APPLYING GRIDPIX AS A 3D PARTICLE TRACKER …...the need for constructing a 3D particle tracker. In...

APPLYING GRIDPIX AS A 3D PARTICLE TRACKERFOR PROTON RADIOGRAPHY

Master Thesis

of

Panagiotis C. Tsopelas

Supervisors:

prof. dr. ir. Els Ko�eman

dr. Jan Visser

prof. dr. Sytze Brandenburg

Utrecht, 21 November 2011

Abstract

Proton therapy is one of the possible treatments in the �ght against cancer,mostly applied to patients with tumors located near sensitive organs as theseorgans can be better spared with proton beam irradiation compared to X-rayirradiation. X-ray computed tomography images are used to provide informationabout the proton range inside the human body with an error of approximately of4%. Miscalculations in the proton range can lead to delivering a high amount ofdose in healthy tissue and not in the tumor so reducing these errors is essential.Acquiring images with proton computed tomography, the precision in range canbe improved by a factor of 2.5[1]. To successfully use protons for radiography theconstruction of a 3D particle tracker is needed. This apparatus should provideus with precise information about the proton tracks and the proton energy.

The detector R&D group of Nikhef, in Amsterdam is collaborating withthe KVI, in Groningen in research for proton computed tomography. The Grid-Pix, a gas-�lled detector combined with a CMOS pixel chip used at Nikhef forhigh energy physics experiments, is an ideal candidate for the needs of protonradiography. A 3D particle tracker consisting of GridPix detector and a BaF2

scintillating calorimeter was designed and irradiated with 55 - 144 MeV protonswith the proton beam of the AGOR cyclotron in KVI. A number of samples wasalso tested in di�erent con�gurations. The 3D reconstruction of proton tracksfrom the GridPix detector and the measurement of the energy of the protonsfrom the BaF2 are presented and compared with simulations in GEANT4.

III

Contents

1 Introduction 1

2 Hadrons in Medicine 5

2.1 Hadron Therapy . . . . . . . . . . . . . . . . . . . . . . . . . . . 52.1.1 History & Perspectives . . . . . . . . . . . . . . . . . . . . 52.1.2 Hadrons and the physics behind them . . . . . . . . . . . . 52.1.3 Facilities . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

2.2 Proton CT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92.2.1 Motivation for Proton Radiography . . . . . . . . . . . . . 92.2.2 Image reconstruction . . . . . . . . . . . . . . . . . . . . . 10

3 Proton CT set-up 11

3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113.2 GridPix based TPC . . . . . . . . . . . . . . . . . . . . . . . . . . 11

3.2.1 The Timepix chip . . . . . . . . . . . . . . . . . . . . . . . 123.2.2 Field Cage . . . . . . . . . . . . . . . . . . . . . . . . . . . 153.2.3 Ingrid . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 153.2.4 3D Track Visualization . . . . . . . . . . . . . . . . . . . . 153.2.5 Gas Mixtures . . . . . . . . . . . . . . . . . . . . . . . . . 163.2.6 Hit Resolution . . . . . . . . . . . . . . . . . . . . . . . . . 203.2.7 Read-out . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

3.3 BaF2 scintillating detector . . . . . . . . . . . . . . . . . . . . . . 243.4 Trigger . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

4 Experiment at the KVI 29

4.1 AGOR cyclotron . . . . . . . . . . . . . . . . . . . . . . . . . . . 294.2 Experimental set-up . . . . . . . . . . . . . . . . . . . . . . . . . 304.3 Monitoring the Set-Up . . . . . . . . . . . . . . . . . . . . . . . . 314.4 Calibration of the BaF2 detector . . . . . . . . . . . . . . . . . . . 314.5 Synchronizing GridPix and the BaF2 detector . . . . . . . . . . . 324.6 Collected Data Sets . . . . . . . . . . . . . . . . . . . . . . . . . . 324.7 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34

5 Results & Analysis 35

5.1 GridPix Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 355.1.1 Projection on the XY plane . . . . . . . . . . . . . . . . . 355.1.2 Y-coordinate distribution of all hits . . . . . . . . . . . . . 375.1.3 Time Measurement & Z reconstruction . . . . . . . . . . . 395.1.4 Energy dependence of Ionization . . . . . . . . . . . . . . 43

V

Contents

5.1.5 Intensity e�ects . . . . . . . . . . . . . . . . . . . . . . . . 435.2 Calorimeter Data . . . . . . . . . . . . . . . . . . . . . . . . . . . 44

5.2.1 Calibration . . . . . . . . . . . . . . . . . . . . . . . . . . 445.2.2 Shifted Energy spectrum during Runs . . . . . . . . . . . . 47

5.3 Correlation of GridPix and Calorimeter data . . . . . . . . . . . . 495.4 Simulations in GEANT4 . . . . . . . . . . . . . . . . . . . . . . . 50

5.4.1 Interaction of GridPix with the proton beam . . . . . . . . 515.4.2 Simulation of the Copper samples . . . . . . . . . . . . . . 53

6 Conclusions 55

6.1 Performance of the 3D particle tracker . . . . . . . . . . . . . . . 556.2 Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56

Bibliography IX

A XI

A.1 Principles of a Time Projection Chamber . . . . . . . . . . . . . . XIA.2 Field Cage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . XII

B XIII

B.1 Binary resolution . . . . . . . . . . . . . . . . . . . . . . . . . . . XIII

C XV

C.1 Trigger . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . XV

D XVII

D.1 Measurements with Cosmic Rays . . . . . . . . . . . . . . . . . . XVII

VI

Chapter 1

Introduction

In cancer treatment X-ray irradiation is the most common technique, causingin many cases serious side e�ects on the patient. During an X-ray irradiationthere is an interaction probability between photons and tissue of about 3% percm with an exponential reduction of the dose deposited in the tissue. To deliverthe highest total dose to the tumor in cancer treatment with X-Rays, one cannotirradiate from one direction only as the tissue before and after the tumor alongthat direction will be severely damaged. Therefore multiple irradiations fromdi�erent angles must be done, making it impossible to exclude areas near thetumor from getting irradiated (Fig. 1.1(a)).

In proton therapy, the energy deposition at the beginning and along the trackis low while the proton energy is close to the initial energy. The highest relativedose is deposited at the end of the proton range in a small and well de�ned areaavoiding further damage in the body (Fig. 1.1(b)). This property of protonscan be e�ectively used by matching the area where protons mainly deposit theirenergy with the location of the tumor. In cancer therapie, the desired radiationdose should have a broad, �at peak, where the �at section corresponds to theextent of the tumor being treated. In order to do so, the dosage is deliveredin a broad, �at distribution de�ned as Spread Out Bragg Peak1 (SOBP) whenthe radiation is naturally deposited in narrow peaks as illustrated in Fig. 1.2.Miscalculating the range of protons through the body of the patient will lead toa high amount of dose delivered in healthy tissue and a too low or no amount ofdose delivered in part of the tumor. A similar deviation in the case of X-rays hasmuch less implications.

To successfully use protons for cancer treatment detailed information aboutthe patient is needed. Most of this information can be obtained by X-ray com-puted tomography imaging. However, the quality of data from X-ray computedtomography (CT) is not su�cient. By using X-ray CT, the Houns�eld numbersare converted to proton stopping power relative to water2 using a calibrationcurve. This conversion can lead to errors in the calculation of proton range in

1The SOBP can be produced in several ways: by using a range modulation wheel, a degraderand energy selection system or a syncrhotron that directly delivers the beam at the desiredenergies.

2Human body is simulated as water.

1

Chapter 1 Introduction

(a) Treatment of spinal cancer using X-rays.While the tumor is irradiated the most, sur-rounding organs are signi�cantly a�ected.

(b) Same treatment using protons. Radia-tion is concentrated on the tumor.

Figure 1.1: X-rays vs protons. Courtesy of Francis H. Burr, proton Therapy Center,

Massachussets.

Figure 1.2: X-ray and proton energy deposition as a function of the body depth.

2

patients of approximately of 4%. Using protons in making a CT image, one canreduce these errors by a factor of 2.5 as stated in [1].

The concept of using protons for imaging has started since 1960 . Sincethen, in research for proton radiography and CT irradiation of samples of simplegeometries have been made ([2],[3] and [4]) as well as more complex geometriesrepresenting a human head[5]. Silicon strip and pixel detectors have been usedas part of a 3D particle tracker for protons in order to obtain information aboutthe proton beam. The use of a gaseous detector for a 3D particle tracker is alsoan option that can o�er advantages over the solid state detectors. A gaseousdetector is radiation hard and has low interference with the beam. A GridPixbased time projection chamber is a gaseous detector used in high energy physicsexperiments that o�ers the possibility to reconstruct charged particle tracks in3D. The aim of this project is to demonstrate that a 3D particle tracker basedon a GridPix detector for tracking a proton beam for radiography purposes ispossible.

The �rst chapter presents the current status in cancer treatment with protonsand introduces the concept of proton radiography and CT. The second chapterfocuses on the physics behind the therapy and the radiography and explainsthe need for constructing a 3D particle tracker. In chapter 3 the principles ofthe GridPix based time projection chamber and the BaF2 scintillating detectorused as a 3D particle tracker are explained. The next two chapters describe thetesting of the 3D particle tracker with a proton beam, the results obtained andcomparison with simulations made in GEANT4. The thesis ends with conclusionsand suggestions for future experiments.

3

Chapter 2

Hadrons in Medicine

2.1 Hadron Therapy

2.1.1 History & Perspectives

The �rst treatments with protons of patients with cancer were performedat Berkeley Radiation Laboratory in 1954 and at Uppsala (Sweden) in 1957[6].Until 2010, about 75,000 patients have been treated according to [7] mainly inUSA, Japan and also in Germany, Switzerland, Russia while the current annualtreatment capacity is about 7,500 patients. Proton therapy is applied to patientswith tumors located near sensitive organs that need to be treated such thatradiation of these organs is avoided[8].

One of the countries that is interested in adopting proton therapy is theNetherlands. The insurance package in the Netherlands plans to cover futureproton therapy since there appear to be around 7,000 patients annually whocould bene�t from such treatment, so the construction of even 3 new facilitiesis under investigation. The KVI in Groningen in collaboration with Nikhef inAmsterdam are working on this �eld of research . The KVI institute is involvedin a project of constructing a clinical proton therapy facility and is collaboratingwith Nikhef in the research of proton CT and radiography.

2.1.2 Hadrons and the physics behind them

A hadron is a particle with rest mass much larger than that of an electron[9].A charged particle will lose kinetic energy through coulomb interactions withthe charge of the electrons and the nuclei when traversing a material. Sinceelectrons are more plentiful than charged nuclei, interactions with electrons aredominant[10]. During an interaction between the two particles, most of the en-ergy is transfered from the heavier hadron to the lighter electron. Among thehadrons, the ones that are used in cancer therapy are protons, neutrons andions (positively charged atoms containing both protons and neutrons). Althoughions are not literally hadrons, they are also included in the particles of hadrontherapy. Charged particles and photons interact di�erently with matter and the

5

Chapter 2 Hadrons in Medicine

di�erences and properties of these physical interactions make up the basis forhadron therapy.

