HIGH-RESOLUTION ROOM TEMPERATURE RADIATION

DETECTION USING CORRELATED SCINTILLATION AND

CHARGE MEASUREMENTS

by

NATHAN J. PRICE

B.S. GENERAL ENGINEERING, SPRING 2010

UNIVERSITY OF ILLINOIS

M.S. SYSTEMS ENGINEERING, SPRING 2012

UNIVERSITY OF ILLINOIS

Ph.D. Proposal

May, 2019

Abstract

This work will focus on characterization of room temperature radiation detection materials which

result in simultaneous scintillation and charge collection. Methods previously applied to

cryogenically cooled xenon will be applied to room temperature solid materials. High resolution

room temperature devices would provide great value in experimental, national security, and space

applications. Hg2Br2 and CdMgTe samples provided by Brimrose Technology Corporation will

be explored for gamma resolution enhancements through correlation of scintillation and charge

collection spectra. Optimal methods for combining these spectra will be explored, as well as pulse

shape discrimination methods based on rise time, decay time, and other methods. Additional

measurements of these materials will include: electron/hole mobility, scintillation light spectra,

optical range material absorption, scintillation detection resolution, charge detection resolution,

and a combined resolution from scintillation and charge collection.

Optimal methods of combining coincident measurements of charge collections and scintillation

emissions will be explored, as well as pulse shape discrimination methods based on rise time,

decay time, and other methods.

i

Table of Contents

Abstract ............................................................................................................................................ i

List of Figures ................................................................................................................................ iii

List of Tables .................................................................................................................................. v

1 Introduction ............................................................................................................................. 6

2 Gamma Detection Overview ................................................................................................. 10

2.1.1 Gamma Ray Interactions ....................................................................................... 10

3 Scintillation Based Gamma Detection .................................................................................. 15

3.1.1 Scintillation Mechanisms ...................................................................................... 15

3.1.2 Poisson Limits ....................................................................................................... 16

3.1.3 Scintillation Characteristics for Gamma Spectroscopy ........................................ 19

4 Charge Collection Based / Solid State Gamma Detection .................................................... 21

4.1.1 Charge Collection Mechanisms ............................................................................ 21

5 Dual Mode Collection ........................................................................................................... 29

6 Prior Work on Sample Materials .......................................................................................... 34

6.1 Prior Work on Mercurous Bromide .............................................................................. 34

6.2 Prior Work on CdMgTe ................................................................................................ 36

7 Proposed Work ...................................................................................................................... 38

7.1 Samples ......................................................................................................................... 38

7.2 Scintillation Measurements ........................................................................................... 42

7.2.1 Visible Spectrum ................................................................................................... 42

7.2.2 Gamma Spectrum Measurements ......................................................................... 44

7.3 Charge Measurements ................................................................................................... 47

7.4 Dual Collection Measurements ..................................................................................... 49

7.5 Analysis Methodology .................................................................................................. 50

7.5.1 Spectrum Generation ............................................................................................ 51

7.5.2 Preliminary Combined Spectrum Estimates ......................................................... 53

8 Results ................................................................................................................................... 56

8.1 Hg2Br2 .......................................................................................................................... 56

8.1.1 Scintillation Measurements ................................................................................... 56

ii

8.1.2 Charge Collection ................................................................................................. 57

8.1.3 Dual Mode Collection ........................................................................................... 57

8.2 CdMgTe Results ........................................................................................................... 57

8.2.1 Scintillation Collection ......................................................................................... 57

8.2.2 Charge Collection ................................................................................................. 57

8.2.3 Dual Mode Collection ........................................................................................... 57

9 Conclusion ............................................................................................................................ 58

10 Works Cited ...................................................................................................................... 59

iii

List of Figures

Figure 1 Estimates on the probability of correct classification with varying detector resolutions (from [2]) ........................................................................................................................................ 6 Figure 2 Resolution comparison of (blue) NaI (red) HPGe and (black) CZT [3] .......................... 8 Figure 3 The relative importance of the three major types of gamma-ray interactions. The lines show the values of Z and 𝒉𝝊 for which the two neighboring effects are just equal (from [6]) .... 11 Figure 4 K-Shell Energy as Function of Atomic Number [7] ...................................................... 12 Figure 5: Photon Cross Sections in Hg2Br2 [10] ........................................................................... 14 Figure 6 Electron Band structure of NaI (left) and LaBr3(right) calculated from [11] ................ 15 Figure 7 Doping Electron Shells within the Band Gap of the Bulk Material [14] ....................... 16 Figure 8: Statistical Limit of FWHM of Poisson Events Based on Quantity Detected Overlaid with Reported Values from [16]: Color indicates peak emission wavelength. Legend depicts chemical formula and reference as designated in [16]. ................................................................. 17 Figure 9: Statistical Limit of FWHM of Poisson Events Based on Quantity Detected Overlaid with Reported Values from [16] ................................................................................................... 18 Figure 10 Measured Energy Resolutions and Theoretical Poisson Statistical Resolution Limit for a Variety of Scintillation Materials [9] ......................................................................................... 19 Figure 11: Band structure of Au (left) and Si (right) calculated using density functional theory (DFT) [13] ..................................................................................................................................... 22 Figure 12 Temperature Influence on Conduction of Material with Bandgap, 𝑬𝒈 and Fermi Energy, 𝐸𝑓 .................................................................................................................................... 23 Figure 13 Density of states calculation for Germanium showing conductivity at room temperature ................................................................................................................................... 24 Figure 14 Electron-hole Pair migration in Material (left) Initial electron hole creation (right) Migration of Electrons and Holes through material ..................................................................... 25 Figure 15 Sample Induced Charge Pulse Shape Due to Differing Electron Hole Mobilities ....... 25 Figure 16 Measured Induced Charge fit to Hecht Relation for Mobility Lifetime Measurement (from [21]) .................................................................................................................................... 28 Figure 17: Measurements of the Anti-Correlation of Charge and Light Collection in Liquid Xenon (from [25]) ......................................................................................................................... 30 Figure 18 Spectral Resolution Improvement from Charge and Light Combination (from [25]) . 31 Figure 19 Summary of results from Aprile et. al. showing measurement trends with varying electric field .................................................................................................................................. 32 Figure 20 Sample of Single Crystal Mercurous Bromide with 2" diameter made by PVT [29] .. 34 Figure 21 Transmission spectrum measured through 48 mm Mercurous Bromide Sample (from [29]) ............................................................................................................................................... 35 Figure 22: Room Temperature Hg2X2 charge collection spectra reported in [31] ....................... 36 Figure 23 Electron Mobility Lifetime found for CdMgTe by fitting to Hecht Relation .............. 37

