CZT device with improved sensitivity for medical imaging and homeland security applications
Author(s):
H. Chen;
S. A. Awadalla;
P. Marthandam;
K. Iniewski;
P. H. Lu;
G. Bindley
Show Abstract
A special CdZnTe (CZT) device on THM grown crystal has been developed. The device has different work function
metals on opposite electrodes yet operates at room temperature like a conventional back-to-back symmetric MSM
detector and not a one directional Schottky diode device. Aiming at creating a big breakthrough in CZT imaging device
technology, the special CZT device presented in this study is capable of increasing the photopeak count by up to 50%
compared to conventional CZT imaging device while maintaining good room temperature energy resolution by not
significantly trading off detector leakage current. Pixel pad size and interpixel gap on a 20x20x5 mm3, 8x8 pixel pattern
that result in optimum detector efficiency and interpixel resistance are presented. Sensitivity improvement impact on
other device configuration will also be discussed. The design is highly practical, reliable and suitable for mass
production.
Dual anode contact geometries for x-ray and gamma-ray spectroscopy and 3D localization
Author(s):
J. W. Martin;
A. B. Garson III;
Q. Li;
K. Lee;
M. Groza;
V. Buliga;
A. Burger;
H. Krawczynski
Show Abstract
We report on the continued development and testing of unique types of Cadmium Zinc Telluride (CZT) detectors.
Using large volume (10×20×20 mm3) CZT crystals, we contact various "dual anode" detector designs. We
incorporate a segmented cathode with five regions so that the charge on all seven contacts can be used to
determine the energy and the 3-D interaction location of detected X-ray and gamma-ray photons. We describe
the status of the detector development program, emphasize strengths and weaknesses of the different contact
configurations, and discuss possible applications of Dual Anode Detectors in radiation detection applications.
Front-end ASIC for pixilated wide bandgap detectors
Author(s):
Emerson Vernon;
Gianluigi De Geronimo;
Jack Fried;
Cedric Herman;
Feng Zhang;
Zhong He
Show Abstract
A CMOS application specific integrated circuit (ASIC) was developed for 3D Position Sensitive
Detectors (PSD). The preamplifiers were optimized for pixellated Cadmium-Zinc-Telluride (CZT)
Mercuric-Iodide (HgI2) and Thallium Bromide (TlBr) sensors. The ASIC responds to an ionizing
event in the sensor by measuring both amplitude and timing in the pertinent anode and cathode
channels. Each channel is sensitive to events and transients of positive or negative polarity and performs
low-noise charge amplification, high-order shaping, peak and timing detection along with analog
storage and multiplexing. Three methodologies are implemented to perform timing measurement
in the cathode channel. Multiple sparse modes are available for the readout of channel data. The ASIC
integrates 130 channels in an area of 12 x 9 mm2 and dissipates ~330 mW. With a CZT detector
connected and biased, an electronic resolution of ~200 e- rms for charges up to 100 fC was measured.
Spectral data from the University of Michigan revealed a cumulative single-pixel resolution of ~0.55
% FWHM at 662 KeV.
Uniformity of charge collection efficiency in Frisch collar spectrometer with THM grown CdZnTe crystals
Author(s):
Alireza Kargar;
Adam C. Brooks;
Mark J. Harrison;
Henry Chen;
Salah Awadalla;
Glenn Bindley;
Bob Redden;
Douglas S. McGregor
Show Abstract
A 4.7×4.7×9.5 mm3 Frisch collar device was fabricated from CdZnTe materials grown by the Traveling Heater
Method (THM). The device was then characterized through probing with a highly collimated 662 keV gammaray
source of 137Cs. In a systematic series of experiments, the detector, at its best design, was probed using a
collimated 137Cs source. The results were confirmed through simulating the charge collection efficiency (CCE)
maps of the device under the operated condition. It is proved that, unlike the planar configuration, the charge
collection efficiency profile along the length of Frisch collar device is considerably improved. It is also shown
that enhancement in spectral performance occurs due to the application of the Frisch collar to a planar device.
This enhancement is due to the fact that the Frisch collar alters the nonuniform CCE profile in a planar device
to a more uniform distribution in a Frisch collar device. Additionally, a technique to optimize this uniform
distribution is investigated for a 5.0 × 4.7 × 19.6 mm3 Frisch collar device, while the experimental results
are confirmed though numerical simulation. Based on this technique, there exists an optimum dielectric layer
thickness for the CdZnTe Frisch collar device, for which the CCE profile has its best (most uniform) distribution
and shows its best spectroscopic performance. The CdZnTe materials for this study were grown by THM at
Redlen Technologies and the CdZnTe devices were fabricated and characterized at the S.M.A.R.T. Laboratory
at Kansas State University.
Characterization of a 15-mm-long virtual Frisch-grid CZT detector array
Author(s):
A. E. Bolotnikov;
S. Babalola;
G. S. Camarda;
Y. Cui;
S. U. Egarievwe;
P. M. Fochuk;
M. Hirt;
A. Hossain;
K. Kim;
O. V. Kopach;
N. D. Sferrazza;
J. Sturgess;
K. Polack;
B. Raghothamachar;
G. Yang;
R. B. James
Show Abstract
We detail our new results from testing an array of 15-mm long virtual Frisch-grid CdZnTe detectors with a cathode
signal readout-scheme intended to improve spectral response by correcting for electron trapping. We designed a novel
electrode configuration for these long-drift detectors that ensures an energy resolution close to the statistical limit, and
high detection efficiency. However, in reality, the quality of the crystals limits the performance of this type of device.