The mean loss rate per unit thickness 〈dE/dx〉 of charged particles in [MeVg−1cm2] is well described by the Bethe-Bloch1 equation[11]:

−⟨dE

dx

⟩= Kz2

Z

A

1

β2

[1

2ln

2mec2β2γ2Tmax

I2− β2 − δ(βγ)

2

]

Where K/A = 0.307 MeV g−1 cm2, A and Z the atomic mass and atomic numberof the absorber, z the charge of the incident particle, β the velocity, γ the Lorentzfactor, mec

2 the rest mass of the electron, Tmax the maximum kinetic energy thatcan be transfered to a free electron with a single collision, I the mean excitationenergy in eV and δ(βγ) the density e�ect correction to the ionisation energyloss.

Important information we extract from the Bethe-Block formula is that thelinear energy loss of a particle through a material depends on the nature of thematerial and the velocity of the particle. As a charged particle travels througha medium, it's stopping power follows the Bethe-Bloch curve. Since the humanbody is made of water for about 80%, simulations and calculations of the energyloss and the range of protons in water are used as a reference. The Bethe-Blochcurve for a proton traversing through water is shown in Fig. 2.1(a). The particleloses energy in small steps and gradually moves to low energy range in the Bethe-Bloch curve(Fig. 2.1(a)). As the energy of the particle decreases the stoppingpower increases and a peak occurs called the Bragg peak. This results in theenergy deposited or dose delivered in the patient to be well localized in spacewith only a part of it before the Bragg peak and another part higher than afactor of 3-4 inside the Bragg peak region before the particle stops(Fig. 2.1(b)).Note that by changing the proton energy, the depth of the Bragg peak is shifted.

Some of the di�erences in radiation therapy between protons and ions lie in thehigher biological e�ectiveness of the heavier ions in the Bragg peak region andthe advantage that ions seem to have in the treatment of certain types of cancer(like bone and soft tissue sarcomas, lung and prostate cancer[14]). As shown inFig. 2.2, the protons have a wider spread and stop almost immediately after thepeak. On the other hand ions, although having a sharper shape, continue to ionizebeyond the peak due to nuclear fragmentation of ions so the dose is not decreasedimmediately to zero[16]. Ions have an increased energy loss towards the BraggPeak thus a signi�cant increase in irreparable damage is observed which yieldsa higher relative biological e�ciency[15]. A successful hadron therapy treatmentaims to match the Bragg peak where the tumor is located, so the tumor cells aredestroyed by generating irreparable DNA damage.

1Additional corrections to the Bethe-Bloch formula exist, see [12].

6

2.1 Hadron Therapy

(a) Bethe-Bloch curve for a proton through water.

(b) Ranges and Bragg peaks for di�erent proton energies.

Figure 2.1: Bethe-Bloch curve and corresponding energy loss per unit thickness in

water.

7


Figure 2.2: Attenuation of protons, ions and photons in water.

Figure 2.3: A gantry rotating around the patient.

2.1.3 Facilities

The construction of a proton therapy facility is a huge project where di�erent�elds like physics, biology, engineering, medicine, law, management and �nancecome together. In a modern proton therapy center the accelerator is connected toseveral treatment rooms by means of beam transport lines. The energies requiredfor therapy of deep-seated tumors with hadrons are typically in the range 70to 250 MeV for protons and 120 to 400 MeV/nucleon for carbon ions. Thesebeam energies can be achieved with synchrotrons, cyclotrons or linacs as mainaccelerators.

In existing proton treatment centers, the dose calculations are currently per-formed based on X-ray computed tomography and the patient is positioned withthe help of X-ray radiographs. The accuracy of X-ray CT for proton treatmentplanning is limited due to the di�erence in physical interactions between photonsand protons, which partially obviates the advantage of proton therapy[17]. Theneed to improve the quality of the data for proton therapy and to determine thesafety margin around the tumor volume led to proton CT[1].

8

2.2 Proton CT

Figure 2.4: The famous "lamb chop", the �rst proton radiograph taken by Koehler

at Harvard. The picture taken with protons (down) appears to show more detail than

that with X rays (up), however general conclusions about comparison between X-ray

radiography and proton radiography should not be drawn.

2.2 Proton CT

2.2.1 Motivation for Proton Radiography

The history of heavy charged particle radiography began in 1968 when Koehlershowed (Fig. 2.4) that with parallel-sided objects with a thickness nearly equalto the path length or range of an incident 160 MeV proton beam, proton radio-graphic �lms could be produced with much greater image contrast than that ofX-ray radiographs taken under the same conditions[18]. Although a number ofpublications on the subject exist since then([2],[3] and [4]), not a lot of progresshas been made as the use of X-ray radiography dominated. Due to the progressof proton therapy, the speci�c needs of the treatment in terms of imaging and thedevelopment of proton gantries (Fig. 2.3), proton radiography is being studiedand is of great interest.

Using the beam of proton therapy as an additional tool for radiography andCT would o�er a great advantage in an e�ective treatment against di�cult caseswhere the tumor is located close to radiation sensitive organs like regions closeto the head or the neck[17]. This is now recognized as the major motivation fordeveloping proton CT and also the major clinical application of proton beamsfor imaging. When using X-ray CT, the Houns�eld numbers (the unit systemof measuring attenuation coee�cients of tissues in CT) are converted to protonstopping power relative to water using a calibration curve. This translation canlead to errors in the calculation of the proton range in patients in the order of afew mm for a depth of 100 mm. Proton CT can provide more accurate informationabout the proton energy loss and the straggling of protons through the di�erenttissues.

9


A proton CT system containing a proton gantry and fast image reconstructiontechniques has not yet been developed. The main challenges lie in the develop-ment of techniques for image reconstruction and of a detector system that canmeasure all the information needed for the imaging.

2.2.2 Image reconstruction

In proton therapy the aim is to stop the protons exactly in the tumor. Inproton CT we are interested in irradiating the patient with a proton beam andmeasure precisely the protons before entering and after exiting the body. Lookingback at the physics of the proton interaction with matter, although in therapy wewant to match the Bragg peak region with the location of tumor, in radiographywe have to stay well above that region in terms of the incoming proton energy.Thus, in order to avoid stopping the protons in the patient, a beam of higherenergy than the one used for therapy is required.

Extracting information from the proton tracks for imaging purposes is doneby: (a) measuring the linear energy loss through the material and (b) detectingthe lateral and angular displacements of the proton from its incident position anddirection. These constitute the basis for proton radiography. Given the resolutionof the existent detectors with which we can detect the energy and the stragglingof the protons through matter, there is a limit in using higher relativistic protons.The reason for this is that the divergences in terms of energy loss and stragglingthrough the material, the key elements that will provide the information we need,will become too small to detect. The optimum can only be determined by detailedsimulations including the detector characteristics. The energy regime for protonCT, as de�ned by the characteristics of the accelerators that are based on therequirements for therapy, is shown in Fig. 2.1(a).

A detailed description of the principles of proton CT can be found in [17]. Toperform the above measurements and to retrieve all the necessary data leads tothe design of the proton CT scanner or 3D particle tracker which is the subjectof this project.

10

Chapter 3

Proton CT set-up

3.1 Introduction

Over the years Nikhef has been involved in many projects in high energy physicsin which solid state and gaseous detectors have been used in tracking particles.A new type of gaseous detector, a GridPix based TPC is being developed andwas selected for this project. This detector is an ideal candidate for monitoringa proton beam o�ering some major advantages. Since it is a gas detector it isradiation hard and has low interference with the high ionizing protons due to itslow thickness1. The main advantage is that it o�ers the possibility to obtain 3Dreconstruction of tracks of passing charged particles.

The principle of a proton CT set-up is shown in Fig. 3.1. It's function is basedon tracking single-proton events, stopping them to measure their energy andreconstructing their trajectory. The energy measurement can be done by using acalorimeter while the intersecting points, de�ned A and B in Fig. 3.1(b), are usedto estimate a straight path (black line). That is not however the trajectory theproton has followed. By using the angle (θ1) de�ned by the exiting proton track(red line), additional constraints on the reconstruction of the most likely path theproton has followed (blue line) can be put leading to a higher spatial resolution.Moreover, since the uncertainty in the length becomes smaller, a more accurateestimation of the average stopping power along the trajectory can be made.

Based on the principle shown in Fig. 3.1, a 3D particle tracker using thedetectors available at Nikhef was designed. The components used, both hardwareand software, as well as the di�erent parameters that are important for operatingit are described in the following sections.

3.2 GridPix based TPC

GridPix is a gas-�lled detector in which a Micro Pattern Gas Detector iscombined with a CMOS pixel chip. It follows the principles of a TPC (Time

1Very important especially for tracking the protons that have traversed the body of the patient.

11

Chapter 3 Proton CT set-up

(a) Schematic of a 3D particle tracker. Theexit energy is stored.

(b) Path reconstruction of single-proton event.

Figure 3.1: 3D particle tracker and principle of use.

Projection Chamber2) with the readout being done by a Timepix chip[19]. TheGridPix detector used in this project is shown in Fig. 3.2(c). It consists of aTimepix chip (section 3.2.1) with a layer of 1 µm thick aluminum with holesetched in it called Ingrid (section 3.2.3) attached on a NEXT board with a �eldcage surrounding it (section 3.2.2) as presented in Fig. 3.2(a) and (b). The boardis connected with the readout via a SCSI cable.

3.2.1 The Timepix chip

The Timepix chip has 256 x 256 square pixels with a pitch of 55 µm. Eachindividual element of the pixel matrix is connected to a preampli�er, discriminatorand digital counter integrated on the readout chip[21]. Time is measured by areference counting clock, which is generated by an external clock that can be setto various frequencies up to 100 MHz. The Timepix can be set in three operationmodes:

1. Time over Threshold: measures the time that the signal in the pixel isabove a certain threshold energy. This is an indirect measurement of theamount of charge deposited in the pixel.

2. Time of Arrival: the clock counter is incremented from the moment thesignal goes over threshold until the shutter is closed or until it reaches 11810counts.

3. Medipix: single photon counting mode, which increments a 14-bit counterby one for each hit above a set threshold.

2Information about the principles of a TPC can be found in Appendix A

12


(a) Anatomy of a GriPix. The Timepixchip lies at the bottom of the �eld cagewith the Ingrid on top.

(b) The GridPix used in thisproject. A blob of SU-8 is vis-ible.

(c) The GriPix mounted on a NEXT board with a �eld cage on top.

Figure 3.2: The GridPix detector.

13


(a) A charged particle ionizes the gasand the free electrons drift towardsthe anode.

(b) Avalanche evolution in the ampli�-cation gap.

(c) A charged particle track through GridPix.

Figure 3.3: Principles of a TPC.

14


3.2.2 Field Cage

A �eld cage is placed on top of the NEXT board in order to contain the gasin the system and provide a homogeneous electric �eld. The walls are made ofKapton foil 50 µm thick surrounded by 17 µm thick Copper strips that serve aselectrodes to shape the drift �eld vertically3. The dimensions of the �eld cage�t the Timepix chip (14 mm x 14 mm x 16 mm). The top is sealed by a coppercathode a few mm thick while at the bottom lies the Timepix chip with the Ingridon top (Fig. 3.2(a)).

