

trials being performed using BC-416 with the modules operating in singles and coincidence

modes. There are plans to replace BC-416 with LYSO and GAGG scintillators, as well as to

introduce active andpassive collimators into the experimental set-up. The culmination of the

initial phase of the project will include imaging a so-called “phantom”, in order to test and,

if necessary, “fine-tune” the performance of the experimental set-up. While such work will

enable the SPARRO Group to develop a better understanding of the operational aspect of

the project, a major part of the future work will involve the development of dedicated image-

reconstruction algorithms. In this endeavour, the expertise of members of the Department

of Computer Science at the University of Regina will prove invaluable.

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