Influence of transport properties on energy resolution of planar TlBr and CdZnTe gamma-ray detectors: Monte-Carlo investigation
The response of TlBr- and CdZnTe-detectors to gamma-rays from ⁵⁷Со and ¹³⁷Cs was simulated by Monte-Carlo method via Geant4 package. We studied the influence of transport parameters of electrons and holes on energy resolution of detectors. The modification of photopeaks with a changing the ratio...
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| Zitieren: | Influence of transport properties on energy resolution of planar TlBr and CdZnTe gamma-ray detectors: Monte-Carlo investigation / A.I. Skrypnyk, M.A. Khazhmuradov // Вопросы атомной науки и техники. — 2014. — № 4. — С. 134-138. — Бібліогр.: 6 назв. — англ. |
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irk-123456789-803452015-04-17T03:01:57Z Influence of transport properties on energy resolution of planar TlBr and CdZnTe gamma-ray detectors: Monte-Carlo investigation Skrypnyk, A.I. Khazhmuradov, M.A. Физика радиационных и ионно-плазменных технологий The response of TlBr- and CdZnTe-detectors to gamma-rays from ⁵⁷Со and ¹³⁷Cs was simulated by Monte-Carlo method via Geant4 package. We studied the influence of transport parameters of electrons and holes on energy resolution of detectors. The modification of photopeaks with a changing the ratio of the electron and hole mobilitylifetime products was investigated. All results obtained for TlBr detectors were compared with the results for CdZnTe-detectors. The efficiency for detecting gamma-quanta in the range of energies from 10 keV to 3 MeV by both kinds of detector was researched. С помощью пакета Geant4 методом Монте-Карло промоделирован отклик TlBr- и CdZnTe-детекторов на гамма-излучение от ⁵⁷Со и ¹³⁷Cs . Изучено влияние транспортных свойств электронов и дырок на энергетическое разрешение детекторов. Исследована модификация фотопиков с изменением отношения произведений подвижности на время жизни для электронов и дырок. Все результаты, полученные для TlBr-детекторов, были сравнены с результатами для CdZnTe-детекторов. Исследована эффективность обеих систем детектирования гамма-квантов в диапазоне энергий от 10 кэВ до 3 МэВ. За допомогою пакета Geant4 методом Монте-Карло було промодельовано відгук TlBr- і CdZnTe-детекторів на гамма-випромінювання від ⁵⁷Со і ¹³⁷Cs. Вивчено вплив транспортних параметрів електронів і дірок на енергетичну роздільність детекторів. Досліджено модифікацію фотопіків зі зміною відношення добутків рухливості на час життя для електронів і дірок. Всі результати, які було отримано для TlBr-детекторів, було порівняно з результатами для CdZnTe-детекторів. Досліджено ефективність обох систем детектування гамма-квантів у діапазоні енергій від 10 кеВ до 3 МеВ. 2014 Article Influence of transport properties on energy resolution of planar TlBr and CdZnTe gamma-ray detectors: Monte-Carlo investigation / A.I. Skrypnyk, M.A. Khazhmuradov // Вопросы атомной науки и техники. — 2014. — № 4. — С. 134-138. — Бібліогр.: 6 назв. — англ. 1562-6016 PACS: 29.40.Wk, 85.30De, 07.85.-m http://dspace.nbuv.gov.ua/handle/123456789/80345 en Вопросы атомной науки и техники Національний науковий центр «Харківський фізико-технічний інститут» НАН України |
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Digital Library of Periodicals of National Academy of Sciences of Ukraine |
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Физика радиационных и ионно-плазменных технологий Физика радиационных и ионно-плазменных технологий |
| spellingShingle |
Физика радиационных и ионно-плазменных технологий Физика радиационных и ионно-плазменных технологий Skrypnyk, A.I. Khazhmuradov, M.A. Influence of transport properties on energy resolution of planar TlBr and CdZnTe gamma-ray detectors: Monte-Carlo investigation Вопросы атомной науки и техники |
| description |
The response of TlBr- and CdZnTe-detectors to gamma-rays from ⁵⁷Со and ¹³⁷Cs was simulated by Monte-Carlo
method via Geant4 package. We studied the influence of transport parameters of electrons and holes on energy
resolution of detectors. The modification of photopeaks with a changing the ratio of the electron and hole mobilitylifetime