iv

Figure 24: Image of current inventory of three Hg2Br2 samples (top left) and 2 CdMgTe samples prior to application of electrodes .................................................................................................. 39 Figure 25 Charge Collection Samples of CdMgTe with Gold Electrodes .................................... 39 Figure 26 Dual Mode Samples without Transparent Electrodes .................................................. 40 Figure 27 CSDA Range Estimates for Electrons in Hg2Br2 ........................................................ 41 Figure 28 MCNP Simulation Results Indicating Full Energy Absorption Peak in 5mm x 5mm x 1mm Hg2Br2 Sample ..................................................................................................................... 42 Figure 29 Scintillation Light Yield Spectra .................................................................................. 43 Figure 30 Emission Spectrum of Hg2Br2 and the Quantum Efficiency Spectrum of Two Common PMT Photocathodes ...................................................................................................... 44 Figure 31: PMT Divider Gains [32]. 2520AN used in this work is an implementation of Divider A [33] ............................................................................................................................................ 45 Figure 32: Hardware setup for Scintillation Collections .............................................................. 45 Figure 33 Scintillation Collection Configuration (left) Expanded Assembly (right) Cross section of assembled .................................................................................................................................. 46 Figure 34 Equipment setup which will be used for dual PMT collections ................................... 46 Figure 35 Fixture used for Dual PMT and Dual mode collections ............................................... 47 Figure 36 Charge Collection Fixture: (top left) exterior of fixture (top right) 3D printed fixture without faraday cage (bottom left) cutaway shown fixture (bottom left) cross section view of charge collection fixture ............................................................................................................... 48 Figure 37: Components for Charge Collection Setup ................................................................... 48 Figure 38 Charge Collection Fixture for Reduced Electrical Noise ............................................. 49 Figure 39 Experimental setup for dual mode collection ............................................................... 50 Figure 40 User Interface for Custom Collection Software During 137Cs Scintillation Collection 51 Figure 41 Pulse Shape Identification of Incomplete Charge Collection Due to Defocusing Fields (from [35]) .................................................................................................................................... 53 Figure 42: Potential Optimal Resolutions for Hg2Br2, based on Equation 5.3. ........................... 54 Figure 43: Experimental Relationship between Optimal Angle and Correlation Coefficient ...... 55 Figure 44 Example Scintillation Events into 50𝛀 (left) and 1M𝛀 (right) .................................... 56 Figure 45: Scintillation Spectrum for 137Cs measured on Hg2Br2 Sample (left) Spectra Collected with and without the source present (right) FWHM estimates made on background subtracted spectrum. ....................................................................................................................................... 57

v

List of Tables

Table 1 Test Samples Inventory ................................................................................................... 38 Table 2 Test Sample Fabrication Queue ....................................................................................... 39

6

1 Introduction Radioactive isotopes emit gamma rays at specific energies based upon their radioactive decays.

Through measurement of the gamma-ray energy distribution it is possible to determine the radio-

isotope(s) present. Many factors influence the quality of the measurement that can be made, and

the sensitivity and confidence level of the identification of the radio-isotopes present within a

given sample. Gathering high quality spectrum is essential for enforcement and monitoring of

non-proliferation, ensuring the safety of the boarders and ports of entry, as well as assessing the

safety and operation of nuclear reactors.

Estimates have been made on the influence of detector resolution on the ability to

positively identify radio-isotopes in Reference [2] which suggest that the identification

performance of a detection system improves exponentially as detector resolution improves.

Figure 1 shows the probability of mis-classification of 6 potential isotopes with detector

resolutions varying from 0.2% to 8%.

Figure 1 Estimates on the probability of correct classification with varying detector resolutions (from [2])

demonstrating exponential scaling of information with detector resolution

7

Due to the importance of detector resolution on the information content which can be collected

for a given device, improving detector resolution is an important problem. Some of the best

resolution systems which are commercially available are constructed with High Purity

Germanium which is cooled to liquid nitrogen temperatures. The best resolution room

temperature devices which are commercially available are CdZnTe solid state detectors. S

CdZnTe detectors have been reported to have spectral resolutions as low as 0.8 % using a Frisch

Grid configuration [1] [2]. Sample spectra from each of these detector types, which highlight the

inability of lower-resolution devices to identify Plutonium isotopes in the presence of Iodine are

shown in Figure 2 below.

8

Figure 2 Resolution comparison of (blue) NaI (red) HPGe and (black) CZT [3] gamma detector responses

demonstrating the information loss with lower resolution detectors.

Though HPGe is capable of identifying the Pu source in the presence of Iodine, it requires large

amounts of energy in order to maintain the operating temperature required to make

measurements of that quality. Additionally, manufacturing limitations as well as cooling

requirements limit the physical dimensions of these detectors.

This work seeks to investigate an alternative path to high resolution room temperature gamma

spectroscopy using combining scintillation and charge measurements. Chapter 2 provides an

overview of the mechanisms which determine how gamma radiation is absorbed in materials.

Chapters 3 and 4 describes the mechanisms involved in the scintillation process and charge

collection processes for gamma-ray spectroscopy. Chapter 5 provides an overview of prior work

that has been done on dual collection materials (though not at room temperature).

Chapter 6 gives the background of prior work which has been done on the development and

testing of the materials which will be investigated in this study (CdMgTe and Hg2Br2).

9

Chapter 7 details the experimental setup for this work and experimental results are contained in

Chapter 8.

10

2 Gamma Detection Overview Gamma rays are photons which are generated from the decay of radioactive isotopes or direct

nuclear interactions. These photons can be absorbed in a detection material through a variety of

mechanisms which will be described below. The probability of these gamma rays being

absorbed in materials is defined by the energy dependent quantity, the mass attenuation

coefficient, 𝜇 𝜌* , the density of material, and the linear distance which the gamma ray interacts in

the material according to:

𝑵 = 𝑵𝟎𝒆/𝝁𝝆𝝆𝒙

2.1

Values for 𝜇 𝜌* have been characterized for elemental media with atomic numbers 1 through 92

which are available in Reference [4].

2.1.1 Gamma Ray Interactions

Gamma rays interact within materials in a variety of ways. The three interaction types which are

important in radiation measurement are: photoelectric absorption, Compton scattering, and pair

production [5]. The likelihood of these interactions is dependent upon the atomic number, Z, of

the interacting material as well as the energy of the incident gamma-ray.

11

Figure 3 The relative importance of the three major types of gamma-ray interactions. The lines show the

values of Z and 𝒉𝝊 for which the two neighboring effects are just equal (from [6])

2.1.1.1 Photoelectric Absorption

Gamma rays which are absorbed by an atom and result in an ejected energetic electron, referred

to as a photoelectron, is called photoelectric absorption. The photo-electron will be ejected with

an energy:

𝐸34 = ℎ𝜈 − 𝐸8

2.2

Where 𝐸8is the binding energy of the photoelectron from its original shell. For gamma ray

energies of interest in gamma spectroscopy the photo electron is most likely to be ejected from

the K-shell. The K-shell energy vary from less than 1 to over 100 keV and varies with the

element [7].

12

Figure 4 K-Shell Energy as Function of Atomic Number [7]

This interaction is most probable for incident photon energies up to several hundred eV and

varies proportional to 𝑍: where n varies between 4 and 5 over the relevant gamma ray energies

[8].

These photoelectrons are then ejected into the material and will interact with the detection

material. The likely hood of interaction for a given material is a function of the electron energy

and can be estimated using a variety of empirical elemental and compound results provided by

Reference [9]. Of interest in this study are estimates made using the continuous slowing down

approximation range (CSDA) which is provided in units of g/cm2. Materials which do not have

sufficient dimensions and densities will not capture the energy of photoelectrons, which will

readily escape above certain energy limits.