3.2.3 Ingrid

Since the charge produced in the individual ionization is too small to bedetected by the bare Timepix chip, it has been extended with an integratedgrid to create a charge avalanche with su�cient charge to be detectable. Thisintegrated grid (called Ingrid) is a layer of 1 µm thick aluminum with holes etchedin it (bottom right corner of Fig. 3.2(a)). The Ingrid is attached to the chip by50 µm tall spacers. The individual drift electrons from the ionizations in Fig.3.3(a) are focused into the Ingrid-holes. Between the Ingrid and the chip theelectric �eld of the order of 104 V/cm is large enough to create avalanches. ASi3N4 protection layer is placed over the pixels to avoid any damage from possibledischarges that may occur. These avalanches are detected by the pixels of thechip (blue area at the bottom). In Fig. 3.3(b) the process is simulated.

3.2.4 3D Track Visualization

The 3D positions of single electrons from ionization by charged particlespassing through the detector volume are measured in a drift region with a depthof 16 mm. The pixel hit gives us the (x,y) coordinates of the electron. Thetime that it takes for electrons to arrive at the Ingrid is measured by an internalcounter in the Timepix chip started by an external trigger. Such a trigger isdescribed in section 3.4. Knowing the frequency of the clock, the electric �eldapplied and the type of gas mixture in the TPC the drift time is converted intodistance representing the z-coordinate of the electron generated in the ionizationprocess[22]. By gathering information about all pixels hit and their drift time, atrack can be visualized (Fig. 3.3(c)).

In this project only one GridPix detector was used for proton tracking. Inorder to succesfully reconstruct the trajectory of a particle, at least one additionaltracking detector is required. Thus, it was not possible to reconstruct particletracks in the analysis but only to visualize them. For a detailed method of usingtracking detectors in a telescope to �t charged particle tracks one could look at[23].

3More on the electric �eld con�guration near the electrodes in Appendix A.

15


3.2.5 Gas Mixtures

The gas �lling the �eld cage is a mixture of two gases, one with large primaryionization statistics (called "counting" gas) and the other an organic moleculewith large photoabsorption coe�cients up to large wavelengths (called "quencher"gas) in order to prevent secondary avalanches due to photons emitted in theionization of the counting gas. The key factors for acquiring clear and well de�nedtracks with GridPix are a su�cient number of ionizations in the TPC and lowdi�usion of the free electrons produced from ionization on their way to the grid.The parameters that a�ect these two mechanisms are the gas mixture and theelectric �eld applied on the cathode and on the grid.

Let Dc be the di�usion constant, ud the drift velocity, t the drift time, Tthe temperature, k the Boltzmann constant and µe the electron mobility. FromEinstein's formula[32], the mean deviation in any direction i is given by:

σi =√

2Dct (3.1)

with

Dc =kT

eµe (3.2)

and

ud = µeE =eτ

mE =

L

t(3.3)

where e the electron charge, m the electron mass, τ the mean free time betweencollisions, E the electric �eld and L the drift distance. By solving (3.3) for t andapplying it in (3.1) space di�usion RMS becomes:

σi =

√2Dc

L

ud(3.4)

In order to achieve the minimum di�usion, a gas mixture with high driftvelocity is required. The distance L is also important as the higher an electronstarts drifting, the more it will deviate from drifting straigth to the Grid.

When the di�using body has thermal energy ε = (3/2)kT , Dc has the form:

Dc =2ε

3eµe (3.5)

By rewritting (3.3) as t = L/µeE and subsituting t and (3.5) in (3.1) we get:

σi =

√4εL

3eE(3.6)

The mean deviation in any direction can be written:

16


σi = D√L (3.7)

where D is the di�usion coe�cient for that direction:

D =

√2Dc

µE(3.8)

To detect a particle, the gas mixture chosen is such such that multiple scat-tering is minimized. For low mass gas mixtures it is known that ethane(C2H6),Isobutane(iC4H10) and dimethyl ether (C2H6O) are good quenchers in combina-tion with helium (He), which help increasing the number of primary and total ionpairs per cm for a given density[33]. Two di�erent gas mixtures were tested, (He90%, iC4H10 10%) and (DME 50%, CO2 50%). The reason for this selection wasthe expected di�erence in the behavior of these two gases. The values for D andud (Fig. 3.4 and 3.5) as well as the expected number of ionizations (Fig. 3.6) werecalculated in GARFIELD[23], a computer program for the detailed simulation of2D and 3D drift chambers and gas detectors. In Fig. 3.4 and 3.5, the diagramsof the drift velocity and di�usion(longitudinal and transversal) as a function ofthe electric �eld applied show the di�erences between the two gas mixtures.

The properties for a good gas mixture for the TPC are high drift velocity andsmall di�usion properties according to (3.4) and (3.7). Di�usion in DME/CO2

is an order of magnitude smaller than He/iC4H10. Also, the drift velocity ofDME/CO2 is an order of magnitude larger. The reason why drift velocity di�ersbetween the two gas mixtures is due to the fact that the mean free time τ betweencollisions depends on the density of the electrons in the gas[24] and as shown in(3.3), ud is proportional to τ . DME/CO2 appears to be appropriate for our mea-surements. If we choose our electric �eld to be around 2,000 V/cm, the di�usion isat its lowest value(∼ 65 µm/

√cm) and the drift velocity ∼ 1 cm/µsec. So, for an

electron drifting from the middle and the highest point of the Field Cage4, the de-viation due to di�usion according to (3.7) will be σL/2 = 65

√L/2 [µm] = 58 µm

and σL = 82 µm respectively. This agrees well with the experimental resultsobtained for the same drift heights with σL/2 = 56 µm and σL = 84 µm.

For proton radiography, the YZ-plane of the tracking detector should be largeenough to cover a typical �eld of the proton beam which is about 10 cm x 10cm. The depth of the detector (X-coordinate) should be determined by analysingwhat trajectory length you need to get su�cient accuracy on the angles of theproton path with the Y and Z axis. Typically one would like to have about 1mrad, so 0.1 mm per 100 mm[25]. The YZ-plane of the GridPix based TPC usedin this project is 1.4 cm x 1.6 cm while the depth is 1.4 cm.

There are various factors that play a role in constructing larger GridPixdetectors. The X and Y-coordinates are limited by the dimensions of the Timepixchip (1.4 cm x 1.4 cm). It is possible to "tile" together Timepix chips with

4the height of the Field Cage in the GridPix based TPC used in this project was L=1.6 cm

17


(a)

(b)

Figure 3.4: Drif velocity of (a) He 90%, iC4H10 10% and (b) DME 50%, CO2 50% for

T=300 K and p=1 atm.

18


(a)

(b)

Figure 3.5: Di�usion coe�cients of (a) He 90%, iC4H10 10% and (b) DME 50%, CO2

50% for T=300 K and p=1 atm. The green line indicates the transversal and the orange

the longitudinal di�usion coe�cient.

19


(a) (b)

Figure 3.6: Distributions of number of electrons produced from protons through DME

50%, CO2 50% for T=300 K and p=1 atm. The plots are made in GARFIELD.

Ingrids on top in 2 x N con�gurations. A detector with 2 x 4 GridPix chipshas been constructed in University of Bonn[26]. For making a higher drift gap(Z-coordinate), a higher voltage between the cathode and the grid is required inorder to sustain the electric �eld at 2000 V/cm for DME/CO2. GridPix basedTPCs with a drift gap of even 10 cm have been constructed at Nikhef.

The electrons generated in the ionization process by low energy protonstransversing the DME/CO2 mixture follow the distributions of Fig. 3.6. Theenergies of 190 and 65 MeV are plotted with the �rst corresponding to the high-est energy of the AGOR cyclotron and the second to the remaining energy after20 cm of water, the maximum distance a proton would travel traversing a com-mon human body5. As the energy of the incoming protons is decreasing, moreionizations are expected in the TPC.

3.2.6 Hit Resolution

The error for a single hit in a coordinate i is of the form ∆i2 = σ2pitch + σ2

drift

where σdrift is (3.7). On the x, y and the reconstructed z coordinates, the errorsare given by (3.9) - (3.11):

∆x2 =d2pitch

12+D2

T z (3.9)

5For patients with larger body mass a higher energetic beam is required.

20


Figure 3.7: The time walk e�ect.

∆y2 =d2pitch

12+D2

Lz (3.10)

∆z2 =(tbinudrift)

2

12+D2

Lz + ∆t2timewalk (3.11)

with dpitch=5.5×10−2 mm being the pixelpitch, tbin=10 ns the clockcycle6, udrift

the drift velocity. The values for the longitudinal DL and transversal DT di�usionare extracted from Fig. 3.5. The denominator equal to 12 appears from therelation between the strip pitch and the detector resolution for binary read-out(Appendix B). ∆ttimewalk is an additional error in the z coordinate due to timewalk, one of the e�ects that worsens ToT and ToA measurements.

The rise of a weak signal has a smaller slope than the rise of a strong signal asshown in Fig. 3.7. Thus, the time it takes for a signal to cross the threshold (greenhorizontal line) varies depending on the charge cloud that produced the signal.This e�ect is called timewalk. Timewalk can be prevented in three ways: a) byraising the grid voltage, so that the ampli�ed signal produced in the avalancheis strong enough and will have a fast rise time, b) by lowering the threshold,minimizing the pulseheight gap so weak signals will cross the threshold sooner orc) by measuring the time over threshold which gives the strength of the signal (thestronger the signal the less timewalk). However, each solution has a drawback.Raising the grid voltage might lead to another unwanted e�ect called crosstalk,where the ampli�ed signal is so strong that hits adjacent pixels making it hardto connect the hit to a single pixel. Also, lowering the threshold increases thepossibility of hits due to noise to appear along with the actual hits producedfrom ionization. The appearance of noisy pixels is unwanted cause it will a�ectthe reconstruction of a track when a �t is applied. Lastly, the measurement oftime over threshold cannot be done simultaneously when the the time of arrivalis measured. This limitation can be overrun with the forthcoming version ofTimepix chip that allows simultaneous measurement of time over threshold andtime of arrival.

6The clockcycle depends on the frequency the clock is set. In this case the clock was set at itsmaximum frequency (100 MHz) thus each clockcycle is 10 ns.

21


(a) The RelaxD board.

(b) The Pixelman UI.

Figure 3.8: Harware and software components for the GridPix Read-out.

22


Figure 3.9: Measurement of the ToA. a) The shutter (set by the trigger) is de�ned

between the left dashed line and the right dashed line, b) an analog signal crosses the

threshold and c) the clock of the Timepix starts counting until the end of the shutter.

By substracting the clock counts from the shutter time we acquire the Time of Arrival

(red ellipse).

3.2.7 Read-out

GridPix is read out by the RelaxD board which is connected to a PC via a 1Gb/s Ethernet connection. There is the possibility to connect 1, 2 or 4 Timepixchips to the same board and operate them separately. The current maximumrate of reading data from the RelaxD board is 120 Hz per chip. Increasing thereading speed of the Relaxd is not possible with the current Timepix chip. This isbecause the time needed to read out the chip is about ∼8ms, a dead-time duringwhich the detector cannot collect data. So, reading out more than 120 framesper second events will result in a number of events being lost (not measured).

The data acquisition, calibration/equalization and triggering of GridPix wasmade by the Pixelman software[27]. The user has various options for these taskslike:

• di�erent acquisition modes(individual frames, integral of all frames etc.)