Here, we describe the characterization of the array, show our preliminary results obtained with gamma-ray sources, and
expound on their relation to our material-characterization data.
Simulation of electric field profile in semi insulating Au/CdZnTe/Au detector under flux
Author(s):
J. Franc;
R. Grill;
R. James;
J. Kubát;
E. Belas;
P. Moravec;
P. Hoschl;
P. Praus
Show Abstract
We report our simulations on the profile of the electric field in semi insulating CdTe and CdZnTe with Au contacts under
radiation flux. The type of the space charge and electric field distribution in the Au/CdTe/Au structure is at high fluxes
result of a combined influence of charge formed due to band bending at the electrodes and from photo generated carriers,
which are trapped at deep levels. Simultaneous solution of drift-diffusion and Poisson equations is used for the
calculation. We show, that he space charge originating from trapped photo-carriers starts to dominate at fluxes
1015-1016cm-2s-1, when the influence of contacts starts to be negligible.
Electric field distribution of cadmium zinc telluride (CZT) detectors
Author(s):
G. Yang;
A. E. Bolotnikov;
G. S. Camarda;
Y. Cui;
A. Hossain;
K. Kim;
R. B. James
Show Abstract
Cadmium Zinc Telluride (CZT) is attracting increasing interest with its promise as a room-temperature nuclear-radiationdetector
material. The distribution of the electric field in CZT detectors substantially affects their detection performance.
At Brookhaven National Laboratory (BNL), we employed a synchrotron X-Ray mapping technique and a Pockels-effect
measurement system to investigate this distribution in different detectors. Here, we report our latest experimental results
with three detectors of different width/height ratios. A decrease in this ratio aggravates the non-uniform distribution of
electric field, and focuses it on the central volume. Raising the bias voltage effectively can minimize such nonuniformity
of the electric field distribution. The position of the maximum electric field is independent of the bias voltage;
the difference between its maximum- and minimum-intensity of electric field increases with the applied bias voltage.
Dual gamma neutron detection with Cs[sub]2[/sub]LiLaCl[sub]6[/sub]
Author(s):
Jarek Glodo;
Rastgo Hawrami;
Edgar van Loef;
William Higgins;
Urmila Shirwadkar;
Kanai S. Shah
Show Abstract
Some applications, particularly in homeland security, require detection of both neutron and gamma radiation. Typically,
this is accomplished with a combination of two detectors registering neutrons and gammas separately. Recently, a new
scintillator, Ce doped Cs2LiLaCl6 (CLLC) that can provide detection of both has been investigated for gamma and
neutron detection. This material is capable of providing very high energy resolution, as good as 3.4% at 662 keV
(FWHM), which is better than that of NaI(Tl). Since it contains 6Li, it can also detect thermal neutrons. In the energy
spectra, the full energy thermal neutron peak appears near 3 GEE MeV. Thus very effective pulse height discrimination
can be achieved with this material. The CLLC emission consists of two main components: Core-to-Valence
Luminescence (CVL) spanning from 220 nm to 320 nm and Ce emission found in the range of 350 to 500 nm. The
former emission is of particular interest since it appears only under gamma excitation. It is also very fast, decaying with
a 2 ns time constant. This provides CLLC with different temporal responses under gamma and neutron excitation and it
can be used for effective pulse shape discrimination.
SrI[sub]2[/sub] scintillator for gamma ray spectroscopy
Author(s):
N. J. Cherepy;
B. W. Sturm;
O B. Drury;
T. A. Hurst;
S. A. Sheets;
L. E. Ahle;
C. K. Saw;
M. A. Pearson;
S. A. Payne;
A. Burger;
L. A. Boatner;
J. O. Ramey;
E. V. van Loef;
J. Glodo;
R. Hawrami;
W. M. Higgins;
K. S. Shah;
W. W. Moses
Show Abstract
We are working to perfect the growth of divalent Eu-doped strontium iodide single crystals and to optimize the design of
SrI2(Eu)-based gamma ray spectrometers. SrI2(Eu) offers a light yield in excess of 100,000 photons/MeV and light yield
proportionality surpassing that of Ce-doped lanthanum bromide. Thermal and x-ray diffraction analyses of SrI2 and EuI2
indicate an excellent match in melting and crystallographic parameters, and very modest thermal expansion anisotropy.
We have demonstrated energy resolution with SrI2(4-6%Eu) of 2.6% at 662 keV and 7.6% at 60 keV with small crystals,
while the resolution degrades somewhat for larger sizes. Our experiments suggest that digital techniques may be useful
in improving the energy resolution in large crystals impaired by light-trapping, in which scintillation light is re-absorbed
and re-emitted in large and/or highly Eu2+ -doped crystals. The light yield proportionality of SrI2(Eu) is found to be
superior to that of other known scintillator materials, such as LaBr3(Ce) and NaI(Tl).
Angular resolution obtained with a LaBr[sub]3[/sub]-based rotational modulator
Author(s):
Brent S. Budden;
Gary L. Case;
Michael L. Cherry
Show Abstract
A Rotational Modulator (RM) gamma ray imager, consisting of a single grid of lead slats rotating above an array
of detectors with diameter equal to the slat spacing, has the capability of providing angular resolution significantly
better than the geometric resolution (i.e., the ratio of detector diameter to mask/detector separation). The
sensitivity, weight, and angular resolution are comparable to that of a coded aperture device, but with significantly
less complexity. As the grid rotates, the transmission from a source is modulated on each detector between 0 and
100%. The count profile is cross-correlated with precalculated modulation profiles to produce an approximate
source image. Deconvolution of this image with the known imager response can accurately resolve point sources
and complex emissions. The appropriate deconvolution technique can achieve angular resolution better than
the basic geometrical resolution of the instrument. A prototype RM developed at Louisiana State University
features high sensitivity and energy resolution, functional angular resolution of 15, and a simple readout system.
