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- Front Matter: Volume 6706
- CZT I and CdTe
- CZT II
- Applications I
- Other Materials I
- CZT III and CdTe
- Other Materials II
- Applications II
- CZT IV and CdTe
- Applications III
- Scintillators
- Poster Session
Front Matter: Volume 6706
Front Matter: Volume 6706
Show abstract
This PDF file contains the front matter associated with SPIE Proceedings Volume 6706, including the Title Page, Copyright information, Table of Contents, and the Conference Committee listing.
CZT I and CdTe
Large-volume high-resolution cadmium zinc telluride radiation detectors: recent developments
Show abstract
The excellent room temperature spectral performance of cadmium zinc telluride detectors grown via the Traveling
Heater Method (THM) makes this approach suitable for the mass deployment of radiation detectors for applications in
homeland security and medical imaging. This paper reports our progress in fabricating thicker and larger area detectors
from THM grown CZT. We discuss the performance of such 20x20x10 mm3, and 10x10x10 mm3 monolithic pixellated
detectors and virtual Frisch-Grid 4x4x12 mm3 devices, and describe the various physical properties of the materials.
Optimization of virtual Frisch-grid CdZnTe detector designs for imaging and spectroscopy of gamma rays
Show abstract
In the past, various virtual Frisch-grid designs have been proposed for cadmium zinc telluride (CZT) and other
compound semiconductor detectors. These include three-terminal, semi-spherical, CAPture, Frisch-ring, capacitive
Frisch-grid and pixel devices (along with their modifications). Among them, the Frisch-grid design employing a non-contacting
ring extended over the entire side surfaces of parallelepiped-shaped CZT crystals is the most promising. The
defect-free parallelepiped-shaped crystals with typical dimensions of 5x5x12 mm3 are easy to produce and can be
arranged into large arrays used for imaging and gamma-ray spectroscopy. In this paper, we report on further advances
of the virtual Frisch-grid detector design for the parallelepiped-shaped CZT crystals. Both the experimental testing and
modelling results are described.
Characterization of thick layers of CdTe grown with MBE for the fabrication of radiation detectors
Show abstract
100 μm thick layers of CdTe have been grown by Molecular Beam Epitaxy (MBE) on LEC GaAs (001) substrates. The
intended application for the CdTe thick films is the fabrication of radiation detectors. As recently reported
extensive characterization has been performed. In this paper the results of the previous papers are being summarized,
showing the potential of the CdTe films to be used as radiation detector. Furthermore first investigations on the
application of the layers as radiation detectors are being presented.
Characterizations and measurements of CZT material: novel techniques and results
Show abstract
The performance of current long-drift-length Cadmium Zinc Telluride (CZT) detectors principally is determined by
the material's quality. Hence, the material's limitations must be better understood and potential solutions identified to
grow CZT crystals with the required qualities. Our efforts have focused on developing novel techniques and testing
methods that will allow us to explore the correlations between the crystal's defects and the detector's properties. Local
stoichiometric variations and other local disordering make it very hard to systematically correlate performance and
material defects on a macroscopic scale. Therefore, to delineate the factors limiting the energy resolution of CZT
detectors, we directed our efforts towards micron-scale material characterization and assessments of the detectors using
the National Synchrotron Light Source (NSLS). The NSLS offers us a highly collimated high-intensity X-ray beam that
we employed to undertake detector-performance mapping, and to investigate the association between microscopic
defects and fluctuations in collected charge. In this paper, we illustrate our techniques and results.
CZT II
Vertical Bridgman growth of Cd(1-x)Zn(x)Te for room temperature radiation detectors
Show abstract
Low pressure Electro-Dynamic Gradient freeze (EDG) method has been used to
grow compensated, high resistivity Cd(1-x)ZnxTe for x and gamma ray detectors. All
growths contained excess Tellurium which is added to the growth. Ampoule design and
setup to limit vapor transport was determined to be important. Ingots grown in a Pyrolitic
Coated Graphite crucible are shown to provide a good response to ionizing radiation at
room temperature and can be used multiple times. The highest doping levels of
Aluminum are shown to improve mobility lifetime products for electrons and average
8.7x10-4 cm2/V at 0.5 μsecond shaping fitting the Hecht relation.