2.1.1.2 Compton Scattering

There also exists a mechanism in which the incident gamma ray interacts directly with the

electrons in the incident material and is deflected. In this interaction part of the gamma ray

13

energy is transferred to an electron, called a recoil electron, in the material. The energy of the

deflected gamma ray is expressed as:

ℎ𝜐< =ℎ𝜐

1 + ℎ𝜐𝑚@𝑐B

(1 + cos 𝜃)

2.3

Where 𝑚@ is the rest mass of an electron, and 𝜃 is the deflection angle of the gamma ray in the

lab frame [8]. The cross section for this interaction can be expressed by the Klien-Nishina

formula:

𝑑𝜎𝑑Ω = 𝑍𝑟@B M

11 + 𝛼(1 − 1 − cos 𝜃)O

B

P1 + 𝑐𝑜𝑠B𝜃

2 TP1 +𝛼B(1 − cos 𝜃)B

(1 + 𝑐𝑜𝑠B𝜃)[1 + 𝛼(1 − cos 𝜃)]T

2.4

Where 𝛼 ≝ ℎ𝜈𝑚@𝑐B* and 𝑟@ is the classical electron radius. As can be seen, the total cross

section (integration of above equation) will vary linearly with Z. This effect is dominant at

energies which are roughly 2 MeV [5].

2.1.1.3 Pair Production

When incident gamma rays have sufficient energy (two times the rest mass of an electron), pair

production is possible. In this method the incident gamma ray annihilates and is replaced by an

electron and a positron with equal kinetic energies in opposite directions to conserve energy and

momentum. The kinetic energy of the resulting electron-positron pair will be equal to the

gamma ray energy which was in excess of the mass of the created particles. The positron will

eventually slow, annihilate with an electron, and create two annihilation photons that are emitted

with equal energy in opposite directions.

14

The influence of each of these absorption mechanisms vary with the Z number of the material

and the incident photon energy. The various cross sections for Mercurous Bromide (Hg2Br2) are

shown in Figure 5.

Figure 5: Photon Cross Sections in Hg2Br2 [10] showing dominance of photoelectric absorption, scattering, and pair production at low, medium, and high energies respectively

It can be seen that photoelectric absorption is dominant at low incident photon energies (below ~

400 keV). Compton scattering is dominant in the intermediate energies between approximately

400 keV and 5 MeV. Above 5 MeV pair production is the most probable absorption method in

Hg2Br2.

15

3 Scintillation Based Gamma Detection In some materials the energy that is absorbed in the above processes will result in the generation

of visible and near visible photons in quantities which are proportional to the incident gamma ray

energy. This section will describe the mechanisms for creating these scintillation photons, as

well as high level considerations which apply to using scintillation detectors for gamma

spectroscopy.

3.1.1 Scintillation Mechanisms

When the deposited energy is absorbed into the detector material, electrons are excited to higher

energy states. Scintillation detector materials have a band gap, or a range of electron energies

which cannot be occupied within the crystalline material due to quantum effects. The energies

which can be occupied below this gap are known as the valence band and the energies which can

be occupied above this gap are known as the conduction band. The energy bands for two

common scintillating materials have been calculated using density functional theory (DFT) [11]

[12] [13] in Figure 6.

Figure 6 Electron Band structure of NaI (left) and LaBr3(right) calculated from [11] [12] [13]

The difference in energy between the lowest energy in the conduction band and the highest

energy in the valence band is known as the band gap energy. Electrons in the valence band are

16

essentially fixed to the lattice position of the atom to which they are bound. Electrons which are

excited to the conduction band are free to move through the material. In many scintillators the

light yield is increased by doping the bulk material (on the order of parts per thousand) with a

different atom which have allowed energy states within the band gap of the bulk lattice that lead

to scintillation emissions.

Figure 7 Doping Electron Shells within the Band Gap of the Bulk Material [14]

These dopant sites have been shown to increase light yields within materials significantly and

allow for selection of emission wavelengths which are better matched to the PMT used for

measurement.

3.1.2 Poisson Limits

The number of scintillation events which will be detected can be expressed using Poisson

statistics. The probability of detecting k scintillation photons in a given interval is expressed:

𝑃(𝑘𝑒𝑣𝑒𝑛𝑡𝑠𝑖𝑛𝑖𝑛𝑡𝑒𝑟𝑣𝑎𝑙) = 𝑒/b𝜆d

𝑘!

3.1

Where:

𝜆 = Mean detected counts in the interval

The variance of counts, k, is equal to the mean, 𝜆. As the mean increases this distribution more

closely approximates a normal distribution with mean and standard deviation equal to 𝜆. The

17

resolution of a gamma spectrum is commonly expressed as the FWHM (full width at half

maximum). The FWHM of a normal distribution can be solved analytically and is shown in

Equation 3.2 [15]:

𝐹𝑊𝐻𝑀 = 2√2 ln 2 ≈ 2.355𝜎

3.2

For this reason, the number of photons which are collected will directly affect the resolution of

the gamma spectrum which can be achieved. In practice scintillators perform worse than this

measured limit by several percent as shown using over one hundred published measurements

[16] as shown in Figure 8 and Figure 9.

Figure 8: Statistical Limit of FWHM of Poisson Events Based on Quantity Detected Overlaid with Reported Values from [16]: Color indicates peak emission wavelength. Legend depicts chemical formula and reference

as designated in [16].

18

Figure 9: Statistical Limit of FWHM of Poisson Events Based on Quantity Detected Overlaid with Reported

Values from [16]

Some of the deviation from the Poisson limit is due to inefficiencies in converting the

scintillation emission into a signal which can be measured. The scintillation emissions are

commonly collected using photomultiplier tubes (PMTs) which consist of a photocathode and a

series of amplifying dynodes. The photocathode is a material with a low work function which

will allow electrons to be freed from the surface from scintillation emission photons that are

incident. These freed electrons are called photoelectrons. The photoelectrons are then amplified

in the dynodes of the PMT and are able to linearly amplify the signal on the order of 1 – 10

million times. Through this amplification a few photoelectrons are capable of generating

sufficient currents for measurement. The quantum efficiency, 𝜂, of the PMT photocathode is

defined as:

19

𝜂 =𝑁𝑢𝑚𝑏𝑒𝑟𝑜𝑓𝑝ℎ𝑜𝑡𝑜𝑒𝑙𝑒𝑐𝑡𝑟𝑜𝑛𝑠𝑒𝑚𝑖𝑡𝑡𝑒𝑑

𝑁𝑢𝑚𝑏𝑒𝑟𝑜𝑓𝑖𝑛𝑐𝑖𝑑𝑒𝑛𝑡𝑝ℎ𝑜𝑡𝑜𝑛𝑠

This efficiency varies with the wavelength of the incident photons. Common photocathode

materials have efficiencies below 25% at their highest efficiencies, which are generally most

efficient between 300 – 400 nm.

Due to these inefficiencies, the photoelectrons which can be measured will be fewer than the

photons emitted. Figure 10 shows the resolution variation with number of photoelectrons

(𝜂 × 𝑁𝑢𝑚𝑏𝑒𝑟𝑜𝑓𝐼𝑛𝑐𝑖𝑑𝑒𝑛𝑡𝑃ℎ𝑜𝑡𝑜𝑛𝑠 ) which were measured for numerous detection materials.

Figure 10 Measured Energy Resolutions and Theoretical Poisson Statistical Resolution Limit for a Variety of Scintillation Materials [9]

3.1.3 Scintillation Characteristics for Gamma Spectroscopy

Material properties which are commonly of interest for scintillation gamma detector materials

include [16]:

• Density (g/cm3)

• Luminosity (photons/MeV)

20

• Decay Time (ns)

• Emission Peak (nm)

• Energy Resolution (FWHM %)

These values will be measured for the material samples considered in this investigation.