The detector array consists of 19 1.5 × 1 thick cerium-doped lanthanum bromide (LaBr3:Ce) crystals. LaBr3
produces significantly more light than other common scintillators, offering < 3% FWHM energy resolution at 662
keV. A grid spaced ~1.2 m from the detection plane with slat width 1.5 offers a 13.8° field of view. We present
our reconstruction technique, deconvolution algorithms, and simulated and experimental imaging results.
Development of a novel energy-resolved photon-counting detector for gamma ray imaging applications
Author(s):
L.-J. Meng;
J. W. Tan;
K. Spartiotis;
T. Schulman
Show Abstract
In this paper, we present the design and preliminary performance evaluation of a novel energy-resolved photon-counting
(ERPC) detector for gamma ray imaging applications. The prototype ERPC detector has an active area of 4.4 cm x 4.4
cm, which is pixelated into 128 x 128 square pixels with a pitch size of 350 μm 350 μm. The current detector consists
of multiple detector hybrids, each with a CdTe crystal of 1.1 cm x 2.2 cm x 1 mm, bump-bonded onto a customdesigned
application-specific integrated circuit (ASIC). The ERPC ASIC has 2048 readout channels arranged in a 3264
array. Each channel is equipped with pre- and shaping-amplifiers, a discriminator, peak/hold circuitry, and an analog-todigital
converter (ADC) for digitizing the signal amplitude. In order to compensate for the pixel-to-pixel variation, two
8-bit DACs are implemented into each channel for tuning the gain and offset. The ERPC detector is designed to offer a
high spatial resolution, a wide dynamic range of 12-200 keV and a good energy resolution of 3-4 keV. The hybrid
detector configuration provides a flexible detection area that can be easily tailored for different imaging applications.
The intrinsic performance of a prototype ERPC detector was evaluated with various gamma ray sources, and the results
are presented.
High-speed photon counting processing for CdTe detector
Author(s):
B. Shinomiya;
A. Koike;
H. Morii;
T. Okunoyama;
Y. Neo;
H. Mimura;
T. Aoki
Show Abstract
Recently practical X-ray measurement systems are demanded energy distinction function. Photon-counting CdTe semiconductor detectors have a high energy resolution in a low count rate condition at room temperature. However, the energy resolution is decreased by pile-up phenomenon in a high count rate condition. In conventional signal processing, processing time estimated X-ray photon energy from the pulse waveform is about tens of microseconds. This time is depended on the pulse decay time. This paper purposes to maintain the high energy resolution by changing the signal-processing algorithm which derived the pulse rise height of the output waveform from the CdTe detector in a high count rate condition. As a result, the pulse rise time required to estimate the pulse rise height was short about 100ns at incident X-ray energy 60keV. As the result of energy spectrum by using this data, the FWHM of about 11keV (at 60keV) when the count rate of 500kcps. This result show the possibility that the photon counting sensor has application for the high count rate imaging without decrease of the high energy resolution.
Far-IR reflectance spectra analysis of CdZnTe and related materials
Author(s):
Tzuen-Rong Yang;
Sheng-Hong Jhang;
Yen-Hao Shih;
Fu-Chung Hou;
Yu-Chang Yang;
P. Becla;
Der-Chi Tien;
Zhe Chuan Feng
Show Abstract
Far-infrared (FIR) reflectance spectroscopy has been employed to study the optical properties for a
series of bulk CdZnxTe1-x and CdSexTe1-x wafers. The zone-centre optical phonons for the ternary
alloys show a variety of behavior patterns: they exhibit a "one-mode", "two-mode" or
"intermediate-mode" behavior depending on the vibration characteristics of the end binary members.
The CdSeTe called CST were found to be single-crystal with the zincblende structure. These four
samples labeled with CST5, CST15, CST25, and CST35, which correspond with the composition of Se,
5%, 15%, 25%, 35%, respectively. The intensity of CdTe-like TO band decays with x increasing, and
the peak position increases from 140 cm-1 to 145 cm-1. In the other hand, the intensity of CdSe-like TO
band grows with x increasing, and the peak position of CdSe-like TO band increases from 174 cm-1 to
181 cm-1. We use the model of dielectric function and using Least-Square fit to find the optical and
transport parameters. By the infrared spectra analysis, we found the conductivity of CdZnxTe1-x
increase with increasing of x value and the conductivity of CdSexTe1-x decrease with increasing of x
value.
Characterization of secondary phases in modified vertical Bridgman growth CZT
Author(s):
Martine C. Duff;
Kelvin G. Lynn;
Kelly Jones;
Zurong R. Dai;
John P. Bradley;
Nick Teslich
Show Abstract
CdZnTe or "CZT" crystals are highly suitable for use as a room temperature based spectrometer for the
detection and characterization of gamma radiation. Over the last decade, the methods for growing high quality CZT
have improved the quality of the produced crystals however there are material features that can influence the
performance of these materials as radiation detectors. For example, various structural heterogeneities within the CZT
crystals, such as twinning, pipes, grain boundaries (polycrystallinity), and secondary phases (SP) can have a negative
impact on the detector performance. In this study, a CZT material was grown by the modified vertical Bridgman
growth (MVB) method with zone leveled growth without excess Te in the melt. Visual observations of material from
the growth of this material revealed significant voids and SP. Samples from this material were analyzed using various
analytical techniques to evaluate its electrical properties, purity and detector performance as radiation spectrometers and
to determine the morphology, dimension and elemental /structural composition of one of the SP in this material. This
material was found to have a high resistivity but poor radiation spectrometer performance. It had SP that were rich in
polycrystalline aluminum oxide (Al2O3), metallic Te and polycrystalline CdZnTe and 15 to 50 μm in diameter. Bulk
elemental analyses of sister material from elsewhere in the boule did not contain high levels of Al so there is
considerable elemental impurity heterogeneity within the boule from this growth.
