MPhys Long Report

Gamma-Ray Spectroscopy Using a High-Purirty

Germanium Detector

University of Sussex

Candidate Number: 107695

May 7th 2015

i

Gamma-Ray Spectroscopy Using a High-Purirty Germanium

Detector

Candidate no.: 107695

University of Sussex

Abstract. High-Purity Germanium detectors are used for gamma

spectroscopy. Three reference radionuclides were used to perform an

energy calibration. The radio-background was measured these in

conjunction were used to find a low radioactivity mystery source

radionuclide make up which was determined to be Radium-226

ii

Preface

The data collected for this long report was done by both me and my lab partner under the guidance of

Simon Peeters and Phil Meek

Contents

Abstract i

Preface ii

1 Introduction 1

2 Background 1-6

2.1 Gamma-ray Interactions 2

2.1.1 Photoelectric Effect 2

2.1.2 Compton Effect 2-3

2.1.3 Pair Production 3

2.2 Full Spectrum Effects 4

2.2.1 Secondary Electron Escape 4

2.2.2 Bremsstrahlung Escape 4

2.2.3 Characteristic X-Ray Escape 4

2.2.4 Secondary Radiations 4

2.2.5 Housing Effects 4-5

2.2.6 Summation peaks 5

2.2.7 Background Radiations 5

2.3 Semiconductors 5-6

2.3.1 HGPe Geometry 6

3 Experimental Apparatus 6

4 Results 6-7

4.1 Data Acquisition 6

4.2 Energy Calibration 6

4.3 Background 7

4.4 Mystery Source 7

5 Discussion 7-8

6 Conclusion 8

References 9

Appendix A 10

1 Introduction

Gamma-ray spectroscopy enables the ability to

determine the energy and photon count-rate of

gamma radiation. This is useful as different

radionuclides emit gamma photons at discrete

energies, thereby allowing the ability to infer the

sources of the gamma radiation [1]. In more detail if

a radionuclidal make-up of a source needed to be

investigated a calibrated gamma spectrometer can

create a spectrum of energies and the count-rate of

each of these energies; eliminating any background

noise then referring back to theoretical or measured

values of radionuclides spectra will identify the

source.

Natural radioactivity was discovered

accidentally by Becquerel in 1896 including: alpha,

beta and gamma radiation. In 1900 Villard noticed

that one of the types of natural radioactivity which

path did not bend under the influence of magnetic

fields and was far more penetrative then the other

types of natural radioactivity, distinguishing it and the

name gamma-rays being coined as a label. Both of

these were done with photographic plates, which was

a slow process, (as they had to get developed) and

distinguishing the different types of radiation was

difficult. The process was improved by the creation

of various types of gas-filled counters greatly

improving on many of the pitfalls of the photographic

plate. The first being created by Rutherford and

Geiger in 1908. Generally these detectors could not

directly determine the energy of the photons that

were detected. The next large step in detectors was

made by Hfstadter in 1948, the NaI(Tl) detectors

were chemically and physically stable, boasted a

relatively high resolution and efficiency, and could

detect photons of energies ~1Mev. Other types of

detectors were made including magnetic electron

spectrometers and the diffraction spectrometers each

with their own techniques, advantages and

disadvantages in measuring gamma-ray transitions.

But the next significant jump was the successful

creation of the Ge(Li) semiconductor based detector

in 1962 by Freck and Wakefield. These boasted

resolutions ten-times that of the Na(Tl) detectors!

Working effectively as a diode with its current being

the creation of electron-hole pairs acting as charge

carriers by the absorption and emission of incident

gamma photons. These detectors had one main

disadvantage however they had to be kept very cold

at all times, (Generally liquid nitrogen temperature,

approximately 77 K), due to the creation of electron-

hole pairs which happens at appreciably high

temperatures. This requires a relatively sizable

upkeep. This has been alleviated somewhat in later

years with the development of the high-purity

germanium detector (HPGe) which can be

transported and stored at room temperature [2]

2 Background

Gamma spectrums created by HPGe have certain

distinct characteristics which broadly are due to one

or more of the following categories:

Interaction or lack thereof of gamma

photons with matter inside the HPGe

crystal

Interaction of gamma photons with matter

inside the housing

2

Near simultaneous interactions

2.1 Gamma-Ray Interactions

Photons interact with matter in three

different ways:

The Photoelectric Effect, Compton Effect or by pair

production. The probability that any particular

interaction will take place is a function of both the

energy of the incident gamma photon, E, and the

atomic number of the material the photon is

interacting with, Z, (Fig. 2.1)

.

Fig. 2.1- Dominance of different interactions of

matter [3].

From Fig. 2.1 it is seen that at low incident

gamma photon energies and high atomic number the

photoelectric effect is dominant. At high energy

incident gamma photons and high atomic number

pair production is dominant. For intermediate photon

energies the Compton effect is dominant.

2.1.1 Photoelectric Effect

In the photoelectric effect an incident

photon approaches a material then is wholly absorbed

by an atoms bounded electron of the material

unbounding the electron, now called a photoelectron.