21

4 Charge Collection Based / Solid State Gamma Detection

Charge collection or solid-state detectors are another important method of detecting and

identifying gamma radiation. This chapter will provide an introduction and overview of the

mechanisms which result in charge collection in solid state detectors as well as some examples

of device materials and configurations which are of use presently. Additionally, important

quantities which can be measured which quantify the suitability of a material as a solid-state

charge collection material will also be discussed and will be investigated for the materials in the

present work.

4.1.1 Charge Collection Mechanisms

The band structure of materials as introduced in Section 3.1.1 are equally important for charge

collection. Electrons which are excited to the conduction band (excess electron) have a

corresponding hole (electron deficit) which is created in the valence band. Since these electron-

hole pairs are free to move through the lattice, in the presence of an applied bias these electrons

will move in the direction of positive bias, and the holes will move in the direction of the

negative bias. This moving charge generates a current that can be measured, which is

proportional to the energy absorbed in the material.

In a conductor the highest energy band that is occupied is not full. Electrons need only small

increases in energy to be freed from their lattice position and move through the material. A

simulated band structure for gold is shown in Figure 11 demonstrating the absence of a gap in

the allowed energies in the crystal lattice as well as the presence of a band gap in silicon between

roughly 5 and 6 eV. An electron must increase in energy the by greater than or equal to the band

gap energy to move in the material.

22

Figure 11: Band structure of Au (left) and Si (right) calculated using density functional theory (DFT) [11]

[12] [13]

The population of electrons at given energies can be expressed as a function of the material

purity, the temperature of the material, and the number of allowed states at each energy. This

relationship is expressed [17]:

𝑁(𝐸)𝑑𝐸 = 𝑃(𝐸)𝑆(𝐸)𝑑𝐸

4.1

Where:

• 𝑁(𝐸)𝑑𝐸 = number of electrons per unit volume with energy between 𝐸 and 𝐸 + 𝑑𝐸

• 𝑆(𝐸)𝑑𝐸 = number of allowed electronic energy states, per unit volume, in the energy

interval between 𝐸 and 𝐸 + 𝑑𝐸

• 𝑃(𝐸) = probability that a state of energy E is occupied

𝑃(𝐸) is defined by the Fermi Distribution Function and is expressed:

𝑃(𝐸) = 1

1 + 𝑒yz/z{|

d}~

4.2

Where

• 𝐸� = Fermi Energy, constant dependent on purity of solid

23

• 𝑘𝑇 = Boltzmann constant times temperature

As can be seen, the population probability is dependent upon the temperature as well as the

Fermi Energy of the material. When the probability of occupation extends to states which exist

in the conduction band the material will conduct. In semiconductors the Fermi energy exists

within the band gap of the material, which implies that at T=0 the material would have no

conductivity. At higher temperatures there will be occupation in the conduction band and the

material will conduct. An example of the temperature dependency of the conduction band is

shown in Figure 12.

Figure 12 Temperature Influence on Conduction of Material with Bandgap, 𝑬𝒈 and Fermi Energy, 𝑬𝒇

A common issue with semi-conductor materials is the inherent conductivity of the material from

thermal excitations that produce leakage current in the detector and increase the noise floor. For

this reason, some semiconductor detectors are cooled in order to decrease the thermally

generated conductivity. A mature detector material that is commonly used at liquid nitrogen

temperatures is high purity Germanium (HPGe). This material has excellent gamma resolution

(less than 1% FWHM) but requires that the detector be constantly under cryogenic conditions.

24

Figure 13 Density of states calculation for Germanium showing conductivity at room temperature and not at cryogenic temperatures

When an electron is excited to the conduction band of the material, an electron deficit (hole)

remains in the valence band. In the presence of an electric field, this electron will migrate in

opposite directions from the hole. The velocity, 𝜐, which the electron and the hole are able to

move through the material are proportional to the applied field, 𝔼, and characteristic of the

material:

𝜐� = 𝜇�𝔼 𝜐3 = 𝜇3𝔼

4.3

The proportionality constant is called the electron or hole mobility and is commonly expressed in

units of 𝑐𝑚B𝑉𝑠* . The pulse shape which will be measured is influenced by the differing

electron and hole mobility in the material as well as the distance the electrons and holes must

travel before reaching a collection surface. Figure 14 shows the time evolution of the location of

electrons and holes within the detector material for an event which generated near the center of

the detector.

25

Figure 14 Electron-hole Pair migration in Material (left) Initial electron hole creation (right) Migration of

Electrons and Holes through material over time

Electron hole pairs which initiated near the V+ surface would lead to immediate collection of the

electrons, and the longest possible collection time for holes. Figure 15 shows an ideal induced

charge time signal for specific values of mobility and initial location within the detector volume.

Figure 15 Sample Induced Charge Pulse Shape Due to Differing Electron Hole Mobilities

Based upon the material purity and other factors the electrons or holes may become trapped in

the material or recombine before becoming collected. When this occurs the electron-hole pair

will no longer be contributing to conduction within the material and will be lost for measurement

26

purposes. Since the electron and hole mobilities are not identical, the amount of recombination

which occurs is a function of the distance which electrons and holes must traverse through the

material and will result in pulse shapes which vary depending on the location in the material

where the electron-hole pairs were generated. A key parameter which characterizes the quality

of a spectroscopic radiation semiconductor is the mobility-lifetime product, 𝜇𝜏 [18]. For planar

devices which will be explored in this study, the charge movement which the device caused by

the absorbed radiation can be stated from the Shockley-Ramo Theorem : [19] [20] expressed in

Equation 4.4:

𝑑𝑄∗ = −𝑒𝑁@𝑊

(𝑑𝑥|3 + 𝑑𝑥|�)

4.4

Where:

• 𝑄∗ is the induced charge

• 𝑁@ is the initial number of electron-hole pairs

• W is the semiconductor width

• 𝑑𝑥|3 is the motion of the electrons

• 𝑑𝑥|� is the motion of the holes

In semiconductors the presence of defects will result in some of these charge carriers being

trapped within the material during their motion. Excited electrons created near the V+ boundary

will be trapped in the material less frequently than those created near the V- boundary. The same

reasoning exists for holes. The Hecht relation [21] describes this relationship of position

dependence upon the charge collected and is express in Equation 4.5:

27

𝑄∗ = 𝑒𝑁@ �𝔼𝜇�𝜏�𝑊 �1 − 𝑒/

�𝔼����* � +

𝔼µ3𝜏3𝑊 �1 − 𝑒/

�/�𝔼����* ��

4.5

Where:

• 𝜇�𝜏� is the electron mobility lifetime for holes

• 𝜇3𝜏3 is the electron mobility lifetime for electrons

• 𝔼 is the applied electric field

• 𝑊 is the detector width

• 𝑒 is the charge of an electron

• 𝑥 is the position of the charge carrier within the medium

In order to obtain the measurement of each mobility, 𝜇3/�, radiation which is absorbed near the

electron or hole collection surface can be used. A modification of Equation 4.4 can then be

employed only considering a single charge carrier species as shown in Equation 4.6 [22]

𝜇 =𝑊B

𝔼Δ𝑡

4.6

If it is known that the electron-hole pairs are generated near a collection surface the time from

baseline to peak in an event collection is sufficient for estimation of the electron/hole mobility.

With knowledge of the mobility, a curve fit can be applied for a measured event with Equation

4.5 in order to estimate the mobility life time values. An example of this fit being applied from

[22] is shown in Figure 16.