Synchrotron radiation x-ray absorption fine-structure and Raman studies on CdZnTe ternary alloys
Author(s):
Yu Li Wu;
Yen-Ting Chen;
Zhe Chuan Feng;
Jyh-Fu Lee;
P. Becla;
Weijie Lu
Show Abstract
The synchrotron radiation (SR) X-ray absorption fine-structure spectroscopy (XAFS) technology has been employed to
obtained Zn K-edge absorption spectra for Cd11-xZnxTe alloy with x = 0.03, 0.10, 0.20, 0.30, 0.40, 0.50 and 1.00. Their
Fourier transform spectra were analyzed, which have shown a bimodal distribution of bond lengths, suggesting distortion
of the Te sub-lattice, so that the linear interpolation is true only in an approximate sense. Synthetic CdZnTe crystals can
be used for the room temperature-based detection of gamma radiation. The radiation detection properties of CZT crystals
vary widely. A common defect found in most high-quality CZT crystals is Te secondary phases, often located along
grain boundaries. The secondary phases can be both large inclusions (>50 μm) and smaller precipitates (<50 μm). The
Te secondary phases distributed throughout the crystal can cause changes to the detector leakage current, resulting in
decreased radiation spectrometer performance. This set of Cd1-xZnxTe crystals were also measured by Raman scattering
at room temperature. The two observed peaks at about 125 and 145 cm-1 which can be assigned to Te A1 and E phonon
mode, respectively. The induced damage on the crystal surfaces by Raman laser has been discussed. It is suggested that
in the case of highly Zn doping CdZnTe crystals, the ZnTe bond were broken by laser exposing and then free Te atoms
are migrating to these heated areas which cause Te precipitate. Further, the results of the soft X-ray measurements have
been also presented and this part of the experimental data needs to do more penetrating analysis in the future.
Unseeded growth of CdZnTe:In by THM technique
Author(s):
Utpal N. Roy;
Stephen Weiller;
Juergen Stein;
Andrey Gueorguiev
Show Abstract
Travelling heater method (THM) has been a great success lately for the growth of large CdZnTe
crystals. In this presentation, indium doped CdZnTe crystals have been grown adapting travelling heater
method (THM) in vertical configuration, using three zone custom designed muffle furnace. Crystals have
been grown with different ampoule diameter and size to study the grain growth. Seedless single crystalline
CdZnTe:In crystals have been gown with 4 cm diameter weighing about 650 grams. Crystals have been
characterized by near IR imaging, both microscopic and full wafer. The average resistivity along the length
of the ingot was found to be about 109 ohm-cm. A resolution 3.2% was obtained at 662 keV. The effect of
annealing of the whole wafer in Cd-Zn alloyed vapor on the resistivity and on the Te precipitations will be
discussed.
The ECLAIRs telescope onboard the SVOM mission for gamma-ray burst studies
Author(s):
Henri Triou;
Bertrand Cordier;
Diego Gotz;
Stéphane Schanne;
Thierry Tourrette;
Pierre Mandrou;
Roger Pons;
Olivier Godet;
Nadège Remoué;
Didier Barret;
Jean-Luc Atteia;
Martine Jouret
Show Abstract
The X- and gamma-ray telescope ECLAIRs onboard the future mission for gamma-ray burst studies
SVOM (Space-based multi-band astronomical Variable Objects Monitor) is foreseen to operate in
orbit from 2014 on. ECLAIRs will provide fast and accurate GRB triggers to other onboard telescopes,
as well as to the whole GRB community, in particular ground-based follow-up telescopes. With its
very low energy threshold ECLAIRs is particularly well suited for the detection of highly redshifted
GRB. ECLAIRs consists of a X- and gamma-ray imaging camera (CXG) observing in a field of view
of 2 sr. The CXG is a 2D-coded mask imager with a 1024 cm2 detection plane made of 80 x 80 CdTe
pixels, sensitive from 4 keV to 250 keV, with imaging capabilities up to about 50 keV and a
localization accuracy better than 10 arcmin.
ECLAIRs includes also a triggering electronics which uses the CXG data and detects GRB as countrate
increases or the appearance of a new source in cyclic sky images. GRB alerts are transmitted to
observers within tens of seconds via a VHF network and all detected photons are available hours later.
In this talk we present the lastest ECLAIRs concepts, with emphasis on the expected performances.
A quantum-limited CMOS-sensor-based high-speed imaging system for time-resolved x-ray scattering
Author(s):
Brian Rodricks;
Boyd Fowler;
Chiao Liu;
John Lowes;
Lucas J. Koerner;
Mark W. Tate;
Sol M. Gruner
Show Abstract
The field of ultrafast x-ray science is flourishing, driven by emerging synchrotron sources (e.g., time-slice storage rings, energy recovery linacs, free electron lasers) capable of fine time resolution. New hybrid x-ray detectors are under development in order to exploit these new capabilities.
This paper describes the development of a 2160 x 2560 CMOS image sensor (CIS) system with a 6.5 µm pitch optimized for time-resolved x-ray scattering studies. The system is single photon quantum limited from 8 keV to 20 keV. It has a wide dynamic range and can operate at 100 Hz full-frame and at higher frequencies using a region-of-interest (ROI) readout. Fundamental metrics of linearity, dynamic range, spatial resolution, conversion gain, sensitivity and Detective Quantum Efficiency are estimated. Experimental time-resolved data are also presented.