• selecting the acquisition parameters(counts, time)

• select the chip mode (Medipix, ToT, ToA)

• set the triggering (start/stop by software, start/stop by external or a com-bination of them)

By setting the triggering at start/stop by hardware and applying a triggeron the RelaxD board (details of the trigger in the Appendix), tracks throughthe GridPix can be recorded. Each track is a collection of hits and each hit isdescribed by three numbers X,Y and C. X stands for the row of the pixel hit, Yfor the column of the pixel hit and C is the number of counts of the pixel. The

23


Figure 3.10: The BaF2 detector.

procedure of converting the clock counts to time of arrival is explained in Fig.3.9.

3.3 BaF2 scintillating detector

For measuring the energy of the protons a calorimeter is needed. In ourcase we used a scintillating detector consisting of a BaF2-crystal. The crystal iswrapped with Te�on and an additional layer of aluminum foil as UV-re�ectors andcoupled optically to the quartz window of the photomultiplier tube (HamamatsuR2059-01). Since no documentation or manual of the speci�c detector was found,information from [28] was used as suggested by [34].

By shaping the signal of the photomultiplier tube (PMT) in two di�erentways, a fast and a slow signal are generated. The calorimeter has 3 outputs,giving 1) a fast scintillation component (600 ps) used for triggering the GridPix,2) a slow scintillation component (630 ns) used for measuring the energy and 3)the signal of the last dynode which was not used. The contribution of the fastscintillation component (λ = 220 nm) to the total light output (dominated by theslow scintillation component at λ ∼ 315 nm) diminishes with the increase of theenergy density deposited by the ionizing particles(Fig. 3.11(a) left). The ratio ofboth contributions remains constant over the full dynamic range up to relativisticand even ultra-relativistic energies[28]. In principle, both signals should be usedfor determining the deposited energy of a particle but since the slow componentdominates, only the slow component signal was recorded and used for estimatingthe energy. A typical output of this signal is shown in Fig. 3.12.

The purpose of this scintillating detector is to measure the energy of the scat-tered/outgoing protons. The particles are stopped in the detector and the energythey deposit is converted into scintillation light. The light emitted is captured bythe PMT which produces the three signals mentioned. The scintillating detector

24

3.4 Trigger

(a) Typical signal shape of BaF2 for pho-tons and charged particles(left), Particleidenti�cation based on the correlation ofthe fast and total light output(right) from[28].

(b) Scintillation intensity of the 315 nm (slowcomponent) emission peak as a function of tem-perature from [30].

Figure 3.11: Characteristics of the BaF2 crystal.

must be sensitive enough to distinguish di�erences in the energy of the incidentprotons in order to determine successfully whether or how they interacted. TheBaF2 is not the most appropriate candidate but was the only available scintillat-ing detector. A better detector in terms of resolution would be a LaBr3 crystalthat gives about 60,000 photons of 390 nm[29]. The BaF2 scintillating detectorwill also be referred in this project as calorimeter.

An important e�ect that may a�ect our measurements is the temperaturedependence of the scintillation intensity. The slow 315 nm peak has a temperaturedependence of -1.1% per ◦C as reported in [30] while the fast 220 nm peak does notchange signi�cantly. As shown in Fig. 3.11(b), compared to the room temperatureintensity a factor of 3 in light yield can be gained by cooling down the crystal.Since determining the energy of the protons is based on the measurement of theslow component, this e�ect may smear our energy resolution. The BaF2 detectorshould be temperature stabilized in order to avoid this e�ect to appear.

3.4 Trigger

The GridPix and BaF2 detector were used in order to visualize a chargedparticle track and measure its energy. To do so, the data of the two di�erentdetectors should be synchronized to successfully reconstruct the events. A smallscintillator 1.5 cm x 1.4 cm (not covering completely the surface de�ned by theside of the GridPix TPC) attached to a Hamamatsu H5783 PMT was combinedwith the signal of the fast output of the calorimeter to form the trigger. Theorder in which the components were placed is shown in Fig. 3.13(a). A trackthat passes through the scintillator and deposits energy in the calorimeter willhave inevitably passed through the TPC.

The scintillator signal and the calorimeter fast output are combined to forma coincidence. By this coincidence two time windows are opened(Fig. 3.13(b)).

25


Figure 3.12: Measured signal with cosmic rays recorded by an Agilent oscilloscope.

The �rst window, the shutter, is triggering the RelaxD board, which activatesthe GridPix. The second window, the VETO, is keeping the coincidence inactiveso no other events trigger the RelaxD while it is busy. However, during thetime the shutter is open more than one charged particles may pass through theset-up. This results in multiple tracks being reconstructed with some of themappearing to be outside the detector volume in the Z-coordinate. To minimizethe appearance of such tracks, the length of the shutter should be equal to thelargest drift time. The length of the VETO should be equal to the time thatthe RelaxD and the DAQ PC are busy processing the data in order to increasethe count rate of the particles recorded. The logic modules used with all theparameters set can be found in Appendix C.

The set-up needed to be tested with charged particles with enough energyto traverse the scintillators and the plastic cap of the BaF2 detector and depositenergy in the calorimeter. In order to do so, the set-up of Fig. 3.13 was rotatedfacing towards the sky to detect cosmic rays. Cosmic rays are minimum ionizingparticles meaning that their deposited energy in a material is at the minimumin the Bethe-Bloch curve. Therefore, the detection of such particles would provethat the 3D particle tracker set-up could well be used in detecting the low energyand more ionizing protons planned for a proton CT. Over a period of operationin the lab, MIPs were detected and recorded successfully7 showing us that we are

73D visualizations of cosmic rays are presented in Appendix D.

26

3.4 Trigger

(a) Placement of the GridPix TPC, scintillator and calorimeter.Components and distances among them not in scale.

(b) Triggering on coincidences. Coincidences arriving late as well as ones rejectedby the VETO are also displayed.

Figure 3.13: Triggering on the coincidence of a scintillator and the calorimeter.

27


ready to move to the next stage of the project, testing the set-up with a protonbeam.

28

Chapter 4

Experiment at the KVI

The KVI institute in Groningen is an international institute for atomic andsubatomic physics, focusing on nuclear, theoretical and accelerator research. TheKVI is collaborating with Nikhef in the research for a proton CT. The protonCT scanner of Chapter 3 was prepared and tested with cosmic rays at Nikhef inAmsterdam. It was transfered to Groningen where an experiment with protonsfrom the KVI cyclotron was planned. The experiment at KVI is presented herecontaining the set-up, the calibration of the calorimeter with the proton beam, theruns taken at di�erent energies and di�erent intensities and the samples tested.

4.1 AGOR cyclotron

The AGOR cyclotron (Accelerateur Groningen-ORsay) in KVI/Groningendepicted in Fig. 4.1(a) produces proton beams used for irradiations with primaryenergies of 90, 150 and 190 MeV. The �ux can be from a few particles up to 1014

particles s−1 cm−2. The intensity is up to ∼1013 particles s−1 but mostly around1012 particles s−1. Particles are delivered in bunches with the number of particlesper bunch following the Poisson distribution. The properties of the beam arepresented in Table 4.1. The initial narrow particle beam from the accelerator isbroadened and �attened with scatter foils, while collimators to stop protons thathave been scattered over too large angles can also be placed. The initial beamenergy can be degraded to lower energies by a compact ensemble of 9 aluminumplates of various thicknesses.

Table 4.1: Properties of the KVI cyclotron beam.

Maximum Energy σEnergy Frequency σx,y σbeam190 MeV 300 KeV 55 MHz 2 mm 1.5 mrad

29

Chapter 4 Experiment at the KVI

(a) The cyclotron. (b) Beam line on breadboard tables.

Figure 4.1: The AGOR facility for irradiations of materials.

4.2 Experimental set-up

The 3D particle tracker described in Chapter 3 was placed in the irradiationarea of AGORFIRM (Fig. 4.3). Samples and beam line components were alignedby using a 3D laser positioning system. An XY table (Fig. 4.1(b)) contains alarge mounting rack that allows movement of samples through the beam with arange of 600 mm in the horizontal and 300 mm in the vertical direction with arelative accuracy of 0.01 mm. Special predrilled plates for sample mounting areused that can be quickly �tted to the mounting rack.

The proton primary energy used was 150 MeV (not the maximum energy thatcan be achieved). The intensity was monitored by converting the signal of thefast output of the calorimeter into a digital one with a discriminator and keepingtrack of the number of particles with a counter. After the particles leave thevacuum of the beam pipe through the exit foil, the following components wereplaced: a single scattering foil (1.44 mm Pb) to produce a homogeneous �eld witha diameter of about 3 cm, a degrader (series of aluminum plates) to change theenergy and a 2 x 2 cm collimator limiting the beam divergence to < 3 mrad. TheGridPix based TPC was placed about 3 m downstream from the scatter system,the BaF2 detector was mounted downstream of the TPC and between them the

Figure 4.2: Side view of all the components of the experimental set-up.

30

4.3 Monitoring the Set-Up

Figure 4.3: The 3D particle tracker.

small scintillator used as part of the trigger (Fig. 4.2). The beam is travelingon the X-axis traversing the YZ-plane of the TPC as drawn in Fig. 4.3. Theintersection point of the lasers used for the alignment mark the (0,0,0) of ourreference system located at the center of the GridPix.

4.3 Monitoring the Set-Up

The PC for the data acquisition for the GridPix containing the Pixelmansoftware and the Agilent oscilloscope recording the signals from the BaF2 detectorwere remotely controlled in the counting room. The data from GridPix werestored on the DAQ PC while the signals from the calorimeter were stored in theAgilent oscilloscope, with a limit in the maximum number of waveforms thatcould be stored in the segmented memory of the oscilloscope.

The number of triggers of the GridPix were compared to the number ofrecorded signals in the oscilloscope. For low trigger rates, tested in the lab withcosmic rays and radioactive sources, these two numbers coincided indicating syn-chronization between the two detectors. The set-up had not been irradiated withhigh rates like the ones scheduled for the experiment thus synchronization atthose rates was not tested.

4.4 Calibration of the BaF2 detector

The calibration of the BaF2 detector was done by placing it directly into theproton beam, changing the beam energy and recording all the signals of the slowoutput. The energies, the corresponding output voltages and the thicknesses of

31


Table 4.2: Energy and Voltage values.

Energy [MeV] Al thickness [mm] Voltage [mV] σV [mV]143.6 - 1308 18.6

118.3 20 1181 17.6

88.3 40 975 20.7

54.8 57 611 43.8

Noise - 82 5.8

the aluminum plates are presented in Table 4.2 with the last entry being the noise.The primary energy is not 150 MeV but 143.6 MeV due to energy loss when thebeam passes the scatter foil and travels through the air. Note also the values ofσV as the voltage is decreasing. Since the light output (number of photons) isproportional to the energy, then we expect σV ∝

√V . The reason why σV is not

following this rule is due to the method the primary beam is degraded to lowerenergies. The energy spread of the particles is becoming larger with lower energydue to straggling in the Al degraders. Thus, the observed width is a convolutionof the intrinsic resolution and the energy spread[25].

4.5 Synchronizing GridPix and the BaF2 detector

Synchronization between the GridPix and the BaF2 was lost in the initial runsdue to the ine�ciency of the trigger to keep up with the intensity of the beam.Synchronization is important for our experiment to correlate the data betweenthe the two detectors in order to match a track in the GridPix with the energyrecorded in the calorimeter.