28

Figure 16 Measured Induced Charge fit to Hecht Relation for Mobility Lifetime Measurement (from [21])

29

5 Dual Mode Collection Several works including [23] [24] have shown that liquid Xenon and liquid Argon allow for

simultaneous collection of scintillation emissions and charge. In addition, these works have

shown a relationship between the collected scintillation and charge as a function of the field

strength in the material. The field strength directly influences the migration of electron hole

pairs used in charge collection. As the electric field in the material increased the quantity of

scintillation emission photoelectrons decreased, and the collected current increased. An

analytical expression for this relationship was derived and was fit to experimental results as

expressed in Equation 5.2.

𝑆(𝐸)𝑆@

=1 + 𝑁3� 𝑁�* − 𝑄(𝐸) 𝑄@*

1 + 𝑁3� 𝑁�* − 𝜒

5.1

Where:

• 𝑆@: the charge yield normalized to the charge at infinite electric field.

• 𝜒: the fraction of electrons which do not recombine even at zero field

• 𝑆(𝐸): is the scintillation emission collection at a given electric field strength, 𝐸.

• 𝑁3�𝑁�* : the ratio of excitons to ion pairs produced.

• 𝑄(𝐸): the collected charge at a given electric field.

• 𝑄@: the energy deposited by the 𝛾-ray divided by the average energy required to produce

an electron hole pair in the material

Empirical agreement was found in both liquid Argon and liquid Xenon to this expression. The

agreement to liquid Xenon is shown in Figure 17.

30

Figure 17: Measurements of the Anti-Correlation of Charge and Light Collection in Liquid Xenon (from [25])

Later work in 2007 by Aprile et.al. [25] expanded upon this relationship to demonstrate that a

significantly improved energy resolution can be achieved by combining these anti-correlated

measurements. For measurements on liquid Xenon using 137Cs a standard deviation of 10.3%

was measured for scintillation and a standard deviation of 4.7% was measured for charge. The

standard deviation of the spectrum after combining these measurements was found to be 1.7%,

demonstrating the significant improvement over both methods taken separately.

Measurements from this study depicting the separate and combined spectral resolution of this

method are shown in Figure 18.

31

Figure 18 Spectral Resolution Improvement from Charge and Light Combination (from [25])

The method used for combining will also be applied and alternatives will be explored in the

present work.

Charge and light measurements were combined by defining and angle, 𝜃, which represents the

slope of the correlation between the light and charge measurements. This value was calculated

from the 137Cs peak. The combined signal was then calculated using Equation 5.2:

𝜀� =sin 𝜃 𝜀� + cos 𝜃 𝜀�sin 𝜃 + cos 𝜃

5.2

The resolution which can be achieved with this method is calculated:

32

𝑅�B =sin 𝜃B 𝑅�B + cos 𝜃B 𝑅�B + sin 𝜃 cos 𝜃 𝑅�𝑅�𝜌��

(sin 𝜃 + cos 𝜃)B

5.3

Where:

• 𝑅� is the combined resolution

• 𝑅� is the scintillation resolution

• 𝑅� is the charge resolution

• 𝜌�� is the correlation coefficient between charge and scintillation measurements

• 𝜃 is angle of the correlation between charge and scintillation measurements (as shown in

Figure 18)

Figure 19 Summary of results from Aprile et. al. showing measurement trends with varying electric field

Data from these measurements showed that as electric field increased, though the resolution of

the scintillation and the charge measurements both improved slightly, the correlation between the

33

measurements decreased in magnitude. This decrease in correlation coefficient resulted in lower

resolution combined measurements as shown in Figure 19. Results from these prior

investigations will be used in this work to provide estimates of the resolutions of room

temperature materials which may be achievable (see Section 7.5.2).

34

6 Prior Work on Sample Materials The material samples being investigated in this work have prior work published as solid-state

detectors. This section will provide context of the data which has been collected and analyzed

for Hg2Br2 and CdMgTe.

6.1 Prior Work on Mercurous Bromide

Mercurous Halides were developed for use in polarization optical applications in the 1970s [26]

[27] and later investigated (in addition with numerous other compound powders) for infrared

scintillation properties in work by Moses et.al. [28] in 1998, in which Mercurous Bromide,

Hg2Br2, was shown to have a mild scintillation emission yield.

Work by Brimrose demonstrated samples of Mercurous Bromide could be fabricated to

dimensions on the order of 2 inches by 3 inches using physical vapor transport (PVT) in 2008

[29] with intended applications in acousto-optic devices.

Figure 20 Sample of Single Crystal Mercurous Bromide with 2" diameter made by PVT [29]

In this work the optical transmission characteristics of Mercurous Bromide were measured and

description was given on manufacturing processes which resulted in large single crystal samples

being developed with very few defects and Hg/HgO impurities.

35

Figure 21 Transmission spectrum measured through 48 mm Mercurous Bromide Sample (from [29])

This work was later expanded for use in gamma detection. In 2016 capabilities of growing high

purity Mercurous Halide materials for use as solid-state gamma detectors was shown [30]. In

2016 Brimrose filed a patent application for radiation detectors with the Hg2X2 halogens [31]

describing device configurations and reporting charge collection spectral responses of below 2%

as shown in in Figure 22. Resolutions for this material have been reported as low as 0.48% at

662 keV by the manufacturer. Figure 22 is a logarithmic spectral response graph of 6 mm

pixelated Hg2Br2 material under 1000 V bias at room temperature. The sample was exposed to

137Cs gamma rays. A 662 keV gamma ray peak is clearly resolved at 1.8% FWHM without depth

of interaction (DOI) correction [31].

36

Figure 22: Room Temperature Hg2X2 charge collection spectra reported in [31]

The scintillation emission spectrum for this material has not been reported but has been

conducted in this investigation (see Figure 45). The emission existence in conjunction with the

existing high-resolution charge measurements make this material promising for dual mode

collection applications. Additionally, the material dimensions which have been demonstrated

(several inches) provide a useful material volume for high resolution room temperature devices.

6.2 Prior Work on CdMgTe

Brimrose also has existing work on a STTR (Small Business Technology Transfer) program in

which CdMgTe was investigated for use in radioisotope identification applications. In this work

a Frisch grid detector with ohmic contacts was constructed and tested. A gamma resolution of

3.4% FWHM at 662 keV was found at room temperature [30]. Additionally, due to the

application of the Frisch grid the collected spectrum provided a method to measure the mobility

life time, µ3𝜏3, which was found to be 5.3 ×10/¡ 𝑐𝑚B𝑉* . The experimental results for this fit

are shown in Figure 23.

37

Figure 23 Electron Mobility Lifetime found for CdMgTe by fitting to Hecht Relation

CdMgTe also has shown a scintillation emission spectrum, though the material is less transparent

to this scintillation than Hg2Br2 (limiting useable scintillation and dual mode dimensions).

Brimrose has developed fabrication capabilities of depositing transparent Iridium Tin Oxide

(ITO) electrodes on these CdMgTe samples. For this reason, this material is being explored due

to its early availability for dual mode collection.

38

7 Proposed Work This section provides an overview of the samples and experimental setups which will be used in

this investigation.

7.1 Samples

Numerous samples will be fabricated and used in this investigation. Several samples have been

fabricated and are presently available for testing. The summary of samples which are on-hand is

summarized in Table 1.