Characterisation of an asymmetric AGATA detector
Author(s):
C. Unsworth;
A. J. Boston;
H. C. Boston;
S. Colosimo;
J. Cresswell;
M. R. Dimmock;
F. Filmer;
D. Judson;
S. Moon;
P. J. Nolan;
M. J. Norman;
M. Slee
Show Abstract
The Advanced GAmma Tracking Array (AGATA) is a next-generation gamma-ray spectrometer for nuclear
physics being developed as part of a Europe-wide collaboration. AGATA aims to vastly improve upon the
sensitivity of today's arrays by removing the BGO shields used to suppress the Compton background and
instead, tracking gamma rays through a complete 4π shell of Germanium using Gamma Ray Tracking (GRT).
In order to facilitate this, Pulse Shape Analysis (PSA) must be used to accurately locate the position of each
gamma-ray interaction within each detector.
The preferred approach to PSA relies on the generation of a database of typical pulse shapes produced by
interactions at each position on a grid throughout the detector. This paper details current progress at the
University of Liverpool toward validating the electric field simulation, which will be used to generate the pulse
shape database, with experimental data from an asymmetric AGATA detector. The field simulation is discussed
and some comparisons are made between this and a two dimensional raster scan of the detector with a highly
collimated source.
Neutron detector based on TimePix pixel device with micrometer spatial resolution
Author(s):
Jan Jakubek;
Stanislav Pospisil;
Josef Uher
Show Abstract
The semiconductor pixel detector Timepix (256 x256 pixels, 55 um pitch) is successor of the Medipix2 device. The
detector is adapted for neutrons by deposition of a converter layer onto its surface. Thermal and cold neutrons are
converted into heavy charged particles in a layer containing 6Li or 10B. Fast neutrons are detected via protons, again
heavy charged particles, recoiled from a polyethylene convertor. The heavy charged particles are detected by the
Timepix with submicron spatial resolution. An important advantage of this detection technique is the full suppression of
gamma background which is always accompanying experiments with neutrons.
Neutron detection with single crystal organic scintillators
Author(s):
Natalia P. Zaitseva;
Jason Newby;
Sebastien Hamel;
Leslie Carman;
Michelle Faust;
Vincenzo Lordi;
Nerine J. Cherepy;
Wolfgang Stoeffl;
Stephen A. Payne
Show Abstract
Detection of high-energy neutrons in the presence of gamma radiation background utilizes pulse-shape
discrimination (PSD) phenomena in organics studied previously only with limited number of materials, mostly
liquid scintillators and single crystal stilbene. The current paper presents the results obtained with broader varieties
of luminescent organic single crystals. The studies involve experimental tools of crystal growth and material
characterization in combination with the advanced computer modeling, with the final goal of better understanding
the relevance between the nature of the organic materials and their PSD properties. Special consideration is given to
the factors that may diminish or even completely obscure the PSD properties in scintillating crystals. Among such
factors are molecular and crystallographic structures that determine exchange coupling and exciton mobility in
organic materials and the impurity effect discussed on the examples of trans-stilbene, bibenzyl, 9,10-
diphenylanthracene and diphenylacetylene.
Praseodymium activation detector for measuring bursts of 14MeV neutrons
Author(s):
B. T. Meehan;
E. C. Hagen;
C. L. Ruiz;
G. W. Cooper
Show Abstract
A new, accurate, neutron activation detection scheme for measuring pulsed neutrons has been designed and
tested. The detection system is accurate and sensitive to neutrons with energies above 10 MeV;
importantly, it is insensitive to gamma radiation and to lower-energy (e.g., fission and thermal) neutrons. It
is based upon the use of praesodymium, an element that has a single, naturally occurring isotope (Pr-141), a
significant (n,2n) cross section, and decays by positron emission. Neutron fluences are measured by using
the sum-peak method to count gamma-ray coincidences from the annihilation of the positron decay
product. The system was tested using 14 and 2.45 MeV neutron bursts produced by NSTec Dense Plasma
Focus Laboratory fusion sources. Comparisons with lead, copper, beryllium and silver activation detectors
have been performed. The detection method allows measurement of 14 MeV neutrons with a total error of
± 10%.
Neutron energy measurements in emergency response applications
Author(s):
Sanjoy Mukhopadhyay;
Paul Guss;
Michael Hornish;
Scott Wilde;
Tom Stampahar;
Michael Reed
Show Abstract
We present significant results in recent advances in the measurement of neutron energy. Neutron
energy measurements are a small but significant part of radiological emergency response applications.
Mission critical information can be obtained by analyzing the neutron energy given off from
radioactive materials. In the case of searching for special nuclear materials, neutron energy information
from an unknown source can be of importance. At the Remote Sensing Laboratory (RSL) of National
Security Technologies, LLC, a series of materials, viz., liquid organic scintillator (LOS), Lithium
Gadolinium Borate (LGB) or Li6Gd(BO3)3 in a plastic matrix, a recently developed crystal of Cesium
Lithium Yttrium Chloride, Cs2LiYCl6: Ce (called CLYC)[1], and normal plastic scintillator (BC-408)
with 3He tubes have been used to study their effectiveness as a portable neutron energy spectrometer.