products was investigated. All results obtained for TlBr detectors were compared with the results for
CdZnTe-detectors. The efficiency for detecting gamma-quanta in the range of energies from 10 keV to 3 MeV by
both kinds of detector was researched. |
| format |
Article |
| author |
Skrypnyk, A.I. Khazhmuradov, M.A. |
| author_facet |
Skrypnyk, A.I. Khazhmuradov, M.A. |
| author_sort |
Skrypnyk, A.I. |
| title |
Influence of transport properties on energy resolution of planar TlBr and CdZnTe gamma-ray detectors: Monte-Carlo investigation |
| title_short |
Influence of transport properties on energy resolution of planar TlBr and CdZnTe gamma-ray detectors: Monte-Carlo investigation |
| title_full |
Influence of transport properties on energy resolution of planar TlBr and CdZnTe gamma-ray detectors: Monte-Carlo investigation |
| title_fullStr |
Influence of transport properties on energy resolution of planar TlBr and CdZnTe gamma-ray detectors: Monte-Carlo investigation |
| title_full_unstemmed |
Influence of transport properties on energy resolution of planar TlBr and CdZnTe gamma-ray detectors: Monte-Carlo investigation |
| title_sort |
influence of transport properties on energy resolution of planar tlbr and cdznte gamma-ray detectors: monte-carlo investigation |
| publisher |
Національний науковий центр «Харківський фізико-технічний інститут» НАН України |
| publishDate |
2014 |
| topic_facet |
Физика радиационных и ионно-плазменных технологий |
| url |
http://dspace.nbuv.gov.ua/handle/123456789/80345 |
| citation_txt |
Influence of transport properties on energy resolution of planar TlBr and CdZnTe gamma-ray detectors: Monte-Carlo investigation / A.I. Skrypnyk, M.A. Khazhmuradov // Вопросы атомной науки и техники. — 2014. — № 4. — С. 134-138. — Бібліогр.: 6 назв. — англ. |
| series |
Вопросы атомной науки и техники |
| work_keys_str_mv |
AT skrypnykai influenceoftransportpropertiesonenergyresolutionofplanartlbrandcdzntegammaraydetectorsmontecarloinvestigation AT khazhmuradovma influenceoftransportpropertiesonenergyresolutionofplanartlbrandcdzntegammaraydetectorsmontecarloinvestigation |
| first_indexed |
2025-07-06T04:18:41Z |
| last_indexed |
2025-07-06T04:18:41Z |
| _version_ |
1836869770630135808 |
| fulltext |
134 ISSN 1562-6016. ВАНТ. 2014. №4(92)
INFLUENCE OF TRANSPORT PROPERTIES ON ENERGY
RESOLUTION OF PLANAR TlBr AND CdZnTe GAMMA-RAY
DETECTORS: MONTE-CARLO INVESTIGATION
A.I. Skrypnyk, M.A. Khazhmuradov
National Science Center “Kharkov Institute of Physics and Technology”, Kharkov, Ukraine
E-mail: belkas@kipt.kharkov.ua
The response of TlBr- and CdZnTe-detectors to gamma-rays from
57
Со and
137
Cs was simulated by Monte-Carlo
method via Geant4 package. We studied the influence of transport parameters of electrons and holes on energy
resolution of detectors. The modification of photopeaks with a changing the ratio of the electron and hole mobility-
lifetime products was investigated. All results obtained for TlBr detectors were compared with the results for
CdZnTe-detectors. The efficiency for detecting gamma-quanta in the range of energies from 10 keV to 3 MeV by
both kinds of detector was researched.
PACS: 29.40.Wk, 85.30De, 07.85.-m
INTRODUCTION
For many years, investigation of wide band-gap
semiconductors (CdZnTe, TlBr, HgI2 and other) is
directed to the development of gamma-ray detectors
working at room temperatures without additional
cooling. However, some features of these
semiconductor materials create problems in determining
a detector’s main operating characteristics. Considerable
non-uniformity of electrophysical characteristics of
single-crystals is one of the most important factors
restraining progress in achieving this goal. The most
unstable characteristics include specific resistance of
detector and product of mobility µ and mean drift time τ
for electrons and holes – (µτ)e,h (transport parameters of
charge carriers). Planar gamma-ray detectors based on
wide band-gap semiconductors have considerable
spread of (µτ)e,h values even if they are produced from
one ingot [1]. At the same bias voltage, Ub, the
characteristics of such detectors with the same sizes
such as sensitivity to the registered radiation δ and
charge collection efficiency (CCE) will be different.