After lowering the intensity the two detectors and by reducing the numberof events per run, the data from the two detectors were �nally synchronized.However, the cost is that the amount of data taken in those runs is limited.

4.6 Collected Data Sets

A number of data sets was collected with the beam at maximum energy (144MeV), the beam at low energy (55 MeV) and with two di�erent kind of samplesplaced between the collimator and the GridPix (Fig. 4.4 and 4.5). The beamintensity was 500 - 7,000 protons per second. The samples used were a number ofcopper plates and two copper wedges of di�erent size. The plates were 2.80 mmthick each and the wedges had a base of 30 mm x 30 mm while the maximumheight of the small one was 12 mm and of the big one 24 mm.

32

4.6 Collected Data Sets

(a) Set-up of Fig. 4.3 with the Cu plates. (b) Beam view of the set-up.

Figure 4.4: The set-up with the Copper plates.

(a) The small wedge used in the experiment. (b) Set-up of Fig. 4.3 with a wedge.

Figure 4.5: The set-up with the wedge.

33


Table 4.3: Parameters and properties of the Runs.

Runs

Plates Wedges

Con�gurations Initial Degrader 1 4 Small Big

Full Energy√ √ √ √ √

Intensity [protons×kHz] 7 0.7 1 0.5 0.5 0.5No samples

√ √

Sample Full YZ cover√ √

Sample Half YZ cover√ √

Num. of GriPix hits 267,455 260,296 68,882 66,419 322,558 148,112Num. of GriPix events 3,911 2,018 1,024 1,024 4,864 2,048Num. of Calo events 3,026 1,056 1,026 1,025 4,864 2,048Synchronization

√ √

4.7 Overview

The Runs and the di�erent con�gurations are summarized in Table 4.3.Around 15,000 tracks were recorded during the runs that lasted for a day. Sinceone of the main goals of the experiment was to investigate the correlation betweenthe GridPix and the calorimeter data, synchronization between the two detectorswas the key part. To achieve that however, the intensity of the beam had to below which means less events per unit time.

34

Chapter 5

Results & Analysis

The data collected from the experiment at KVI are presented in this chapter.First the data collected with the GridPix will be discussed and then the data fromthe BaF2 detector. Finally, we will combine the data of the two detectors.

5.1 GridPix Data

5.1.1 Projection on the XY plane

To start with the analysis of the data collected at KVI, we looked at theXY plane information. The XY plane is the pixel matrix of the GridPix. Wecompared the XY data obtained from the proton irradiation with data collectedfrom radioactive sources and cosmic rays at Nikhef. Looking only at the XYvalues of the GridPix data, an integral plot of the pixels hit is shown in Fig.5.1(a). In Fig. 5.1(b) the same plot is reproduced by irradiating GridPix with aSr-90 source in the lab.

In both pictures we notice a number of artifacts with most obvious a dead areain the top right quadrant. The grid of a GridPix detector is supported by pillarsof SU-8. In the production process, the SU-8 between the pillars is removed afterthe grid is made. In the case of our detector, not all the SU-8 was removedsuccessfully. As a result electrons drifting to the grid over the area covered withSU-8 will not proceed into the multiplication gap. Therefore no signal will beinduced on the pixels making this part of the detector inactive. In addition tothe artifact due to the SU-8 there is an inactive column in the right side as wellas some inactive areas on all four sides of the pixel matrix. This is due to thefact that when the �eld cage was placed and glued on the GridPix, a portion ofthe glue covered parts of the grid. Electrons close to the walls of the Field Cagedrifting towards the Grid will stop in the glue and no pixels will appear to be hit.The white areas in Fig. 5.1(a) and (b) represent inactive pixels.

Looking at the top of the XY planes of Fig. 5.1(a) and (b), we notice adi�erence in the hit density above Y = 200. Many pixels in Fig. 5.1(a) above Y= 200 have zero or a small number of hits recorded in contrast to Fig. 5.1(b),where this part of the detector appears to behave similar to the rest of the active

35

Chapter 5 Results & Analysis

(a) Irradiation with protons. (b) Irradiation with Sr-90 source.

Figure 5.1: Integral plots of pixels hit. In white are all the inactive pixels.

(a) Histogram of the Hits per pixel. (b) Mapping of "good" pixels. Only 41,818pixels (in red) out of 65,536 are consideredreliable for our measurements.

Figure 5.2: Discrimination of e�ective pixels through the hit spectrum.

area. We conclude that the scintillator was not covering the whole XY plane butonly until Y = 200, thus any hits above this line are caused by scattered protonsor delta-rays.

By integrating over all pixel hits, the hit spectrum of the initial runs is shownin Fig. 5.2(a). The peak at zero indicates that around 20,000 pixels (almost1/3 of the total pixels) remain silent appearing not having been hit. Most of therest of the pixels appear to have been hit around 6 times during the experiment.From Fig. 5.2(a) a low cut value at 2 (for silent pixels) and a high cut value at20 (in case of noisy pixels) from the pixel hits can be extracted. Applying thesecuts to the pixels of Fig. 5.1(a), a map of the "good" pixels, meaning the pixelsinside the cut limits that should be considered as e�ective ones, is drawn in Fig.5.2(b). The number of "good" pixels of this chip (around 64%) agrees with otherGridPix detectors[23] which varies from 60-75%. In our experiment, 41,818 pixels

36

5.1 GridPix Data

Figure 5.3: 3D view of the samples placed before GridPix.

(marked with red in Fig. 5.2(b)) of the total 65,536 can be considered e�ectivewith 23,718 silent (marked with white in Fig. 5.2(b)) and 0 noisy pixels. Nonoisy hits appeared since the noisy pixels were masked before the experiment.Comparing to the number of pixels hit from the electrons of the Sr-90, thereappear to be 17% more pixels hit in the case of the irradiation with the source.This is reasonable since the integral plot of Fig. 5.1(b) was made by pointingthe radioactive source perpendicular to the pixel matrix for about 5 minutesin contrast to the proton irradiation where a part of the pixel martix was notactive (due to the fact that the trigger scintillator was not covering the entiredetector).

5.1.2 Y-coordinate distribution of all hits

Since the samples were placed perpendicular to the beam as shown in Fig.5.3, the distribution of the Y-coordinate of the pixels hit can provide us withsome interesting information. By using the 3D Display1, proton tracks can bevisualized in 3D (Fig. 5.4(a)). The proton beam is perpendicular to the YZ-planeso the histogram of the Y-coordinate of a single track will appear as a peak (Fig.5.4(b)). Fitting the distribution with a gaussian, we can have an idea for thedi�usion. The σ from the �t is 1.2 pixels which corresponds to 66 µm while,from section 3.2.5, the expected σ0.75cm = 56 µm. The 15% divergence from theexpected value of σ is due to the fact that the electric �eld in the �eld cage is nothomogeneous so electrons drifting towards the grid will not di�use uniformly. Inthe runs with no samples, the hits are mainly distributed between pixels in the

1The 3D Display and the Histogram Algorithm used to present the tracks of GridPix wereimplemented and kindly shared by Wilco Koppert. Additions to the Histogram Algorithm

were made by the author.

37


(a) Track visualized with the 3D Display.

(b) Y distribution of the hits forming the track.

Figure 5.4: A proton track and the Y distribution of the hits forming it.

38

5.1 GridPix Data

rows 30-200 (Fig. 5.5(a) and (b)). The peaks and valleys correspond to the e�ectof adding a small number of individual tracks. This means that the summingof the individual tracks causes the appearance of peaks in Fig. 5.5 while theirabsence appears as valleys.

In the case of the copper plates covering half of the YZ surface (Fig. 5.5(c) and(d)), the part of the detector not blocked by the sample (pixel Y: 0-125 roughly)should be similar to Fig. 5.5(a). Although the events in the runs with the copperplates are less, the shape of both histograms appear to be the same. A sharpdrop in the number of events around pixel row 100 with 1 plate and pixel row 70with the 4 plates indicates that less hits are recorded in GridPix from the sidecovered by the plates. As protons traverse the plates, the copper reduces theirenergy and causes signi�cant scattering. This results in less tracks traversing theplates to be detected since these protons will not have traversed the triggeringscintillator. This e�ect is more clear in the case of the 4 plates as the drop issharper than the case with 1 plate. Although there appear to be less hits wecannot draw conclusions from this plot about the proton tracks. The assumptionthat a number of protons is scattered due to the copper plates could be easilyveri�ed by placing a second GridPix based TPC before the samples. By doingthis, one could compare the tracks recorded before the sample and the tracksrecorded after the sample and know precisely the �ux, the ratio of the scatteredprotons over the total number of protons and the e�ect of the di�erent thicknessof the samples on this ratio.

For the runs with the wedges (Fig. 5.5(e) and (f)), the same e�ect due toscattering described in the case of the copper plates is observed. The wedgeswere placed with the base parallel to the (X=0, Y=0) - (X=0, Y=256) of theGridPix as shown back in Fig. 5.3, with the tip of the wedge starting at (0, 0)and the thickness increasing towards (0, 256). The distributions have no majordi�erences except the region after pixel row 190 where the distribution of the bigwedge appears to drop with a constant rate.

5.1.3 Time Measurement & Z reconstruction

The main advantage of the GridPix based TPC, the reconstruction of the zcoordinate, was already described in Fig. 3.9. The time of arrival is calculatedby subtracting the clock counts of a single pixel from the shutter time. Knowingthe drift velocity, this time is converted to distance. This process is repeated forall the hits in one event and eventually a track is reconstructed.

By looking at the tracks through the 3D Display, many hits and in somecases even whole tracks appear to be higher than 1.6 cm - the height of the driftvolume of the GridPix. These are what we call "late" electrons. The de�nitionof "late" electrons or tracks is based on the way the time of arrival measurementis being done. The electric �eld applied is 2000 V/cm so the drift velocity, ascalculated from Fig. 3.4(c), is 10 µm/ns. The time an electron needs to driftfrom the highest point of the �eld cage is 1,600 ns. The Timepix clock was set

39


(a) Full beam. (b) With degrader.

(c) With 1 copper plate. (d) With 4 copper plates.

(e) With small wedge. (f) With big wedge.

Figure 5.5: Projections on Y.

40

5.1 GridPix Data

Figure 5.6: Secondary "late" track appearing higher than the �eld cage volume.

to 100 MHz, meaning that one clock count from a pixel is equal to 10 ns. Thus,the maximum drift time or time of arrival in terms of clock counts is 160. Ourshutter was 20 µs so 2000 clock counts. Subtracting the maximum drift timefrom the shutter clock counts, the limit of 1840 clock counts is set. This meansthat if the clock counts of a hit or of a collection of hits that form a track are lessthan 1840, then these hits will appear higher than the Field Cage volume. Anexample of this scenario is displayed in Fig. 5.6, where the upper proton track isaround Z=10 cm while the height of the �eld cage is only 1.6 cm.

Many hits below 1840 counts were recorded. By excluding these late hits,integral scatter plots of the YZ-plane of GridPix can be drawn (Fig. 5.7). Lookingat these plots, we notice:

1. the absence of hits at the edges of the chips (below Y=10 and above Y=235)due to the e�ect of the glue and shown also in Fig. 5.5. This is more obviousin (a), (b) and (d).

2. a small gap in the drift volume from Z=0 mm to Z=1 mm. This means thatno hits occurred at this region, a fact that is highly unlikely though. Thebest explanation would be that there is a signi�cant delay from the triggerwhich results in missing the electrons generated close to the grid.