The kinetic energy of this photoelectron, Ee-, is

Ee-=hv - Eb where hv is the energy of the incident

gamma photon and Eb is the workfunction to liberate

the electron from its initial energy level. This is

depicted in Fig. 2.2

Fig 2.2- Pictorial representation of photoelectric

effect [3].

This results in a photopeak (full-energy peak)

in the spectrum which is seen in Fig 2.3

Fig. 2.3- Characteristic signature of photopeak with

E being energy and

being the count-rate.

2.1.2 Compton Effect

Compton scattering is the scattering of photons off a

quasi-free electron of mass me, decreasing the energy

of the incident photon, hv, at an angle and

producing a recoil electron at an angle . Depicted in

Fig. 2.4.

3

Fig. 2.4- Pictorial representation of Compton Effect

[4]

By energy conservation it can be shown that

the energy of the recoil electron is

= =

2

1cos()

1+(

2)(1cos())

. (1)

As any between -180 to 180 degrees is

realised this leads to the Compton continuum and

Compton edge in the spectrum. Fig 2.5

Fig 2.5- Compton effect with E being energy and

being the count-rate. [3]

The two extremes of (1) are when =0 and

=. When =0 the scattered photon retains all of

its energy and the recoil electron receives zero. When

= the photon is backscattered; this is the case of

maximum energy transfer.

2.1.3 Pair Production

Pair production is the creation of an electron

of energy Ee_ and a positron of energy Ee+. This

happens when an incident photon of energy, hv, is in

the strong electric field near the nucleus of a materials

atom. There is a minimum energy contingent for this

interaction to take place the minimum energy is

simply the mass-energy of two electrons 2mec2 this

leads to the following equation,

Ee_+ Ee+= hv -2mec2. (2)

This would lead to a peak at the point at hv -2mec2

called the Double-escape peak In an ideal detector

these all the gamma interactions would be absorbed

by the detector however it is not always the case.

Upon pair production a positron is created. In an

electron rich environment this would almost

instantaneously annihilate creating two 0.511 MeV

photons. Sometimes one of the photons escapes and

this leads to the single escape peak. Fig. 2.6

Fig. 2.6- Real life detector showing all the different

ways gamma photons can interact with the HPGe

crystal [3]

Due to the fact that pair production only happens

above a certain threshold energy theres a splitting in

the spectrums as depicted below. Fig 2.7

4

Fig. 2.7- two possible spectrums from the

interactions stated above [3]

2.2 Full Spectrum Effects

There are more subtle effects to also look out for:

2.2.1 Secondary Electron Escape

For high energy gamma photons, unbound electrons

created by the gamma-ray interaction (secondary

electrons) will also have a high energy. Therefore a

higher chance of escaping the detector. This will

change the spectrum by shrinking the photopeak and

creating a bias in the Compton continuum and other

sources of low-energy counts such that they will be

more highly counted relative to the higher energy

features.

2.2.2 Bremsstrahlung Escape

Bremsstrahlung photons are emitted by an electron

scattering off other nuclei. This is problematic when

Bremsstrahlung photons are not reabsorbed by the

detector. This changes the spectrum the same way

secondary electrons do however can be minimised by

using materials with a small atomic number.

2.2.3 Characteristic X-ray Escape

After the photoelectric effect takes place electrons in

the atom reconfigure causing the binding energy to

also be liberated as a characteristic X-ray. If this X-ray

does not get detected an extra peak forms on the

spectrum of the photopeaks energy minus the

characteristic X-rays. Fig 2.8.

Fig 2.8- pictorial representation and resulting

spectrum due to X-ray escape peak [3]

2.2.4 Secondary Radiations

Secondary radiation are beta minus and beta plus that

are emitted by the source instead of the intended

gamma ray. With beta minus Bremsstrahlung

radiation is created that can penetrate the detector.

Beta plus will annihilate creating gamma Rays.

2.2.5 Housing Effects

Often gamma-rays do not interact with the detector

however do interact with the detectors housing.

These in turn may interact with the detector and get

absorbed. There include:

Housing photoelectrically absorbs photon

emitting a characteristic X-ray which is then

absorbed

Photon backscatters of housing at an angle

such that it goes into the detector and is

absorbed

Pair production creates a positron which

annihilates creating a 0.511 MeV photon

which is absorbed. Fig. 2.9

5

Fig. 2.9- Pictorial demonstration of housing effects

and adding effects of housing on spectrum [3]

2.2.6 Summation Peaks

Summations peaks occur when the source emits two

gamma photons in a very small period of time such

that they are detected as being the one signal.

2.2.7 Background Radiations

Other peaks can come from background radiation.

Contributions from the background include:

Natural radioactivity from materials in the

detector

Natural radioactivity of objects near the

detector

Radiation from the Earths surface

Radiation from the air

Cosmic Radiation

2.3 Semiconductors

The periodic lattice of crystalline materials creates a

continuum of energies that electrons can exist in

comparison to discrete energy levels. These

continuums are called bands. Forbidden energies

exist between bands. These are aptly called band gaps.