Experiments conducted by Suzuki et al. [1] showed that,
for example, in CdZnTe single-crystals, the ratio of
charge transport parameters for electrons and holes
within the same ingot may vary profoundly from 10 to
100. Furthermore, modification of the ingot’s (μτ)e,h
product can be due to technological processing of the
material during manufacturing into a detector, or may
result from the accumulation of defects during growth
or operation [2].
For a study of features of room-temperature
semiconductor devices which are manufactured for
detecting nuclear radiation and measuring the
characteristics of the radiation fields Monte-Carlo
simulation can be used. A computer experiment helps to
overcome the difficulties that are present as at the study
of features of semiconductors as at the development of
detectors based on them. In the present work, we
studied the influence of the (µτ)e,h products and
(µτ)e/(µτ)h ratio on the spectroscopic characteristics of
CdZnTe- and TlBr-detectors using Geant4 simulation
package. The detailed Monte-Carlo investigation
allowed us to model the response function of planar
spectrometers in the gamma-ray energy range to 3 MeV.
The dynamics of response function of TlBr-detectors for
gamma-ray energies of 122, 136 (
57
Co source) and
661.7 keV (
137
Cs source) was explored and compared
with the dynamics of response function of CdZnTe-
detectors for the same gamma-ray energies. We
presented how change of (µτ)e/(µτ)h ratio influences on
the characteristics of all simulated photopeaks. Also, we
investigated the dynamics of theoretical energy
resolutions of TlBr- and CdZnTe-detectors.
1. DESCRIPTION OF THE MODEL
We simulated the passage of gamma-quanta through
the detector by Monte-Carlo method via the user
program code described detail in [3], embedded in
Geant4 package – universal toolkit for the simulating
the passage of charged particles, neutrons and gamma-
quanta through matter. The simulation procedure is
divided into 2 parts. Initially, the program calculates the
value of the ionization energy, Ei, transferred to the
detector by the absorbed gamma-quantum with the
initial energy of Eγ. Then we calculate the value of
charge induced on the detector’s contacts for every
interacted photon. The computer model of the detector
is approximated as much as possible to a real
spectrometric device. It takes into account the statistical
effects of pair generation within the detector’s volume
and the modification in the amplitude of the output
pulse under the influence of the electronic noise and
charge-carrier capture [3].
To verify the described model we applied
experimental data from 6×6×3 mm planar Cd0.9Zn0.1Te-
detectors, equipped with ohmic contacts. The bias
voltage, Ub, was 300 V. The electron mobility-lifetime
product ()e was selected as 3∙10
–3
cm
2
/V. We
specified the total level of noise in the CdZnTe
spectrometry systems (Equivalent Noise Charge – ENC)
at about 300 e
–
(electron charge units). The detector’s
dark current was taken as 3 nA. CdZnTe-detector was
irradiated by
137
Cs. Fig. 1 presents examples of
calculated and experimental response functions of
investigated CdZnTe-detectors from
137
Cs source.
Overall, it is evident that used model is in good
agreement with the experimental measurements.
ISSN 1562-6016. ВАНТ. 2014. №4(92) 135
Fig. 1. 661.67 keV photopeak
137
Cs spectrum
2. ANALYSIS OF EFFICIENCY OF CHARGE
COLLECTION
One of the main problems of wide band-gap
semiconductor detectors results from considerable
spread of (μτ) values for electron and holes. Transport
parameters for electrons and holes directly influence on
charge collection efficiency. The change in the charge
collection efficiency, by turn, leads to distortions to the
pulse height spectrum.
Therefore, to interpret correctly the investigated
spectra it is necessary to analyze CCE [3]. We will
consider uniformity of distribution of electric field
within detector. In this case, the efficiency of charge
collection in the planar detector irradiated by gamma-
quanta from the negative contact is described by Hecht
model [4]:
2
2
( , , , , , , )
1 exp
1 exp .
ind
e e h h
gen
be
be
bh
bh
Q
z d U
Q
U d z d
Ud
U zd
Ud
(1)
Here, η is the charge-collection efficiency; Qind is the
charge induced on the detector contacts; Qgen is the
average charge created at absorption energy Ei,
Qgen = Ei/ε; d is the detector’s thickness; and z is the
depth of the gamma-quantum interaction within the
detector’s material (0 < z < d).