3. the majority of the hits are above Z=4 mm, marking the shadow of thescintillator. This is more obvious in Fig 5.7 (a), (c) and (d) where a largenumber of events was collected. Since the scintillator was not coveringcompletely the YZ-plane of the �eld cage as mentioned in section 3.4, notall of proton tracks entering GridPix could have been detected. By usingalso the information for the e�ective area in the XY plane obtained fromFig. 5.2(a) and (b), we have a 3D aspect of the active volume of the GridPixbased TPC due to the scintillator.

41


(a) Initial runs at 144 MeV. (b) Runs with the Degrader at 55 MeV.

(c) Runs with the 4 copper plates. (d) Runs with the small wedge.

Figure 5.7: Integral scatter plots of the YZ-plane.

42

5.1 GridPix Data

5.1.4 Energy dependence of Ionization

Comparing the scatter plots of Fig. 5.7(a) and (b), some general conclusionsabout the energy dependence on ionization can be drawn. The expectancy ofmore ionizations as energy decreases was already introduced in section 3.2.5 anddepicted in Fig. 3.6. The GridPix based TPC is designed to detect single electronsionized in the �eld cage and drifting towards the grid. This way we can connecta hit of the Timepix chip with a single electron. Therefore, looking at the hitsrecorded we have an estimation of the ionization. In Fig. 5.8 two tracks ofdi�erent energies are presented. Both tracks traverse the GridPix at the sameheight so di�usion is expected to be the same. The number of electrons is largerin the track with the lower energy however the divergence of the Z coordinateremains the same in both tracks. The number of electrons per cm for eachenergy in Fig. 5.8 is within the limits of the distributions expected like the onesintroduced in Fig. 3.6.

In the initial runs at 144 MeV, 3,911 events were recorded producing 267,455hits. Taking into account the count clock limit of the previous subsection, 203,265of these hits are inside the drift volume of the �eld cage. In the runs with thedegrader, 2,018 events were recorded producing 260,296 total hits with 208,976of these hits inside the drift volume. So, although in the runs with the degraderthe number of events were two time less than the events in the initial runs, thenumber of hits (which corresponds to the number of ionized electrons) is not onlyin the same order of magnitude but almost equal. This is a rough estimate forthe energy dependence of ionization made by looking at the total number of hits.A more accurate method would involve the application of an algorithm lookingat the hits of single tracks and discriminating slow hits and double proton tracks.Then distributions like Fig. 3.6 can be plotted for each energy and comparisonwith the expected number of electrons per cm can be made in more detail. Duringthe writing of this thesis the development of such an algorithm was not made.

5.1.5 Intensity e�ects

The intensity of the proton beam was changed in order to achieve synchro-nization between the GridPix and the calorimeter. Looking at the histograms ofthe pixel clock counts (Fig. 5.9), it shown that the intensity a�ects the numberof late hits. The probability of having a second proton within the 20 µs at 7,000Hz is 3 x 10−4 while at 700 Hz the probaility is 3 x 10−5, a factor of 10 lower. Inboth histograms the red line and arrow indicate the limit of 1840 counts. In Fig.5.9(a) many hits are distributed below 1840 while in Fig. 5.9(b) the late hits aresigni�cantly reduced.

The main cause for this discrimination is the reduction of late proton tracksarriving when the shutter is already open as described in Fig. 3.13(b). Thecyclotron frequency is 55 MHz and protons are delivered in bunches so everysecond there are 55 x 106 bunches[34]. With a beam intensity of 7 kHz, most

43


Figure 5.8: Energy dependence of Ionization.

of these bunches are empty and the protons are distributed in the rest followingPoisson statistics. With a beam intensity of 0.7 kHz, even more bunches areempty so the probability that a second proton traverses the GridPix while theshutter is open is really low. That is why in Fig. 5.9(b) the appearance of latehits is suppressed.

5.2 Calorimeter Data

5.2.1 Calibration

The process of the calibration was described in 4.4. The histogram of Fig.5.10(a) contains the oscilloscope signals of the calibration. The �ve di�erentbeam energies from 44 - 150 MeV can be seen as peaks in the histogram withthe noise creating an extra high peak right after zero. However, only four of thepeaks corresponding to energies were taken into account and �tted as the �fthone (around 450 mV) was considerably smeared. An additional �t was applied tothe noise to have an estimation on the mean value of the noise level. The valuesof the energy and the voltage are plotted and �tted with a 3nd order polynomialcurve (Fig. 5.10(b)) in order to �nd the relation between them. This curve(V (E) = −479 + 27E− 0.15E2 + (3× 10−4E3) will be used to convert the signalsof the BaF2 to energy.

44


(a) Initial runs at 144 MeV and 7 kHz.

(b) Runs with the Degrader at 55 MeV and 0.7 kHz.

Figure 5.9: Pixel clock counts.

45


(a) Histogram of the signals from the di�erent beam energies.

(b) Voltage - Energy correlation.

Figure 5.10: Calibration of the BaF2.

46


Table 5.1: Shifted Voltage values and corresponding Energies from the runs.

Run Energy[MeV] Voltage [mV] Deviation from calibration [%]

Full Energy 144 1,218 6.9Degrader 55 600 1.8Plates 144 1220 6.7Wedges 144 1180 9.8

(a) Largely shifted peak in the initial runs. (b) Partially shifted peak in the runs with thedegrader.

Figure 5.11: Calorimeter data. The dashed line represents the value obtained at the

calibration.

5.2.2 Shifted Energy spectrum during Runs

For the calorimeter data of the initial runs at 144 MeV and the runs withthe degrader at 55 MeV, we expect to see a peak for each energy at the samevoltage as was recorded in the calibration and plotted in Fig. 5.10. By lookingat the histograms of Fig. 5.11 we notice that this is not the case. In the initialruns (Fig. 5.11(a)) there is a large shift of 90 mV to lower energy comparedto the corresponding 144 MeV peak of the calibration. In the runs with thedegrader, the shift is smaller being 15 mV below the corresponding peak of 55MeV of the calibration(Fig. 5.11(b)). One possible explanation could be thetemperature dependence of the scintillation intensity mentioned in 3.3 . Althoughthe temperature in the irradiation area was monitored and stayed constant at18◦C, the temperature of the BaF2 detector was not measured.

The shifted values of the calorimeter output voltage are presented in Table5.1. The shifted peak at 144 MeV is also observed in the two runs with theCopper plates. Since half of the detector was covered with 1 plate in the �rstrun and with 4 plates in the second, we expect two peaks in each histogram - onecommon peak indicating the full energy of the beam and a second peak due tothe presence of the plates. The results in Fig. 5.12(a) verify our prediction. Thecommon peak rises at 1,220 mV, while the secondary peak is located at 1,160 mVin the run with 1 plate and at 860 mV in the run with the 4 plates. In the runswith the wedges, the wedges were covering all of the YZ-plane so the even largershift in Fig. 5.12(c) is due to the energy loss as the beam traverses the tip of the

47


(a) Energy spectrum with the copper plates.

(b) Energy spectrum with the wedges.

Figure 5.12: Calorimeter data of the runs with the samples.

48

5.3 Correlation of GridPix and Calorimeter data

Figure 5.13: Voltage - Energy correlation with shifted voltage values.

wedge.

An attempt to estimate the energy loss due to the copper plates from theoutput voltage of the calorimeter fails. Protons of 144 MeV should deposit 10MeV when traversing 2.80 mm of copper and 43 MeV when traversing 11.2 mmof copper so their �nal energy should be 134 MeV and 101 MeV respectively.However, using the calibration plot of Fig. 5.10(b), the secondary peaks of Fig.5.12(a) correspond to 115 MeV and 75 MeV (14% and 25% o� of the expectedenergy). We conclude that since the whole energy spectrum is shifted after thecalibration, we cannot use the curve of Fig. 5.10(b) to convert voltage to energy.

Using the new voltage values for 144 MeV and 55 MeV and adding the voltagevalues from the secondary peaks due to the copper plates, a new Voltage - Energycurve (V (E) = −514 + 26E− 0.11E2 + (9× 10−5E3) can be drawn (Fig. 5.13).

5.3 Correlation of GridPix and Calorimeter data

So far the data of the GridPix and the BaF2 detector were presented individ-ually. The synchronization between the two detectors achieved during the runswith the wedges allows us to combine the two di�erent data sets in order to drawadditional conclusions.

In Fig. 5.14 the successful result of combining information for the tracks withthe voltage they induced in the calorimeter is presented. The Y-coordinate oftracks that traversed the GridPix are plotted versus their corresponding voltageoutput (which is correlated to their energy). In the point close to the thin partof the wedge, tracks have their maximum energy. Moving along the Y-coordinaterepresents a thicker part of the wedge. It is clear that for tracks traversing thethicker parts of the wedges, their energy is decreasing. For the big wedge, this

49


Figure 5.14: Y-coordinate - Voltage correlation of tracks through the two wedges.

drop is sharper indicating that the proton tracks have lost more energy thus theyhave traversed more distance though the material.

The density of copper is about 9 times larger than the density of water. A145 MeV proton would deposit around 7 MeV when traversing 2 mm of copperand would require 1.2 cm of water to deposit the same amount of energy. Theuse of thin copper samples (plates and wedges) introduces to the incident par-ticles comparable scattering and energy loss with water samples of an order ofmagnitude thicker.

5.4 Simulations in GEANT4

Simulations of the set-up including the GridPix, the scintillator used forthe trigger and the calorimeter were made in GEANT4. GEANT4 is a Monte-Carlo program operated in C++, a powerful tool used for simulating the passageof particles through matter that can provide information about the trajectoryof the particles, the creation of secondary ones and the energy deposition of aparticle. The 3D particle tracker set-up implemented in GEANT4 is presented inFig. 5.15(a). By irradiating the set-up of Fig. 5.15(a) with 145 MeV protons weextract this information. The blue lines in Fig. 5.15(b) correspond to positivecharged particles (the protons), the red lines to negative charged particles (a

50


(a) Simulated set-up. (b) Simulated run (collection of events).

Figure 5.15: Simulation made in GEANT4.

delta ray in this case) and the yellow dots to steps (the points where a particleinteracted with the volume it is traversing).

5.4.1 Interaction of GridPix with the proton beam

A number of simulations was developed to examine the interaction of GridPixwith the protons and compare it with the beam divergence and energy spectrumof the proton beam. The information we are interested in is the energy of protonsdeposited in the GridPix and the divergence of the protons tracks after interactingwith the gas and the walls of the �eld cage. The 145 MeV proton beam has anangular displacement with a σbeam of 1.5 mrad (Fig. 5.16(c)). When the beamtraverses the 57 mm degrader, the σbeam increases to 30 mrad due to the stragglingof protons through the thick Al plate (Fig. 5.16(a)). Looking at the simulateddata of the GridPix detector, Fig. 5.16(e) shows the divergence of the point ofexit from the point of entrance of all proton tracks due to the walls and gas of the�eld cage. The order of magnitude is in the order of 2 mrad with a σGridP ix = 0.40mrad proving that the interaction of protons with the GridPix causes a signi�cantsmall divergence from their initial trajectory comparing with the σbeam of Fig.5.16(a) and (c). The plot of Fig. 5.16(f) shows the distribution of the energydeposited by 145 MeV protons in the GridPix. By applying a Landau �t the mostprobable value is 70 keV. The energy loss through the GridPix appears to be toosmall to lead to errors when calculating the proton energy loss in case samplesare placed before the 3D particle tracker.