Comparisons illustrating the strengths of the various materials will be provided. Of these materials,
LGB offers the ability to tailor its response to the neutron spectrum by varying the isotopic
composition of the key constituents (Lithium, Gadolinium [Yttrium], and Boron). All three of the
constituent elements possess large neutron capture cross section isotopes for highly exothermic
reactions. These compounds of composition Li6Gd(Y)(BO3)3 can be activated by Cerium ions Ce3+.
CLYC, on the other hand, has a remarkable gamma response in addition to superb neutron
discrimination, comparable to that of Europium-doped Lithium Iodide (6LiI: Eu). Comparing these two
materials, CLYC has higher light output (4500 phe/MeV) than that from 6LiI: Eu and shows better
energy resolution for both gamma and neutron pulse heights. Using CLYC, gamma energy pulses can
be discriminated from the neutron signals by simple pulse height separation. For the cases of both LGB
and LOS, careful pulse shape discrimination is needed to separate the gamma energy signals from
neutron pulses. Both analog and digital methods have been applied to obtain a clear gamma and
neutron energy spectrum in a mixed radiation field. A waveform digitizer manufactured by Agilent
Technology Inc. has been successfully used to digitize the signal and separate the gamma and neutron
signals to obtain a high gamma rejection ratio. These results along with some interesting data from a
plastic (BC-408) and 3He dual gamma-neuron detector will be presented.
Multi-element neutron energy spectrometer
Author(s):
Sanjoy Mukhopadhyay;
Richard Maurer;
Ronald Wolff;
Stephen Mitchell;
Alexis Reed
Show Abstract
In the area of nuclear radiological emergency response and preparedness applications, interest in
neutron detection stems from several factors. Unlike gamma rays, which are abundant in nature and
present serious difficulties in differentiating a signal from a changing background, whose values are
location specific, neutrons are rare and nearly homogenous in spatial distribution. Additionally, many
special nuclear materials (SNM) emit neutrons either directly by spontaneous fission or produce
neutrons indirectly through (α, n) reactions in nearby light elements. Also of importance in detection
scenarios is the fact that neutrons are not easily attenuated. Typically neutron detection is done by
simply counting the low energy thermal neutrons by employing pressurized helium tubes operated at
high voltages. Not much emphasis is put on determining the energy of the incident neutrons. However,
critical information can be obtained by analyzing the neutron energy given off from radioactive
materials. In the detection of an SNM, neutron energy information from an unknown source can be of
paramount importance.
We have modeled, designed, and prototyped multi-element neutron energy spectrometers that contain
three to five pressurized helium tubes of dimensions 2" diam. x 10" in length. Each individual helium
tube has a specific amount of high density plastic neutron moderators to slow down the incident
energetic neutrons to an accurately estimated energy. A typical spectrometer is a set of moderator
cylinders surrounding detectors that have high efficiency for detecting thermal neutrons. The larger the
moderator, the higher the energy of incident neutrons for which the detector assembly has matched
detection efficiency. If all the detectors are exposed to the same radiation field and the efficiency as a
function of energy (response function) of each of the detectors is known, the neutron energy spectrum
can be determined from the detector count rates.
Monte Carlo simulation results of response function calculations for different arrays of helium tubes
with varying amount of moderators will be shown. Experimental evidence of effectiveness of a set of
moderated helium tubes to measure the hardness of the incident neutrons will be demonstrated.
Properties of layered crystals for radiation detectors: TlGaSe[sub]2[/sub] system
Author(s):
D. J. Knuteson;
N. B. Singh;
D. Kahler;
B. Wagner;
A. Berghmans;
S. McLaughlin;
K. Schwartz
Show Abstract
In this paper we report the thermal and electronic characteristics of the ternary compound
TlGaSe2, grown from the melt. The phase-diagram, congruency of material and phase transition
were studied by differential thermal analysis. The material melted congruently at 812.9C and did
not show any other phases between room temperature and the melting point. Results of crystal
growth, and effect of alloying and its effect on crystal growth and properties are also discussed.
In high purity material it is easier to cleave the crystal since layers of successive rows of
tetrahedrons are turned away from each other by 900. However, by suitable alloying, the angle
of the tetrahedrons can be changed and the tendency to cleave can be decreased. This effect will
be evaluated by comparing the microstructural properties of pure and alloyed crystals. By
measuring the optical transmission band edge we studied details absorption characteristics to
evaluate the effect of impurities on recombination characteristics. Results indicated that there
were no donor-acceptor recombinations at or above room temperature.
Effect of Te inclusions on internal electric field of CdMnTe gamma-ray detector
Author(s):
Oluseyi Stephen Babalola;
Aleksey E. Bolotnikov;
Stephen U. Egarievwe;
Anwar M. Hossain;
Arnold Burger;
Ralph B. James
Show Abstract
We studied two separate as-grown CdMnTe crystals by Infrared (IR) microscopy
and Pockels effect imaging, and then developed an algorithm to analyze and
visualize the electric field within the crystals' bulk. In one of the two crystals the
size and distribution of inclusions within the bulk promised to be more favorable in
terms of efficiency as a detector crystal. However, the Te inclusions were arranged
in characteristic 'planes'. Pockels imaging revealed an accumulation of charges in
the region of these planes. We demonstrated that the planes induced stress within
the bulk of the crystal that accumulated charges, thereby causing non-uniformity of
the internal electric field and degrading the detector's performance.