In the following, we suppose that the values of ()e,
Ub, and d are constant. We use the notations
be
e
U
d
corresponding to the electron’s mean-
free-path which is assumed constant in this analysis, and
, 0h
e
. Then, equation (1) can be
rewritten in the equivalent form [3]:
( , ) 1 exp
1 exp .
e
e
e
e
d z
z
d
z
d
(2)
The first derivative of equation (2)
const
,
z
d z
d
is
equal to
( , )
expe e
z const e
d z z z
d d d d
. (3)
The first derivative (3) is positive in the whole range,
0
e
z
, so that the efficiency of charge collection is a
monotonically increasing function of the ratio and
respectively, ()h. It is correct for all z in the range
from 0 to d.
3. INFLUENCE OF TRANSPORT
PARAMETERS OF TlBr-
AND CdZnTe- DETECTORS ON ITS
SPECTROSCOPIC CHARACTERISTICS
In the present work, we simulated 2.7×2.7×2 mm
planar TlBr-detectors equipped with ohmic contacts.
Given thickness of the TlBr crystal was selected as
typical for detectors based on this material [5]. TlBr-
detector with such thickness gives higher efficiency for
detecting gamma-quanta with 122 and 136 keV energies
from
57
Co ( 97 and 93%, respectively) compared
with lower thickness detector (Fig. 2).
Fig. 2. Efficiency of gamma-ray TlBr-detectors with
different thicknesses
In the range of energies of gamma-quanta less than
1 МeV, the gamma-ray detector efficiency of 2 mm
TlBr-detector slightly exceeds this efficiency of 3 mm
CdZnTe-detector (Fig. 3). At the same time, as evident
from Fig. 3, efficiency of detection of 661.7 keV
gamma-quanta from
137
Cs radioactive source does not
exceed 15% for both materials with above mentioned
thicknesses.
136 ISSN 1562-6016. ВАНТ. 2014. №4(92)
Fig. 3. Comparison of gamma-ray detector efficiency
for TlBr- and CdZnTe-detectors
Fig. 4 shows the transformation of
57
Co spectrum
obtained by simulation of TlBr-detector with decreasing
value. The bias voltage, Ub, was 400 V. We specified
the total level of noise in the TlBr spectrometry systems
at about 400 e
–
. It was assumed that the detector’s dark
current was 3 nA. We considered the material with
values of the electron and hole mobility of 30 and
4 cm
2
/(V·s), respectively, which reflect the measured µτ
products for electrons and holes [5].
Fig. 4. Transformation of
57
Co spectrum obtained by
TlBr-detector with decreasing value
The electron mobility-lifetime was fixed at
5∙10
–4
cm
2
/V. The mobility-lifetime values for holes
were varied from 1∙10
–4
[5] to 5∙10
–6
cm
2
/V.
10
7
gamma-quantum trajectories were simulated. From
Fig. 4, we observe a little shift of the centroids of
122 and 136 keV photopeaks in the direction of lower
energy with decreasing a value of κ i. e. with decreasing
a value of (μτ)h at const (μτ)e. Moreover, the theoretical
energy resolution of the studied TlBr-detector for
gamma-quantum energy at 122 keV declines from 5.7 to
7.6% with increasing 1/κ value from 5 to 100. 136 keV
photopeak tends to full degeneration. 14.4 keV
photopeak corresponded to third principal gamma-ray
line of
57
Co almost does not change.
Spectrum of gamma-quanta from
57
Co source
obtained by simulation of TlBr-detector was compared
with such spectrum received by simulation of CdZnTe-
detector [3].