An additional simulation of the 145 MeV proton beam traversing a watersample of 10 cm thickness was developed (Fig. 5.16(g) and (h)). The divergenceof the protons due to scattering in the water is about 50-60 times larger thanthe divergence of the proton beam due to the gas and the walls of the �eld cage.The most probable value of the proton energy loss through the GridPix is 120keV. This larger energy deposition is expected since the protons, after havingtraversed the water, will have lost around 68 MeV. Thus, the energy deposition

51


(a) Divergence of the 145 MeV proton beamthrough the 57mm Al degrader.

(b) Energy distribution of the 145 MeV pro-ton beam through the 57mm Al degrader.

(c) Divergence of the initial 145 MeV pro-ton beam.

(d) Energy distribution of the initial 145MeV proton beam.

(e) Divergence of 145 MeV protons due tothe gas and the walls of the �eld cage.

(f) Energy depostition in GridPix of 145MeV protons.

(g) Divergence of 145 MeV protons aftertraversing 10 cm of water.

(h) Energy depostition in GridPix of 145MeV protons after traversing 10 cm of wa-ter.

Figure 5.16: Simulated e�ect of GridPix on the proton beam.

52


in the GridPix of these lower energetic protons will be larger as predicted by theBethe-Bloch.

5.4.2 Simulation of the Copper samples

The results of Fig. 5.7(c) showed that in the part of the �eld cage coveredwith the copper, less hits are recorded. This is probably due to the fact the anumber of the protons traversing the copper is scattered and not triggering thescintillator, therefore not detected in the GridPix. A simulation of the set-up withthe copper samples covering half of the YZ-plane was made in order to obtainadditional information of the e�ect of the copper plates on the proton beam (Fig.5.17). An estimation of the number of ionizations cannot be made by GEANT4so this process was not simulated.

According to Fig. 5.18(a) and (b), the �ux through GridPix is a�ected bythe copper. In the simulation, all the protons of the beam were generated totraverse randomly within the boundaries of the GridPix YZ-plane. In the case ofone plate, 34% of the protons detected traversed the sample and 33.5% belongto the initial beam. The rest 32.5% of the particles is not detected since theyare scattered from the copper plates. In the case of the four plates, 30% of theprotons traversed the samples, 39.3% belong to the initial beam and 30.7% isnot detected due to the scattering through the copper. The simulated energyspectra in Fig. 5.18(c) and (d) contain two peaks, one representing the energy ofthe protons that traversed the copper and the other representing the energy ofthe particles that did not interact with the samples. The lower energetic peak inboth histograms of Fig. 5.18(c) and (d) is wider due to the energy straggling ofthe protons through the copper.

The simulated energy spectra agree with what was recorded in the experimentand was was expected, the appearance of the two di�erent peaks and the widerspread of the secondary peak due to energy straggling of the lower energeticprotons. According to the simulations, the number of scattered protons is nota�ected by the di�erent thickness of the copper. However, the statistics taken isquite low (only 1024 events in each case2) in order to draw de�nite conclusions.

2The number of simulated protons traversing the copper plates was the same as the numberof protons recorded in the experiment.

53


Figure 5.17: Simulation of protons traversing a copper plate and the 3D particle

tracker.

(a) Simulated scatter plot of the YZ-planewith 1 copper plane.

(b) Simulated scatter plot of the YZ-planewith 4 copper planes.

(c) Simulated energy spectrum with 1 cop-per plate.

(d) Simulated energy spectrum with 4 cop-per plates.

Figure 5.18: Simulation of the 3D particle tracker interacting with the Copper plates.

54

Chapter 6

Conclusions

6.1 Performance of the 3D particle tracker

The application of the GridPix based TPC as a 3D particle tracker with145 MeV protons for the proof of principle was investigated. Proton tracks weredetected and visualized and their energy was recorded with a BaF2 scintillatingdetector. A number of samples was also tested to examine how the GridPixand the BaF2 correspond to the the �uctuations introduced in the beam by thesamples.

The loss of synchronization between the two detectors prevented us fromcorrelating each track in GridPix with a signal from the calorimeter except thedata collected with the wedges. Nevertheless, by carefully analyzing the datathe point of the syncronization loss can be detected and a correlation betweenthe two di�erent data sets is possible. The limitations of the RelaxD readoutrate and the ine�ciency of the trigger to keep up with the intensity of the beammake data acquisition slow. The constant progress by people working at Nikhefin increasing the RelaxD readout spead can lead to a faster acquisition.

The capability of using the 3D particle tracker in order to examine howprotons interact with matter is very promising. By combining the GridPix and thecalorimeter data, the density of the sample changing with respect to the distancecan be detected as shown in the case of the wedges. By using only the data fromthe calorimeter, conclusions can still be drawn as in the case of the copper plateswith the appearance of two di�erent peaks in the energy spectrum. However,the shifted energy values during the runs prevented us to use the energy-voltagecurve made by the calibration. The temperature dependence of the scintillationintensity cannot have caused this e�ect since it would take a temperature changeof around 20 degrees for the deviations recorded. Two possible causes might bethe degradation of the optical couplant in time (the coupling must be renewedevery few years[35]) or the saturation of the photomultiplier (the fairly largenon-linearity in the energy-voltage correlation points in that direction since non-linearity is one of the e�ects caused by saturation[34]).

The major advantage that GridPix o�ers in visualizing tracks in 3D waspresented. The late hits and secondary proton tracks are discriminated in the

55

Chapter 6 Conclusions

analysis but can also be reduced by setting the shutter time equal to the maximumdrift time. The active area of the GridPix is currently limited due to the size ofthe chip. It is feasible to tile chips in a 2xN con�guration (with most common a2x2 area), increasing the detection area in the XY plane. To succesfully increasethe drift volume, a higher voltage must be applied in order to sustain the electric�eld. Thus, the construction of a larger GridPix based TPC is possible.

6.2 Outlook

The 3D particle tracker designed and tested in this project constituted of onlyone GridPix based TPC. In proton CT the beam has to be monitored before ittraverses the patient and after it exits. Thus, the addition of a second GridPixbased TPC along the beam is neccessary (formation of a telescope with twoGridPix detectors). With such a set-up, track reconstruction will be possible andthe trajectory of a track after it exits a sample can be examined in order to de�nethe divergence from the incoming trajectory as explained in 3.1.

In terms of analysis, the implementation of new algorithms is important. Suchalgorithms could be used to �t the hits of each event in order to reconstruct atrack, calculate the divergence in Z-coordinate and Y-coordinate to investigatee�ciently how di�usion a�ects the hits, study the ionization distribution in dif-ferent energies.

The behaviour of the BaF2 detector should be thoroughly examined to detecthow the shift in the energy spectrum occurs. In order to look at the scenarioof the BaF2/photomultiplier combination, the crystal should be repackaged witha new optical coupling to a photomultiplier with a well known behavior. Forthe saturation scenario, the proton energy versus the output voltage spectrumshould be checked more thoroughly on line. Also, by irradiating the detectorwith protons as well as gamma sources one could show whether the e�ect is timedependent and how it corresponds to the nature of the incident particle.

The option of using an other scintillating detector should be also taken intoaccount since the BaF2 crystal might not be the ideal candidate for this project.For example, the properties of LaBr3 look very promising in terms of better energyresolution and the fact that it has only one light component as an output (insteadof the two output scintillation components of the BaF2).

A new design of the trigger is essential. The expected new version of theTimepix chip on 2012 will allow a faster acquisition by the Relaxd readout. Moresimulations in GEANT4 will give an insight in how the experiment compares withtheory especially in cases of sample testing.

Research in the Nikhef R&D Group for improving GridPix is ongoing toimprove the detector. For constructing a 3D particle tracker for proton CT andRadiography, that is not enough. A combination of detector technology andacquisition system, programming (analysis & simulations) and a close contact

56

6.2 Outlook

with the latest research in proton CT is needed to successfully implement protonsin imaging.

57

Acknowledgements

The making and writing of a research project is a procedure that demands agreat deal of work, constant questioning of ones actions and sometimes a bit ofluck. The results of such a task are not restricted only in the project itself butbear fruit for future research.

While working on my Master thesis, I had the opportunity to enjoy mywork in an environment that supplied me with everything I needed. Without thesupport, help, suggestions, comments I received during the last year this projectwould be completely di�erent. The least thing I can do is spent the followinglines to mention the people that I owe my gratitude.

I would like to thank prof. Els Ko�eman for giving me the chance to workon this project. I wish her all the best and hope to see her soon back at Nikhef.The leader of the R&D group Nield van Bakel for supplying me with a GridPixdetector in no time when the one I was working with suddenly broke down.Martin van Beuzekom and Bas van der Heijden for spending many hours helpingme in the lab with the RelaxD, the trigger and the calorimeter. Matteo and thePhD students of the R&D group, Martin, Francesco, Enrico, Marten and Rolf fortheir continuous support. A special thanks to Wilco Koppert for sharing manyuseful ideas and algorithms and his patience for having to put with my endlessquestions and problems.

Last but not least, I would like to thank dr. Jan Visser and prof. Sytze Bran-denburg for the time they spent in making corrections, comments and suggestionsto help me improve not only this thesis but also myself. The scienti�c characterand sincerity of their remarks will guide my future steps as a physicist.

I would like to end this thesis with my wish that more e�ort will be put onthis project so it may prove to be bene�cial in research on proton radiographyand CT. The best of luck to Brent Huisman in improving the 3D particle trackerset-up and the people in the R&D group working on the GridPix detector.