In situ characterization of crystal growth and heat treatment in semiconductor materials
Show abstract
In situ characterization methods are being developed at the Idaho National Laboratory that can be used to characterize
the atomic lattice structure of materials used for semiconductor and scintillation detectors during the crystal growth and
heat treatment processes, which have been shown to be critical for the development of optimized semiconductor and
scintillation radiation detectors. Multiple methods for implanting positrons into the material have been developed and
integrated with measurement techniques including Doppler broadening, coincidence Doppler broadening and positron
lifetime measurement. The INL developed induced positron technique allows positron measurements to be performed at
depth up to 10 cm inside crystal boules. Also, a portable measurement system suitable for field use has been developed
that is suitable for assessing heat treatments at depths up to 1 cm inside a material in an industrial environment. Results
of measurements that address the effects of composition and heatup/melting/cool down on material lattice structures are
discussed along with plans for the in situ crystal studies.
Differential aperture x-ray microscopy near Te precipitates in CdZnTe
Show abstract
We report the results of Differential Aperture X-ray Microscopy (DAXM) measurements near Te precipitates in CdZnTe
grown via low-pressure Bridgman. White-beam Laue patterns were acquired with 3-D spatial resolution (with 0.25 μm
resolution in the scanning directions and 1 μm resolution in depth) at depths of up to 35 μm deep normal to the surface.
We find very little crystal strain (< 10-3) or rotation (<0.05 degrees) near Te precipitates. We also examine local
deformations in the vicinity of a microhardness indent, and find that although significant rotations exist, the spatial
extent is limited to a few tens of microns. Furthermore, observed crystal strains are limited to 5 x 10-3 or less in regions
near the microhardness indent.
Applications I
Wide-field hard x-ray survey telescope: ProtoEXIST1
Show abstract
We report our progress on the development of pixellated imaging CZT detector arrays for our first-generation balloon-borne
wide-field hard X-ray (20 - 600 keV) telescope, ProtoEXIST1. Our ProtoEXIST program is a pathfinder for the
High Energy Telescope (HET) on the Energetic X-ray Imaging Survey telescope (EXIST), a proposed implementation of
the Black Hole Finder Probe. ProtoEXIST1 consists of four independent coded-aperture telescopes with close-tiled (~0.4
mm gaps) CZT detectors that preserve their 2.5mm pixel pitch. Multiple shielding/field-of-view configurations are
planned to identify optimal geometry for the HET in EXIST. The primary technical challenge in ProtoEXIST is the
development of large area, close-tiled modules of imaging CZT detectors (1000 cm2 for ProtoEXIST1), with all readout
and control systems for the ASIC readout vertically stacked. We describe the overall telescope configuration of
ProtoEXIST1 and review the current development status of the CZT detectors, from individual detector crystal units
(DCUs) to a full detector module (DM). We have built the first units of each component for the detector plane and have
completed a few Rev2 DCUs (2x2 cm2), which are under a series of tests. Bare DCUs (pre-crystal bonding) show high,
uniform ASIC yield (~70%) and ~30% reduction in electronics noise compared to the Rev1 equivalent. A Rev1 DCU
already achieved ~1.2% FWHM at 662 keV, and preliminary analysis of the initial radiation tests on a Rev2 DCU shows
~ 4 keV FWHM at 60 keV (vs. 4.7 keV for Rev1). We therefore expect about ≤1% FWHM at 662 keV with the Rev2 detectors.
Application of CdTe photon-counting x-ray imager to material discriminated x-ray CT
Takuya Nakashima,
Hisashi Morii,
Yoichiro Neo,
et al.
Show abstract
We proposed that material discriminated X-ray CT with conventional X-ray tube and energy differentiation type 64ch
CdTe radiation line sensor. Distribution of Atomic number was obtained by using dual-energy X-ray CT. In this study,
problem of conventional X-ray tube was reduced by the collimator and measurement time. So line attenuation coefficient
was obtained depend on theory. Atomic number was calculated with two different methods. We could obtain atomic
number within about three error margin.
Other Materials I
GaSe and GaTe anisotropic layered semiconductors for radiation detectors
Show abstract
High quality detector grade GaSe and GaTe single crystals have been grown by a modified vertical Bridgman
technique using high purity Ga (7N) and in-house zone refined (ZR) precursor materials (Se and Te). A state-of-the-art
computer model, MASTRAPP, is used to model heat and mass transfer in the Bridgman growth system and to predict the
stress distribution in the as-grown crystals. The model accounts for heat transfer in the multiphase system, convection in
the melt, and interface dynamics. The crystals harvested from ingots of 8-10 cm length and 2.5 cm diameter, have been
characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), Raman spectroscopy, low temperature
photoluminescence (PL), atomic force microscopy (AFM), and optical absorption/transmission measurements. Single
element devices up to 1 cm2 in area have been fabricated from the crystals and tested as radiation detectors by measuring
current-voltage (I-V) characteristics and pulse height spectra using 241Am source. The crystals have shown high promise
as nuclear detectors with their high dark resistivity (≥109 Ω.cm), good charge transport properties (μτe ~ 1.4x10-5 cm2/V and μτh ~ 1.5x10-5 cm2/V), and relatively good energy resolution (~4% energy resolution at 60 keV). Details of numerical modeling and simulation, detector fabrication, and testing using a 241Am energy source (60 keV) is presented
in this paper.