Two bands that are of particular interest are the

valences band, corresponding to the outer shell

electrons in the crystal. The other band of interest is

the conduction band which is the band after the

valence band. Electrons in the conduction band and

holes (lack of electrons where one could exist)

contribute to the materials electrical conductivity. Let

us name the gap between the valence band and

conduction band Eg. For semiconductors the valence

band is full and has a small Eg this distinguishes it

from insulators which have a big Eg .Fig 2.10. This has

the effect that small excitations can liberate electrons

in the valence band to the conduction band and in

turn a hole be created in the valence band in

semiconductors.

Fig. 2.10- Band structures for insulators (left) and

semiconductors (right)

6

Unfortunately thermal excitations can also have this

effect so the apparatus needs to be immersed in liquid

nitrogen (77 K) temperatures.

2.3.1 HPGe Geometry

In an ideal detection no gamma photons escape. This

would require a large detector which would be

expensive to run and cumbersome. To compromise

in order to maximise the detection of gamma photons

the HPGe has a coaxial geometry

3 Experimental Apparatus

To see block diagram see Appendix A

The settings for the DPP-PHA were:

General Energy filter

DC offset 36 Decay time 50 s

In range 0.6vpp Rise time 1 s

Self trig Enabled Flat top 3 s

Polarity positive Baseline mean 1024

Digital gain 1 Trapezoid gain 1

Decimation 1

TTF

Threshold 150 LSB

4 Results

4.1 Data Acquisition

The Spectrums of three reference materials: Co-60,

Ba-133 and Na-22 were found. (860,000 counts each)

Fig. 4.1- Gamma spectrum for Co-60

Fig. 4.2- Gamma spectrum for Ba-133

Fig. 4.3- Gamma spectrum for Na-22

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4.2 Energy calibration

The energy calibration was found using three

reference known radionuclides: Co-60, Ba-133 and

Na-22.

Fig. 4.4- Energy calibration, Bin no. vs reference

energies of Co-60, Ba-133 and Na-22

4.3 Background

The radioactive background was measured (200,000

counts) as to not measure fake peaks on the

identification of the mystery source which is relatively

a weak radioactive source.

Fig. 4.5- Background spectrum

The Compton continuum was taken away so a step

function could be fitted to easily find all the peaks at

once.

Fig. 4.6- Peaks found and energies assigned to each

on background

4.4 Mystery Source

The mystery sources spectrum was measured and

identified the mystery source as Radium-226

Fig. 4.7- Mystery source spectrum

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Fig. 4.8- Uranium-238/Radium-226 decay chain

(shaded regions show corresponding gamma

spectrum found)

5 Discussion

With all three reference the Compton continuum and

edge was identified; some showing shifting suggestive

if one of the full spectrum effects. we fitted Gaussians

to the peaks and obtained we obtained all the spectral

lines in the NAI catalogue later cross referencing with

a more accurate catalogue which covers the most

abundant peaks. With all of them you could also

identify an annihilation peak. They were all checked

beforehand by the oscilloscope for pile-up

When making the energy calibration we did a

goodness of fit () and after identifying an over

estimation of the errors due to using the standard

deviation of the Gaussian. The error technique was

changed as the centre of the peak was always very easy

to distinguish, so the bin error went down from ~7.5

bins to 1 bin obtaining a reasonable of 0.65.

The background showed what was expected with 21

radionuclides ranging from the Th-232, U-238 and K-

40 families. With the biggest peak not being able to

be identified as only 7 counts above the baseline after

a full days detecting and only a few above the noise.

The first single-escape and double-escape peaks were

also seen.

The mystery source showed very clear peaks against

the noise however finding out what the mystery

source was proved difficult as the NAI Catalogue

does not show gamma emission values of

radionuclides of certain half-life ranges. However

after this revelation finding out the mystery source

was quick using the other catalogue.

6 Conclusion and Suggestions

The mystery source was demystified as radium-226.

Going further in this experiment could include seeing

how the Gaussian changes as a function of energy the

bias voltage could also be changed to see whether it

has an effect.

In retrospect I feel a longer detection time of both the

background and the mystery source with that the

smaller peaks could have been discerned.

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References

[1]- Knoll, G. F. (1979). Radiation Detection and

Meaurement. 4th ed. United States of America: Wiley &

Sons. Page 321

[2]- Debertin, K. and Helmer, R. G. (1988). Gamma-

and X-Ray Spectrometry with Semiconductor Detectors.

Amsterdam: Elsevier Science. Pages 8-12

[3]- Rittersdorf, I (2007). Gamma Ray Spectroscopy. Michagan: University of Michigan. Pages 5-44.

[4]-

http://hyperphysics.phyastr.gsu.edu/hbase/quantu

m/comptint.html

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Appendix A

Fig A.1- Block diagram of setup

Fig A.2- Diagram of HPGe

and Dewar

Computer MCA (CLEN 075730)

HPGe Detector

Oscilloscope

MCA Counts

MCA Data

Detector Signal

Low voltage power

High voltage power