Fabrication of TlBr strip detectors
Author(s):
Keitaro Hitomi;
Youhei Kikuchi;
Mohammad Nakhostin;
Tadayoshi Shoji;
Keizo Ishii
Show Abstract
TlBr strip detectors with four anode strip electrodes were fabricated in this study. The strip electrodes were formed on
TlBr wafers 1 mm thick with a vacuum evaporation process through a shadow mask. Each strip had a width of 0.8 mm
and a length of 3.5 mm approximately. The gap between the strips was approximately 0.1 mm. Inter strip resistance more
than 1 GΩ was obtained with the process. Energy resolutions of 7.0% and 3.8% FWHM were recorded for 511 keV and
1.33 MeV gamma-rays, respectively, with the TlBr strip detector at room temperature.
Rare-earth tri-halides methanol-adduct single-crystal scintillators for gamma ray and neutron detection
Author(s):
L. A. Boatner;
D. J. Wisniewski;
J. S. Neal;
Z. W. Bell;
J. O. Ramey;
J. A. Kolopus;
B. C. Chakoumakos;
R. Custelcean;
M. Wisniewska;
K. E. Pena
Show Abstract
Cerium activated rare-earth tri- halides represent a well-known family of high performance inorganic rare-earth
scintillators - including the high-light-yield, high-energy-resolution scintillator, cerium-doped lanthanum tribromide.
These hygroscopic inorganic rare-earth halides are currently grown as single crystals from the melt - either by the
Bridgman or Czochralski techniques - slow and expensive processes that are frequently characterized by severe cracking
of the material due to anisotropic thermal stresses and cleavage effects. We have recently discovered a new family of
cerium-activated rare-earth metal organic scintillators consisting of tri-halide methanol adducts of cerium and lanthanum
- namely CeCl3(CH3OH)4 and LaBr3(CH3OH)4:Ce. These methanol-adduct scintillator materials can be grown near
room temperature from a methanol solution, and their high solubility is consistent with the application of the rapid
solution growth methods that are currently used to grow very large single crystals of potassium dihydrogen phosphate.
The structures of these new rare-earth metal-organic scintillating compounds were determined by single crystal x-ray
refinements, and their scintillation response to both gamma rays and neutrons, as presented here, was characterized using
different excitation sources. Tri-halide methanol-adduct crystals activated with trivalent cerium apparently represent the
initial example of a solution-grown rare-earth metal-organic molecular scintillator that is applicable to gamma ray, x-ray,
and fast neutron detection.
High-resolution x-ray and γ-ray imaging using a scintillator-coupled electron-multiplying CCD
Author(s):
David Hall;
Andrew Holland
Show Abstract
Over the last decade the rapid advancements in CCD technology have lead to significant developments in the field of
low-light-level, Electron-Multiplying CCDs (EM-CCDs). The addition of a gain register before output allows signal
electrons to be multiplied without increasing the external noise. This low effective readout noise, which can be reduced
to the sub-electron level, allows very small signal levels to be detected. Caesium iodide is one of the most popular
scintillation materials due to its many desirable properties. Approximately 60 photons are produced per keV of incident
X-ray or γ-ray with wavelengths peaking at 550 nm (dependent on doping), matching the peak in the quantum efficiency
of the back-illuminated CCD97 of over 90%. Using a scintillator coupled to an EMCCD it is possible to resolve
individual interactions inside the scintillator. Multiple frames can be taken in quick succession with hundreds of
interactions per frame. These interactions can be analysed individually using sub-pixel centroiding and the data
compiled to create an image of a much higher resolution than that achieved with a single integrated frame. The
interaction mechanism inside the scintillator is discussed with relation to the spatial and spectral resolution of the camera
system. Analysis of individual events opens up the possibility of energy discrimination through the profiling of each
interaction.
Semiconducting material property relationships: trends and their impact on design of radiation detection materials
Author(s):
Kim F. Ferris;
Summer K. Lockersbie;
Bobbie-Jo M. Webb-Robertson;
Dumont M. Jones
Show Abstract
Materials properties important to the design and performance of semiconducting gamma detectors, such as band gap,
density, mobility, and crystal cell anisotropy, can depend on similar underlying physics. The resulting property
correlations limit the number of design variable and the place effective bounds on the range of physical properties
available to gamma-detection materials However, trend correlations can also limit the dependence of error in
structure-property relationships and information gaps when considering new candidate materials. Trend analysis
complements property estimation via data regression techniques, increasing the generality and certainty of
information-based conclusions.
Flexible radiation dosimeters incorporating semiconducting polymer thick films
Author(s):
Christopher A. Mills;
Akarin Intaniwet;
Maxim Shkunov;
Joseph L. Keddie;
Paul J. Sellin
Show Abstract
Flexible radiation dosimeters have been produced incorporating thick films (>1 μm) of the semiconducting polymer
poly([9,9-dioctylfluorenyl-2,7-diyl]-co-bithiophene). Diode structures produced on aluminium-metallised poly(imide)
substrates, and with gold top contacts, have been examined with respect to their electrical properties. The results suggest
that a Schottky conduction mechanism occurs in the reverse biased diode, with a barrier to charge injection at the
aluminium electrode. Optical absorption/emission spectra reveal a band gap of 2.48 eV for the polymer. The diodes have
been used for direct charge detection of 17 keV X-rays, generated by a molybdenum source. Using operating voltages of
-10 and -50 V respectively, sensitivities of 54 and 158 nC/mGy/cm3 have been achieved. Increasing the operating
voltage shows that the diodes are stable up to approximately -200 V without significant increase in the dark current of
the device (<0.2 nA).