Fig. 5. Transformation of
57
Co spectrum obtained by
CdZnTe-detector with decreasing value
From Figs. 4 and 5, it is evident that these spectra of
gamma-quanta obtained by both detectors have similar
tendency to decreasing and broadening of 122 and
136 keV photopeaks and similar shape in this range of
energies with decreasing a value of hole mobility-
lifetime product. The theoretical energy resolution of
CdZnTe-detector for gamma-quantum energy at
122 keV drops from 1.8 to 2.3% when the value of 1/κ
changes from 10 to 60. The spectrum obtained by
CdZnTe also has almost constant 14.4 keV photopeak
(it is not shown in Fig. 5). This reflects the fact that the
depth of absorption of the main part of the gamma-
quanta with 14.4 keV energy in both TlBr- and
CdZnTe-materials is near to a hole-drift-length even at
the worst (μτ)h values. Qualitative changes in the
simulated spectra of CdZnTe-detectors agree with the
experimental data of Sato et al. [6].
However, irradiation of investigated TlBr-detector
by gamma-quanta from
57
Co source gives high enough
peaks in the spectrum in the range between about
30…50 keV in contrast with CdZnTe-detector. The
centroids of these photopeaks also are shifted in the
direction of lower energy with decreasing a value of κ.
We suppose that they are escape peaks corresponding to
gamma-radiation from K-shells of Tl. Also, our
simulation results agree with results of real experiment
for TlBr-detectors [5].
Basing on the data of the computer experiment, it
can be concluded that for gamma-quantum energies less
than 150 keV, planar CdZnTe-detectors of 3 mm
thickness retain satisfactory spectrometric properties in
the ratio range ()e/()h below 30. Energy resolution
of the investigated TlBr-detectors of 2 mm thickness
approximately in two times worse compared with
CdZnTe-detectors in this range of gamma-quanta
energies. It agrees with experimental data for TlBr-
detectors which are obtained to the present time.
Fig. 6 presents the simulated spectrum of gamma-
quanta from
137
Cs source obtained for investigated TlBr-
detector.
ISSN 1562-6016. ВАНТ. 2014. №4(92) 137
Fig. 6. Transformation of
137
Cs spectrum obtained by
TlBr-detector with decreasing value
Because of low efficiency for detecting the gamma-
quantum energy at 661.7 keV we needed to increase the
number of simulated gamma-quantum trajectories of
137
Cs source compared with
57
Co to 10
8
to collect
statistics. The theoretical energy resolution of the
investigated TlBr-detector at a gamma-ray energy of
661.7 keV drops from 2.6 to 4.8%, when the value of
1/κ changes from 5 to 20.
From Fig. 6 we observe faster degeneration of
661.7 keV photopeak in TlBr-compared with 122 keV
photopeak. These results agree with results of
simulation of CdZnTe-detector irradiated by
137
Cs
source. Fig. 7 shows the changes that occur around the
661.7 keV photopeak with the simulated spectrum of
the
137
Cs source for a CdZnTe-detector [3]. The value of
1/ = 20 can be considered as the threshold level. The
theoretical energy resolution of the investigated
CdZnTe-detector at 661.7 keV declines from 1.1 to
1.5% in the range of 1/ values from 10 to 20. The
planar CdZnTe-detectors with higher value of 1/ are
unsuitable for the spectrometry of high-energy gamma-
quanta, because even low accumulation of radiation
traps can lead to the disappearance of the photopeak.
Fig. 7. Transformation of
137
Cs spectrum obtained
by CdZnTe-detector with decreasing value
The faster degeneration of the 661.7 keV photopeak
in CdZnTe- and TlBr-detectors compared with the
122 keV photopeak is connected with the fact that in the
simulated detectors the interaction of 122 keV gamma-
quanta within the detector material mainly occurs in the
first one-third of its thickness. At the same time,
gammas with energy of 661.7 keV uniformly interact
with detector throughout its entire thickness. The
efficiency of charge collection, Eq. (1), depends on
interaction depth. Therefore, decreasing the hole-drift-
length relative to the electron-free path more strongly
reduces (, z) and the pulse amplitude at greater
depths. The full absorption cross-section of CdZnTe and
TlBr is small in the energy region Eγ above 100 keV.
Therefore, this small total pulse-number from the full
absorption of 661.7 keV gamma-quantum is spread
therewith over a wider range of amplitudes.
It was determined that planar CdZnTe-detectors
theoretically can ensure an energy resolution of better
than 2% at 661.7 keV provided that the value of 1/ is
less than 20. In the range 1/ from 20 to 60, the
detector’s resolution quickly deteriorates to 10…12%
after the complete disappearance of the 661.7 keV
photopeak.