Table 1 Test Samples Inventory

Material Purpose Quantity Dimensions (mm)

Finish

Hg2Br2 Scintillation 3 5x5x1 Polished, no electrodes

Hg2Br2 Scintillation 1 5x5x2 Polished, no electrodes

Hg2Br2 Scintillation 1 4x4x4 1 face polished, 5 faces coated with TiO2

CdMgTe Charge Collection 2 7x4x1 Electrodes on opposing 7x4 faces

39

Figure 24: Image of current inventory of three Hg2Br2 samples (top left) and 2 CdMgTe samples prior to application of electrodes

Figure 25 Charge Collection Samples of CdMgTe with Gold Electrodes

Additional samples will be required for the completion of this work. These samples are being

discussed and being placed in the assembly queue with the material manufacturer. Samples in

the fabrication queue are shown in Table 2

Table 2 Test Sample Fabrication Queue

Material Purpose Finish Status

40

Hg2Br2 Charge Collection Electrodes In process

Hg2Br2 Scintillation Transparent Electrodes (ITO)

In process

CdMgTe Charge Collection Polished, no electrodes In process

CdMgTe Dual Mode Transparent Electrodes (ITO)

Active development

Brimrose has experience and proven capability of application of transparent electrodes to

CdMgTe materials using Iridium Tin Oxide (ITO), which will allow for scintillation collection

out of the same faces for which charge collection is conducted. Presently ITO applied to Hg2Br2

does not remain attached to the crystal. This manufacturing process is being investigated by the

manufacturer presently. If transparent electrodes cannot be successfully achieved in Hg2Br2

samples will be fabricated with gold electrodes on opposing faces. The two additional pairs of

opposing faces will be polished and coated with a reflective material such as TiO2 respectively.

An illustration of these samples is shown in Figure 26

Figure 26 Dual Mode Samples without Transparent Electrodes

Due to the small dimensions of the samples which have been obtained, simulations have been

performed to ensure that the electron stopping distance within the material is sufficiently short in

41

order for a photoelectron peak to be present. If the electron stopping distance is larger than the

material dimensions, photo-electrons will leave the material without depositing the full gamma-

ray energy and a photoelectron peak cannot be measured for gamma-spectroscopy.

Electron stopping distances were calculated using a NIST database (ESTAR [10]) and it was

found that the 1mm dimension is larger than the continuous slowing down approximation

(CSDA) range for the material as shown in Figure 27.

Figure 27 CSDA Range Estimates for Electrons in Hg2Br2

An additional simulation was performed using Monte Carlo N-Particle code (MCNP) using a

5x5x1 mm sample of Hg2Br2 experiencing a flux of 137Cs gamma spectrum. A spectrum of

energy deposited in the sample material was calculated and is shown in Figure 28 indicating that

the material dimensions should be sufficient for full energy deposited (photopeak) within the

material.

42

Figure 28 MCNP Simulation Results Indicating Full Energy Absorption Peak in 5mm x 5mm x 1mm Hg2Br2

Sample

Samples are being handled with gloves to prevent contact with heavy metals, and to protect the

samples from oils. Presently these samples are not being hermetically sealed, though the risk of

oxidation is known as stated in [29] “Hg2Cl2 and Hg2Br2 are extremely difficult to purify,

because in the presence of trace impurities they very easily oxidize.” Further care into sealing the

samples may be required.

7.2 Scintillation Measurements

This section describes the experimental setup which will be used for measuring the scintillation

spectra of the materials being investigated.

7.2.1 Visible Spectrum

A visible spectrum has been measured during prior work on several materials which are grown

by Brimrose. It was found that the emission spectra for Hg2Br2 has an emission peak near 800

nm and CdMgTe has an emission peak near 650 nm as shown in Figure 29. [ADD

43

REFERENCE] Add image of scintillating materials. REFERENCE IMAGE BELOW

Figure 29 Scintillation Light Yield Spectra

44

In order to measure as many photoelectrons from the scintillation of these materials, a PMT

photocathode which is compatible with this emission has been selected. The PMT photocathode

which is most compatible and is commercially available is a Tri-Alkali. The quantum efficiency

for Bi-Alkali and Tri-Alkali has been plotted and compared with the emission spectra of Hg2Br2

in Figure 30.

Figure 30 Emission Spectrum of Hg2Br2 and the Quantum Efficiency Spectrum of Two Common PMT

Photocathodes

As can be seen, even the Trialkali photocathode will be inefficient at converting scintillation

photons into photoelectrons which can be collected. Though inefficient this photocathode

selection will result in significantly (~70X) higher measurements than Bialkali would provide.

the Trialkali PMT appears to be the appropriate selection for both Mercurous Bromide and

CdMgTe.

7.2.2 Gamma Spectrum Measurements

For gamma spectrum measurements the sample will be optically coupled to the PMT face.

Optical grease will be used only if necessary, to avoid altering the samples. The PMT will be

biased to achieve a gain of 1e6 to 3e6 using a LV control over the PMT base.

45

Figure 31: PMT Divider Gains [32]. 2520AN used in this work is an implementation of Divider A [33]

The signal coming from the PMT will have preamplification applied as needed using an Ortec

113. This signal will then be shaped and amplified using an Ortec 672. This signal will then be

measured using an oscilloscope.

Figure 32: Hardware setup for Scintillation Collections

46

Figure 33 Scintillation Collection Configuration (left) Expanded Assembly (right) Cross section of assembled

Later work will also be performed to increase the collected light using a dual PMT configuration.

Signal routes will be duplicated of a second PMT on the opposing face of the sample.

Figure 34 Equipment setup which will be used for dual PMT collections

A 3D printed housing has been designed and constructed which will:

• position the PMTs on either side of the sample,

• hold radiation sources at a fixed position,

• provide electrical connections for charge collection (discussed in Section 7.4), and

47

• provide a light barrier.

This fixture is shown in Figure 35.

Figure 35 Fixture used for Dual PMT and Dual mode collections

7.3 Charge Measurements

Charge measurements will be conducted on these samples in order to determine the gamma

spectrum resolution which can be achieved for the materials. Additionally, the charge only setup

will provide for maturation of experimental setup prior to attempting dual mode collections. For

these setups, a custom fixture has been designed which will attach to an SHV connector. A 3D

printed fixture with two copper electrodes will be attached to an SHV connector inside of a

Faraday cage to reduce signal noise. The images showing the fixture which has been designed

are shown in Figure 36.

48

Figure 36 Charge Collection Fixture: (top left) exterior of fixture (top right) 3D printed fixture without

faraday cage (bottom left) cutaway shown fixture (bottom left) cross section view of charge collection fixture

Within this fixture the sample will be placed between the electrical contacts, and the faraday

cade will be sealed. The SHV connector will then be connected using 93Ω cable to an Ortec 142

Preamplifier. The amplified signal will then be shaped (as needed) and run to an oscilloscope.

The setup block diagram for these measurements is shown in Figure 37.

Figure 37: Components for Charge Collection Setup

49

Software control is present on the HVPS to allow for automated collections at a range of electric

fields within the material. This will allow for efficient spectral collection over a range of applied

biases. Signals from the charge collection will have curve fitting applied to determine the rise

and fall time of the signals. Various thickness samples can be tested and compared to allow for

calculation of charge mobility lifetimes.

Initial indications from this fixture show that there may be issues with noise coupled from the

testing environment. For this reason, an additional test fixture is being built which will reduce

any loop area for induced charge. This additional fixture consists of a tapped brass pipe which

will thread directly to the return of SHV or BNC connector. The center pin of the connector will

be soldered to a gold plated pogo pin which will connect to the electrode of the sample. A copper

return can will then close the return path.

Figure 38 Charge Collection Fixture for Reduced Electrical Noise (left) Isotropic cross section showing

assembled fixture (top right) cross section of assembly (bottom right) exterior view of fixture

Several views of the charge fixture under construction are shown in Figure 38.