Bibliography

[1] U. Schneider, E. Pedroni, Proton Radiography as a tool for quality controlin Proton Therapy Med. Phys. 22 353 (1995)

[2] K. M. Hanson et al, Computed tomography using proton energy loss Phys.Med. Biol. Vol. 26, Los Alamos Scienti�c Laboratory (1981)

[3] J. Seco, N. Depauw, Proof of principle study of the use of a CMOS activepixel sensor for proton radiography Med. Phys. DOI: 10.1118/1.3496327,Francis H. Burr Proton Therapy Center (2010)

[4] C. Talamonti et al, Proton radiography for clinical applications NuclearInstruments and Methods in Physics Research A, Vol. 612 Issue 3 (2010)

[5] PSI website, Proton Radiography on the PSI gantry radmed.web.psi.ch

[6] M. Regler, M. Benedikt, K. Poljanc, Medical Accelerators forHadrontherapy with Protons and Carbon Ions CERN Accelerator School -Seville, Spain (2002)

[7] Particle Therapy Co-Operative Group website

[8] Saverio Braccini, Scienti�c and Technological Development of HadronTherapy arXiv:1001.0860v1 (2010)

[9] Donald H. Perkins, Introduction to High Energy Physics CambridgeUniversity Press (1972)

[10] Edward L. Alpen, Radiation Biophysics Academic Press (1998)

[11] Particle Data Group (2010)

[12] J. F. Ziegler, The Stopping of Energetic Light Ions in Elemental MatterRev. Appl. Phys., 85, 1249-1272 (1999)

[13] Jan Visser, Particle Detection Lectures (2010)

[14] Daniela Schulz-Ertner et al., Results of Carbon Ion Radiotherapy in 152patients DOI:10.1016 Elsevier (2004)

[15] G. Kraft, Tumortherapy with ion beams Nuclear Instruments and Methodsin Physics Research A 454 (2000)

[16] Advanced Cancer Therapy center website

[17] M. A. Hayat, Cancer Imaging: Instrumentation and Applications Vol. 2Academic Press (2007)

IX

[18] A.M. Koehler, Proton Radiography Journal DOI: 10.1126/160.3825.303science New York (1968)

[19] Harry van der Graaf, GridPix: An integrated readout system for gaseousdetectors with a pixel chip as anode Nuclear Instruments and Methods inPhysics Research A, Vol. 580 Issue 2, Nikhef (2007)

[20] F.W.N. de Boer et al, Delft Internal Conversion Experiment 1st

International Conference on Micro Pattern Gaseous Detectors, JINST 4P11021 (2009)

[21] Xavier Llopart, TIMEPIX Manual v1.0 (2006)

[22] Wilco Koppert et al, High precision 3D Measurements of Single Electronswith GridPix Detectors Poster, Nikhef (2010)

[23] Wilco Koppert, Testbeam Data Analysis Report, Nikhef (2011)

[24] W. Blum, W. Riegler, L. Rolandi, Particle Detection with Drift Chambers2nd edition, Springer (2008)

[25] Sytze Brandenburg, Needs for Proton Radiography and Proton CT Privatediscussion (2011)

[26] M. Lupberger, Ethernet-driven readout system for gaseous detectors,Presentation, University of Bonn (2011)

[27] http://aladdin.utef.cvut.cz/ofat/others/Pixelman/Pixelman_download.html

[28] R.Novotny, Performance of the BaF2-calorimeter TAPS Nuclear Physics B,Vol. 61 Issue 3, University Giessen (1998)

[29] E.V.D. van Loef et al., Scintillation properties of LaBr3:Ce3+ crystals: fast,

e�cient and high-energy-resolution scintillators , Nuclear Instruments andMethods in Physics Research A, 486:254-258, (2002)

[30] P. Schotanus et al, Temperature dependence of BaF2 scintillation light yieldNuclear Instruments and Methods in Physics Research A, Delft Universityof Technology (1985)

[31] KVI, Groningen AGOR Facility for IRradiations of Materials Brochure

[32] Claude Leroy, Pier-Giorgio Rancoita, Principles of Radiation Interaction inMatter and Detection 2nd edition, World Scienti�c (2009)

[33] Archana Sharma, Properties of some gas mixtures used in trackingdetectors SLAC-JOURNAL-ICFA-16-3, Darmstadt (1998)

[34] Reint Ostendorf, Properties of the AGOR cyclotron beam Privatediscussion (2011)

[35] Paul Schotanus, Temperature dependence of BaF2 Private discussion (2011)

X

Appendix A

A.1 Principles of a Time Projection Chamber

Drift chambers have in common that the drift of the ionization electrons inthe gas is used for a coordinate determination by measurement of the drift time.In a drift chamber, the position of the passing particle is determined by the timedi�erence between the passage of the particle and the arrival of electrons at thedetection element(wire, CMOS detector etc.). Detectors with long drift distancesperpendicularly to a multi-anode prportional plane provide three-dimensionalinformation are called time projection chambers (TPCs). Such a TPC is theMicromegas detector which has an operational principle similar to the operationalprinciple of the GridPix based TPC used in this project.

In a Micromegas detector a thin metal grid is placed on top of a CMOS pixeldetector and is enclosed by a drift chamber. When a charged particle traversesthe detector, it ionizes the gas enclosed in the drift chamber (Fig. A.1). Thedrift chamber consists of a �eld cage with a cathode placed on top. By applyinga high voltage in the cathode, an electric �eld between the cathode and the gridis induced. Thus, electrons produced in the ionization process drift towards thegrid. By applying a high voltage in the grid, when an electron passes through ahole of the grid an avalanche is formed which induces a signal in a pixel of theCMOS detector.

Figure A.1: Operation of a Micromegas detector.

XI

(a) Field cage on top of a GridPix based TPC.The electrodes are pointed by the arrow.

(b) Electric �eld con�guration near the elec-trodes of a �eld cage.

Figure A.2: Field cage.

A.2 Field Cage

The electric �eld in the drift gap has to be as uniform as possible andideally similar to that of an in�nitely large parallel-plate capasitor. A very goodapproximation can be obtained covering the inner surface of the �eld cage witha regular set of conducting strips (electrodes) perpendicular to the electric �eld,with a constant potential di�erence ∆V between two adjacent strips ∆V = E∆,where E the electric �eld and ∆ the pitch of the electrode system. In Fig. A.2(a)these electrodes can be seen as thins strips surrounding the �eld cage. Fig.A.2(b) shows the electric �eld lines (full lines) and the equipotentials (brokenlines) near the strips in a drift space D. The electric �eld very near the electrodesis not uniform. The transverse component decays as exp(−2πt/∆) where t is thedistance from the �eld cage.

XII

Appendix B

B.1 Binary resolution

Figure B.1: Strip pitch p and detector resolution.

The relation between the strip pitch p and the detector resolution (Fig. B.1)for binary read-out follows:

RMS =

√1

x2 − x1

∫ x2

x1

(x− x)2 dx (B.1)

and by setting x=0, x1 = −p/2, x2 = +p/2

RMS =

√√√√ 1

3p

∣∣∣∣+p/2

−p/2

(B.2)

=

√1

p

(p3

24− −p

3

24

)(B.3)

=

√p2

12=

p√12

(B.4)

XIII

Appendix C

C.1 Trigger

Figure C.1: Schematic for the trigger used in the experiment at the KVI.

XV

Appendix D

D.1 Measurements with Cosmic Rays

Figure D.1: The cosmic ray set-up.

The GridPix based TPC was tested in the lab before the irradiation at theKVI. The cosmic ray set-up of Fig. D.1 was assembled. The set-up consists of twoscintillators forming a coincidence and the GridPix based TPC. The scintillatorshave been placed such that their e�ective area covers the YZ-plane of the GridPix.When a particle traverses both scintillators, their coincidence triggers the GridPixand the shutter opens. Using the 3D Display, cosmic rays can be visualised. InFig. D.2 two tracks are shown in the two di�erent con�gurations of the set-up.

(a) YZ-plane perpendicular to cosmics. (b) YZ-plane parallel to cosmics.

Figure D.2: 3D visualization of cosmic rays in two di�erent con�gurations of the cos-

mic ray set-up.

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List of Figures

1.1 X-rays vs protons . . . . . . . . . . . . . . . . . . . . . . . . . . . 21.2 Energy deposition as a function of the body depth. . . . . . . . . 2

2.1 Bethe-Bloch curve and corresponding energy loss per unit thicknessin water. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

2.2 Attenuation of protons, ions and photons in water. . . . . . . . . 82.3 The Gantry in a Proton Therapy facility. . . . . . . . . . . . . . . 82.4 The famous "lamb chop", the �rst proton radiograph taken by

Koehler at Harvard. . . . . . . . . . . . . . . . . . . . . . . . . . . 9

3.1 3D particle tracker and principle of use. . . . . . . . . . . . . . . . 123.2 The GridPix detector. . . . . . . . . . . . . . . . . . . . . . . . . 133.3 Principles of a TPC. . . . . . . . . . . . . . . . . . . . . . . . . . 143.4 Drift velocity of gas mixtures. . . . . . . . . . . . . . . . . . . . . 183.5 Di�usion coe�cients of gas mixtures. . . . . . . . . . . . . . . . . 193.6 Distributions of number of electrons produced from protons through

DME 50%, CO2 50% for T=300 K and p=1 atm. The plots aremade in GARFIELD. . . . . . . . . . . . . . . . . . . . . . . . . . 20

3.7 The time walk e�ect. . . . . . . . . . . . . . . . . . . . . . . . . . 213.8 Harware and software components for the GridPix Read-out. . . . 223.9 Measurement of the ToA. . . . . . . . . . . . . . . . . . . . . . . 233.10 The BaF2 detector. . . . . . . . . . . . . . . . . . . . . . . . . . . 243.11 Characteristics of the BaF2 crystal. . . . . . . . . . . . . . . . . . 253.12 Signal from the BaF2. . . . . . . . . . . . . . . . . . . . . . . . . 263.13 Triggering on the coincidence of a scintillator and the calorimeter. 27

4.1 The AGOR facility . . . . . . . . . . . . . . . . . . . . . . . . . . 304.2 Side view of all the components of the experimental set-up. . . . . 304.3 The 3D particle tracker. . . . . . . . . . . . . . . . . . . . . . . . 314.4 The set-up with the Copper plates. . . . . . . . . . . . . . . . . . 334.5 The set-up with the wedge. . . . . . . . . . . . . . . . . . . . . . 33

5.1 Integral plots of pixels hit. . . . . . . . . . . . . . . . . . . . . . . 365.2 Discrimination of e�ective pixels through the hit spectrum. . . . . 365.3 3D view of the samples placed before GridPix. . . . . . . . . . . . 375.4 A proton track and the Y distribution of the hits forming it. . . . 385.5 Projections on Y. . . . . . . . . . . . . . . . . . . . . . . . . . . . 405.6 Secondary "late" track appearing higher than the �eld cage volume. 415.7 Integral scatter plots of the YZ-plane. . . . . . . . . . . . . . . . . 42

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5.8 Energy dependence of Ionization. . . . . . . . . . . . . . . . . . . 445.9 Pixel clock counts. . . . . . . . . . . . . . . . . . . . . . . . . . . 455.10 Calibration of the BaF2. . . . . . . . . . . . . . . . . . . . . . . . 465.11 Calorimeter data . . . . . . . . . . . . . . . . . . . . . . . . . . . 475.12 Calorimeter data of the runs with the samples. . . . . . . . . . . . 485.13 Voltage - Energy correlation with shifted voltage values. . . . . . 495.14 Y-coordinate - Voltage correlation of tracks through the two wedges. 505.15 Simulation made in GEANT4. . . . . . . . . . . . . . . . . . . . . 515.16 Simulated e�ect of GridPix on the proton beam. . . . . . . . . . . 525.17 Simulation of protons traversing a copper plate and the 3D particle

tracker. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 545.18 Simulation of the 3D particle tracker interacting with the Copper

plates. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54

A.1 Operation of a Micromegas detector. . . . . . . . . . . . . . . . . XIA.2 Field cage. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . XII

B.1 Strip pitch p and detector resolution. . . . . . . . . . . . . . . . . XIII

C.1 Schematic for the trigger used in the experiment at the KVI. . . . XV

D.1 The cosmic ray set-up. . . . . . . . . . . . . . . . . . . . . . . . . XVIID.2 3D visualization of cosmic rays in two di�erent con�gurations of

the cosmic ray set-up. . . . . . . . . . . . . . . . . . . . . . . . . XVII

XX

List of Tables

4.1 Properties of the KVI cyclotron beam. . . . . . . . . . . . . . . . 294.2 Energy and Voltage values. . . . . . . . . . . . . . . . . . . . . . . 324.3 Parameters and properties of the Runs. . . . . . . . . . . . . . . . 34

5.1 Shifted Voltage values and corresponding Energies from the runs. 47

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