From simulation of TlBr-detectors, we concluded
that in the range 1/κ less than 10 we may receive energy
resolution better than 3%. At a value of 1/κ more than
45 the 661.7 keV photopeak cannot be observed by
TlBr-detector.
CONCLUSIONS
Basing on the mentioned model, response of
semiconductor TlBr-detectors to gamma-quanta from
57
Co and
137
Cs sources was simulated and then was
compared with response of CdZnTe-detectors to
gamma-quanta from the same sources. The detection
efficiency of both materials was investigated. We
conclude that TlBr- and CdZnTe-detectors have the
same detection efficiency in the range of energies to
60 keV. At gamma-ray energies of 122 and 136 keV
(from
57
Co source) TlBr-detector has higher detection
efficiency compared with CdZnTe-detector. For
661.7 keV energy for both investigated detectors the
given value does not exceed 15%.
We determined that investigated detectors have
similar tendency to broadening of 122, 136 and
661.7 keV photopeaks and deterioration of energy
resolution with decreasing a value of (μτ)h at a constant
value of (μτ)e. It is concluded that the spectroscopic
properties of the both kinds of detector are maintained
when the range of the (μτ)e/(μτ)h ratio is below 20. For
CdZnTe if the (μτ)e/(μτ)h ratio is above 60, then the
661.7 keV photopeak cannot be observed for planar
detectors, even with very low levels of electronic noise.
In the case of TlBr-detectors, complete degeneration of
661.7 keV photopeak is observed at a little less
(μτ)e/(μτ)h ratio. These criteria establish quality-growth
requirements for spectrometric TlBr- and CdZnTe-
materials.
ACKNOWLEDGEMENTS
We are very grateful to A.V. Rybka and V.E. Kutny
for providing the experimental data for CdZnTe-
detector and their help in the analysis of obtained
results.
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K. Nakazawa, T. Takahashi, Y. Okada, H. Takahashi,
A. Parsons, S. Barthelmy, J. Cummings, N. Gehrels,
D. Hullinger, H. Krimm, J. Tueller. Hard X-ray
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Статья поступила в редакцию 28.05.2014 г.
ВЛИЯНИЕ ТРАНСПОРТНЫХ СВОЙСТВ НА ЭНЕРГЕТИЧЕСКОЕ РАЗРЕШЕНИЕ
ПЛАНАРНЫХ TlBr- И CdZnTe-ДЕТЕКТОРОВ ГАММА-ИЗЛУЧЕНИЯ:
ИССЛЕДОВАНИЕ МЕТОДОМ МОНТЕ-КАРЛО
А.И. Скрыпник, М.А. Хажмурадов
С помощью пакета Geant4 методом Монте-Карло промоделирован отклик TlBr- и CdZnTe-детекторов на
гамма-излучение от
57
Со и
137
Cs. Изучено влияние транспортных свойств электронов и дырок на
энергетическое разрешение детекторов. Исследована модификация фотопиков с изменением отношения
произведений подвижности на время жизни для электронов и дырок. Все результаты, полученные для TlBr-
детекторов, были сравнены с результатами для CdZnTe-детекторов. Исследована эффективность обеих
систем детектирования гамма-квантов в диапазоне энергий от 10 кэВ до 3 МэВ.
ВПЛИВ ТРАНСПОРТНИХ ВЛАСТИВОСТЕЙ НА ЕНЕРГЕТИЧНУ РОЗДІЛЬНІСТЬ
ПЛАНАРНИХ TlBr- ТА CdZnTe-ДЕТЕКТОРІВ ГАММА-ВИПРОМІНЮВАННЯ:
ДОСЛІДЖЕННЯ МЕТОДОМ МОНТЕ-КАРЛО
А.І. Скрипник, М.А. Хажмурадов
За допомогою пакета Geant4 методом Монте-Карло було промодельовано відгук TlBr- і
CdZnTe-детекторів на гамма-випромінювання від
57
Со і
137
Cs. Вивчено вплив транспортних параметрів
електронів і дірок на енергетичну роздільність детекторів. Досліджено модифікацію фотопіків зі зміною
відношення добутків рухливості на час життя для електронів і дірок. Всі результати, які було отримано для
TlBr-детекторів, було порівняно з результатами для CdZnTe-детекторів. Досліджено ефективність обох
систем детектування гамма-квантів у діапазоні енергій від 10 кеВ до 3 МеВ.
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