7.4 Dual Collection Measurements

Simultaneous collection of charge and scintillation emission will be achieved on samples

fabricated with electrodes which are transparent to the scintillation emission. Brimrose has

shown capability at manufacturing these samples using CdMgTe material and is presently

50

developing methods for transparent electrodes on Hg2Br2 samples. Samples of this type will be

placed between two PMTs and electrodes will be attached to a high voltage connection. The

fixture for this work is shown in Figure 35.

Both PMT’s and the AC coupled charge collection measurements will be collected on a single ≥

500 MHz oscilloscope. For each detected event three waveforms will be collected and analyzed

Figure 39 Experimental setup for dual mode collection

These signals will then be analyzed and combined in order to produce gamma spectra. Required

modifications to this fixture are anticipated in order to reduce noise on charge collection spectra

based on preliminary results on charge spectrum.

7.5 Analysis Methodology

This section provides a description of the analysis which will be performed on measured data in

order to determine the gamma-spectrum for the materials being investigated and estimates of the

combined resolution which may be expected based on results which have be measured at the

time of this report.

51

7.5.1 Spectrum Generation

Due to the requirements for correlated spectra from multiple (charge, and one or more

scintillation) data channels multi-channel analyzers which are available for this work are not

sufficient for making these measurements. For this reason, custom software has been developed

by the author which generates these measurements using oscilloscope data. This software is able

to acquire up to 1000 events per second and can store every measurement and/or waveform for

post analysis. The user interface is shown in Figure 40.

Figure 40 User Interface for Custom Collection Software During 137Cs Scintillation Collection

For each channel numerous measurements are calculated including:

• Integrated area (Volt Seconds)

• Peak Value (Volts)

• 10 – 90 % Rise Time

• 90 – 10% Fall Time

52

During the course of investigation additional parameters may be added to this list which are

determined to be relevant. Each event is then logged for post processing. When the dual PMT

setup is used, a calibration factor for each channel will be determined using a photopeak from a

check source. The calibrated channel values can then be combined if it proves to be

advantageous.

Charge and scintillation measurements will be combined using Equation 5.2. A histogram will

then be generated using these derived values.

Analysis will also be performed in order to determine if there are subsets of the data which result

in higher resolution measurements as performed with detection of incomplete charge collections

[34]. Methods applied to CdZnTe detectors for using pulse shape to identify events in which

electron mobility resulted in incomplete energy collections will also be applied in this study [35].

53

Figure 41 Pulse Shape Identification of Incomplete Charge Collection Due to Defocusing Fields (from [35])

For this study, delay times between the scintillation and charge signals, rise and fall times will be

measured and analyzed as well. It is expected that delay time between scintillation and charge

signals will remain relatively fixed between events but will be investigated.

7.5.2 Preliminary Combined Spectrum Estimates

From initial measurements of spectral resolutions for charge (1.8% ) and scintillation (14%) that

have been made (Sections 0 and 8.1.1) estimates of the combined spectra have been conducted

using Equation 5.3. Since the correlation coefficient, 𝜌��, and the angle, 𝜃, between the two

spectra is unknown for this material a range of values has been considered. For 𝜌�� a range of -

0.2 to -1.0, and for 𝜃 a range of 0˚ to 30˚ has been plotted in Figure 42.

54

Figure 42: Potential Optimal Resolutions for Hg2Br2, based on Equation 5.3.

As shown, higher magnitude correlation coefficients will lead to improved optimal resolution of

the material. Also, the angle of interaction, 𝜃, will have a significant influence over the

performance. The optimal angle (though it is not obvious how this could be influenced) can also

be solved for analytically from 5.3 as follows:

𝜕𝑅�𝜕𝜃 = 0

7.1

Solving this for 𝜃 gives the angle which would lead to the minimum combined resolution, 𝜃∗, and is expressed:

𝜃∗ = tan/¦ §−𝑅�(𝑅�𝜌�� − 𝑅�)𝑅�(𝑅� − 𝑅�𝜌��)

¨

7.2

Plugging Equation 7.2 into Equation 5.3 provides a relationship for the minimum combined resolution for a material:

55

𝑅�|©ª©∗ = «−𝑅�B𝑅�By𝜌��B − 1|

𝑅�B + 𝑅�B − 2𝑅�𝑅�𝜌��

7.3

The prior work on combining these spectra found that Liquid Xenon had a measured value for 𝜃

of 24.8˚ and a value of 𝜌�� of -0.87 [25]. These values were very near 𝜃∗ as is shown in Figure

43.

Figure 43: Experimental Relationship between Optimal Angle and Correlation Coefficient

For the present study, if it is found that the correlation coefficient, 𝜌��, has an amplitude larger

than ~0.8 the and correlation angles which are within ~3˚ of 𝜃∗, the values for 𝑅� and 𝑅� which

have been achieved to date will provide a room temperature detector which is below 1% at room

temperature. This resolution would significantly exceed the performance of existing room

temperature detectors (LaBr3:3% and CdZnTe:2%).

56

8 Results This section will document the scintillation, charge, and dual mode collection results for

Mercurous Bromide and CdMgTe materials.

8.1 Hg2Br2

8.1.1 Scintillation Measurements

Using the configuration described in Section 7.2.2 scintillation waveforms have been acquired

and show thousands of photo electrons being collected per event. Example waveforms into a

50Ω and 1MΩ termination are shown in Figure 44.

Figure 44 Example Scintillation Events into 50𝛀 (left) and 1M𝛀 (right)

These signals have then been shaped into semi-gaussian waveforms with a shaping time of 10µs

and collected using an Amtec 8000A MCA and using the software analysis developed for this

work which generates gamma spectrum from oscilloscope waveforms as described in Section

7.5.1.

57

Figure 45: Scintillation Spectrum for 137Cs measured on Hg2Br2 Sample (left) Spectra Collected with and without the source present (right) FWHM estimates made on background subtracted spectrum.

This measurement provides the first scintillation spectrum measured for this material and is a

promising result for use in dual mode collections which will be conducted.

8.1.2 Charge Collection

Charge collection measurements have not yet been conducted on Mercurous Bromide

8.1.3 Dual Mode Collection

Dual Mode collection measurements have not yet been conducted on Mercurous Bromide

8.2 CdMgTe Results

8.2.1 Scintillation Collection

Scintillation Collection measurements have not yet been conducted on CdMgTe.

8.2.2 Charge Collection

Charge collection measurements have not yet been conducted on CdMgTe

8.2.3 Dual Mode Collection

Dual Mode collection measurements have not yet been conducted on CdMgTe

58

9 Conclusion This work will focus on the exploration of Mercurous Bromide and CdMgTe as high-resolution

room temperature detectors which can be grown to moderate volumes (several cubic inches)

through dual collection gamma spectral collection. This work seeks to provide a viable path for

room temperature high resolution detectors which may exceed fabrication size limits of the

current state of the art materials with similar or better spectral resolutions.

59

10 Works Cited

[1] G. S. Cmarda, A. E. Bolotnikov, Y. Cui, A. Hossain, K. T. Kohman and R. B. James, "CdZnTe room-temperature semiconductor gamma-ray detector for national-security applications," in IEEE Long Island Systems, Applications and Technology Conference, Farmingdale, NY, 2007.