Bismuth tri-iodide radiation detector development
Author(s):
Azaree T. Lintereur;
Wei Qiu;
Juan C. Nino;
James E. Baciak
Show Abstract
Bismuth tri-iodide (BiI3), a wide band-gap semiconductor, demonstrates many of the material properties necessary for
high resolution room temperature gamma-ray spectroscopy. These material properties include high density, large bandgap,
and high atomic number. The theoretical intrinsic photopeak efficiency of BiI3 is approximately 2-3 times higher
than CdZnTe over the range of 200-3000 keV. BiI3 has a theoretical intrinsic photopeak efficiency of 19% at 662 keV,
compared to CdZnTe which has a theoretical intrinsic photopeak efficiency of 13% at 662 keV. A modified vertical
Bridgman growth method is being used to grow large, greater than 100 mm3, single BiI3 crystals. Growth parameter
optimization has demonstrated that single crystals can be obtained with temperature gradients of 10°/cm or 15o/cm and a
growth rate of 0.5 mm/hr, or with a temperature gradient of 10o/cm and a growth rate of 1 mm/hr. Polycrystalline
material results from all other growth parameter combinations. X-ray diffraction spectra are used to determine if the
crystals are single crystals or polycrystalline. UV-VIS spectra analysis has revealed that the band-gap of BiI3 is 1.72 eV.
The resistivity of the crystals has been determined by generating I-V curves to be on the order of 108-109 Ω-cm. Zone
refining is being performed to increase the purity of the starting material and the resistivity of the crystals. Detectors
have been fabricated with both gold and palladium electrodes.
Comparison of neutron sensitive scintillators for use with a solid-state optical detector
Author(s):
Sharmistha Mukhopadhyay;
Christopher Stapels;
Eric B. Johnson;
Eric C. Chapman;
Paul S. Linsay;
Thomas H. Prettyman;
Michael R. Squillante;
James F. Christian
Show Abstract
The detection of illicit nuclear sources and SNM requires an ubiquitous network of sensors. While 3He proportional
counters are excellent neutron detectors, there is an insufficient global supply of 3He to create the required number of
detectors. Alternatives to 3He must be efficient, insensitive to gamma radiation, easily manufactured, rugged, and
inexpensive to enable the procurement of a large numbers of sensors. The use of a high sensitivity solid-state optical
detector coupled to scintillation materials, loaded with a neutron absorber such as 6Li or 10B, can fulfill these design
constraints. In this work, we compare the properties of neutron-sensitive scintillation materials utilizing Monte Carlo
simulations and experiments. Cs2LiLaBr6:Ce is compared to commercially available boron-loaded plastic scintillators
and 3He tubes. The scintillators are compared for neutron detection efficiency, limitations on size, gamma-rejection
ratio, neutron detection limits, manufacturing cost, and availability for mass-production.
Generic materials property data storage and retrieval for the semiconducting materials knowledge base
Author(s):
Dumont M. Jones;
Kim F. Ferris;
Bobbie-Jo M. Webb-Robertson;
Joan T. Muellerleile;
Roger W. Hyatt
Show Abstract
Informatics-based identification of candidate semiconducting radiation detection materials depends upon the
development of a robust knowledge base of materials properties. However, the accuracy and integrity of the
knowledge base are often affected by information loss due to incomplete entry and loss of context. We describe our
methods for materials property data storage and retrieval, in support of semiconductor development for gamma
radiation detection materials informatics applications. Analysis-ready data representations vary with each materials
design problem, and are often inconsistent with accurate generic property storage. The proposed approach provides
simple, strongly-typed generic storage for as-measured properties, with tools for assessing as-measured properties
and converting them to analysis-ready representations. This process simplifies property data stewardship, and allows
fine control over the assumptions of data fusion, system characterization, and property representation employed in
property-estimation models.
HgS: a rugged, stable semiconductor radiation detector material
Author(s):
Michael R. Squillante;
William M. Higgins;
Hadong Kim;
Leonard Cirignano;
Guido Ciampi;
Alexei Churilov;
Kanai Shah
Show Abstract
Many materials used in radiation detectors are environmentally unstable and/or fragile. These properties are frustrating
to researchers and add significantly to the time and cost of developing new detectors as well as to the cost of
manufacturing products. The work presented here investigates the properties of HgS. This material was selected for
study based partly on its inherent stability and ruggedness, high density, high atomic number, and bandgap. HgS is found
in nature as the mineral cinnabar. A discussion of the physical properties of HgS, experimental characterization of
natural cinnabar, and initial radiation detection results are presented along with a discussion of potential crystal growth
techniques for producing crystals of HgS in the laboratory.
Design and modeling of a lateral a-Se MSM photoconductor as indirect conversion X-ray imager
Author(s):
Kai Wang;
Feng Chen;
George Belev;
Safa O. Kasap;
Karim S. Karim
Show Abstract
Amorphous selenium (a-Se) has been widely used as a direct conversion X-ray detection material. Vertical structures are
employed in most cases, where >200 μm thick a-Se photoconductor layer is inserted between top and bottom electrodes.
In this paper, we design a lateral metal-semiconductor-metal (MSM) structure in which a relatively thin layer of a-Se (~
8 μm) is coated on top of two lateral electrodes. The simulation results indicate that dark current of such a structure stays
extremely low level and external quantum efficiency (EQE) reaches over 30% with wavelengths ranging from 320 to
680 nm. We further fabricate the lateral MSM photoconductor by a two-mask photolithography process. The fabricated
photoconductor exhibits a dark current below 40 fA under electric fields ranging from 6 V/μm to 9 V/μm, a responsivity
up to 0.06 A/W, a measured EQE of 18% towards a short wavelength of 468 nm, and a high photoresponse speed at 500
Hz with a rise time of 250 μs, fall time of 350 μs, and time constant of 250 μs, respectively. Furthermore, an architecture
of indirect conversion X-ray imager is proposed with the use of such a lateral MSM structure and a blue-emitting
scintillator material atop.