[2] Y. Cui, A. Bolotnikov, G. Camarda, A. Hossain, G. Yang and R. B. James, "CZT Virtual Frish-grid Detector: Principles and Applications," Brookhaven National Laboratory, BNL-82046-2009-CP, Farmingdale, NY, 2009.

[3] Ortec, "Why High-Purity Germanium (HPGE) Radiation Detection Technology is Superior to Other Detector Technologies for Isotope Identification," [Online]. Available: https://www.ortec-online.com. [Accessed 21 March 2019].

[4] J. Hubbell, "X-Ray Mass Attenuation Coefficients," NIST Standard Reference Database 126, 2004.

[5] G. F. Knoll, Radiation Detection and Measurment, Wiley & Sons, 2010.

[6] R. Evans, The Atomic Nucleus, McGraw-Hill Book Company, 1955.

[7] J. Apostolakis, "GEANT4 Low Energy Electromagnetic Models for Electrons and Photons," CERN-OPEN-XX, 1999.

[8] R. Evans, The Atomic Nucleus, New York: Krieger, 1982.

[9] B. M.J., "Stopping-Power & Range Tables for Electrons, Protons, and Helium Ions," NIST Standard Reference 124, 2017.

[10] M. Berger, J. H. Hubbell, S. M. Seltzer, J. Chang, J. S. Coursey, R. Sukumar, D. S. Zucker and K. Olsen, NIST Standard Reference Database 8 (XGAM), NIST, PML, Radiation Physics Division.

[11] A. J. Hautier, C. Moore, S. P. Ong, C. C. Fischer, T. Mueller, K. A. Persson and G. Ceder, "A High-Throughput Infrastructure for Density Functional Theory Calculations," Computational Materials Science, vol. 50, pp. 2295-2310, 2011.

60

[12] A. Jain, S. P. Hautier, S. P. Ong, C. Moore, C. C. Fischer, K. A. Persson and G. Ceder, "Accurate Formation Enthalpies by Mixing GGA and GGA+U Calculations," Physical Review, vol. 84, no. B, p. 045115, 2011.

[13] S. P. Ong, S. Cholia, A. Jain, M. Brafman, D. Gunter, G. Ceder and K. A. Persson, "The Materials Application Programming Interface (API)," Computational Materials Science, vol. 97, pp. 209-215, 2015.

[14] V. J. S. Bora, Photon Statistics in Scintillation Crystals, PhD Dissertation, University of Arizona, 2015.

[15] E. W. Weisstein, "Gaussian Function," MathWorld - A Wolfram Web Resource, [Online]. Available: http://mathworld.wolfram.com/GaussianFunction.html.

[16] S. Derenzo, M. Bozwell, M. Weber and K. Brennan, "Scintillation Properties," [Online]. Available: http://scintillator.lbl.gov.

[17] N. Tsoulfanidis and S. Landsberger, "Semiconductor Detectors," in Measurement and Detection of Radiation, CRC Press, 1938-, pp. 217-240.

[18] S. Uxa, R. Grill and E. Belas, "Evaluation of the mobility-lifetime product in CdTe and CdZnTe Detectors by the Transient Current Technique," Journal of Applied Physics, vol. 114, no. 094511, 2013.

[19] W. Schokley, "Currents to Conductors Induced by a Moving Point Charge," Journal of Applied Physics, vol. 9, no. 635, 1938.

[20] S. Ramo, "Currents Induced by Electron Motion," in Proceedings of I.R.E., 1939.

[21] G. Knoll, "General Operation of Planar Devices," in Radiation Detection and Measurment, Wiley and Sons, 2010, p. 489.

[22] G. Arino-Estrada and et. al, "Measurement of Mobility and Lifetime of Electrons and Holes in a Schottky CdTe Diode," JINST, vol. 9, no. C12032, 2014.

[23] S. Kubota, A. Nakamoto, T. Takahashi and T. Hamada, "Recombination Luminescence in Liquied ARgon and in Liquid Xenon," Physical Review, vol. 17, no. 6, pp. 2762-2765, 1978.

[24] M. Ichige, E. Aprile, T. Doke, K. Hasuike, K. Itoh, J. Kikuchi and K. Masuda, "Measurement of Attenuation Length of Drifiting Electrons in Liquid Xenon," Nuclearl Instruments and Methods in Physics Research, vol. A, no. 333, pp. 355-363, 1993.

61

[25] E. Aprile, K. L. Giboni, P. Majewski, K. Ni and M. Yamashita, "Observation of Anti-correlation between Scintillation and Ionization for MeV Gamma-Rays in Liquid Xenon," Physics Review, vol. B, no. 76, p. 014115, 2007.

[26] C. Barta and C. J. Barta, "Physcial Properties of Single Crystals of the Calomel Group (Hg2X2: X=Cl, Br)," Material Science Forum, vol. 61, pp. 93-150, 1990.

[27] C. Barta, "Preparation of Mercurous Chloride Monocrystals," Kristall und Technik, vol. 5, no. 4, pp. 541-549, 1970.

[28] W. W. Moses, M. J. Weber, S. E. Derenzo, D. Perry, P. Berdahl and L. A. Boatner, "Prospects for Dense, Infrared Emitting Scintillators," IEEE Transactions on Nuclear Science, vol. 45, no. 3, pp. 462-466, 1998.

[29] J.-S. Kim, S. Trivedi, J. Soos, N. Gupta and W. Palosz, "Growth of Hg2Cl2 and Hg2Br2 single crystals by physical vapor transport," Journal of Crystal Growth, vol. 310, pp. 2457-2463, 2008.

[30] S. B. Trivedi, S. W. Kutcher, W. Palzoz, M. Berding and A. Burger, "Next Generation Semiconductor-Based Radiation Detectors Using Cadmium Mangesium Telluride," Department of Energy/11172015-Final, 2015.

[31] H. Chen, J.-S. Kim, F. Jin and S. Trivedi, "Detection of Nuclear Radiation Via Mercurous Halides". United States Patent 0216384, 28 July 2016.

[32] E. T. Ltd., "9113B PMT Datasheet," [Online]. Available: http://et-enterprises.com/images/data_sheets/9113B.pdf. [Accessed 9 March 2019].

[33] E. T. Ltd., "HV2520AN Datasheet," 9 March 2019. [Online]. Available: http://et-enterprises.com/images/data_sheets/HV2520AN.pdf.

[34] A. E. Bolotnikov, G. S. Camarda, Y. Cui and S. Egarievwe, "Array of virtual Frisch-grid CZT detectors with common cathode and pulse-heigth correction," in SPIE, 2010.

[35] A. Bolotnikov, "Performance Characteristics of Frish-Grid CdZnTe Detectors," in Brookhaven Science Associates, Instrument Division Seminar, Upton, 2004.

[36] W. Moses, "Current Trends in Scintillator Detectors and Materials," Nuclear Instruments and Methods in Physics Research, vol. 487, no. A, pp. 123-128, 2002.

[37] P. M. Amarasinghe, J.-S. Kim, H. Chen, S. Trivedi, S. B. Qadri, J. Soos, M. Diestler, D. Zhange, N. Gupta, J. L. Jensen and J. Jensen, "Growth of high quality mercurious halide

62

singel crystals by physical vapor transport method for AOM and radiation detection applications," Journal of Crystal Growth, vol. 450, pp. 96-102, 2016.

[38] K. E. Nelson, T. B. Gosnell and D. A. Knapp, "The effect of energy resolution on the extraction of information content from gamma-ray spectra," Nuclear Instruments and Methods in Physics Research A, vol. 659, no. A, pp. 207-214, 2011.