Mercuric iodide photocell technology for room temperature readout of scintillators
Show abstract
Mercuric iodide (HgI2) is a well known material for the direct detection of gamma rays; however, the largest volume
achievable is limited by the thickness of the detector which needs to be a small fraction of the average trapping length
for electrons. We are reporting here preliminary results of using HgI2 crystals to fabricate photocells used in the readout
of various scintillators. The optical spectral response and efficiency of these photocells were measured and will be
reported. Preliminary nuclear response from an HgI2 photocell that was optically matched to a Ce3+:LaBr3 scintillator will also be presented and discussed. Further improvements will be sought by optimizing the transparent contact
technology.
Deconvolution of gamma-spectra variably affected by space radiation using an evolutionary algorithm
Show abstract
An evolutionary algorithm (ES) for automated deconvolution of γ-ray spectra is described that fits peak shape
morphologies typical of spectra acquired from variably radiation damaged γ-ray detectors. Space radiation effects
significantly impair semi-conductor γ-ray detector efficiency and induce variable degrees of nuclide peak broadening,
distortion in spectra. Mars Odyssey Gamma-ray spectrometer data are used to demonstrate applicability of described
algorithms for three degrees of radiation damage. ES methods accurately identify and quantify the discrete set of
nuclide peaks in an arbitrary spectrum using a nuclide library. A novel method of constraining peak low energy tails,
broadened by detector radiation damage, reduces the peak shape model from six parameters to four yielding a
significant minimization of model complexity. Benefits of this approach include the simple implementation of highly
specific parameter constraints that appropriately define feasible solution spaces. Methods describe peak low energy
tailing descriptors as a continuum of low energy peak tailing curves representing increasing degrees of radiation
damage. Curves are addressable by a single real valued parameter. Results illustrate the use of methods to simply
describe relative radiation dosimetry using this parameter. Analysis of degraded spectra indicates method sensitivity
to low and high levels of space radiation damage prior to and post MO-GRS detector annealings.
Fabrication and performance of mercuric iodide pixellated detectors
Show abstract
The radiation detection efficiency and spectral resolution of mercuric iodide detectors can be improved significantly by
increasing the volume of the detectors and by using a pixellated anode structure. Detector bodies with a thickness of
nominally 10 mm and an active area of approximately 14 mm x 14 mm have been used for these experiments. The
detectors were cut from single crystals grown by the physical vapor transport method. The cut surfaces were polished
and etched using a string saw and potassium iodide solutions. The Palladium contacts were deposited by magnetron
sputtering through stainless steel masks. The cathode contact is continuous; the anode contacts consist of an array of
11 x 11 pixels surrounded by a guard ring. The resistance between a pixel and its surrounding contacts should be larger
than 0.25 Gohm. The detector is mounted on a substrate that makes it possible to connect the anode pixels to an ASIC,
and is conditioned so that it is stable for all pixels at a bias of -3000 Volts. Under these conditions the spectral resolution
for Cs-137 gamma rays (662 keV) is approximately 5% FWHM. When depth sensing correction methods are applied,
the resolution improves to about 2% FWHM or better. It is expected that the performance of the devices can be
improved by the careful selection of crystal parts that are free of structural defects. Details of the fabrication
technologies will be described. The effects of material inhomogeneities and transport properties of the charge carriers
will be discussed.
CZT III and CdTe
Material dependence of bulk leakage current in CdZnTe detectors
Show abstract
The bulk leakage current in a semiconductor detector is an important parameter that affects the noise level and energy
resolution of the detector. For detectors operating with ohmic contacts, the bulk leakage current is determined by the
bulk resistivity of the semiconductor material. However, CdZnTe detectors typically utilize Schottky barrier type
contacts, in which case the bulk leakage current is expected to depend on the contact behavior and not on the bulk
resistivity of the material. We have studied the bulk leakage current and noise of CdZnTe detectors made from
materials supplied by different manufacturers of CdZnTe crystals. The results indicate that there is a marked difference
in bulk leakage currents among materials from different manufacturers and among different samples from the same
manufacturer. In some cases, the bulk leakage current shows no correlation with the bulk resistivity of the materials. In
other cases, the bulk leakage currents tend to be lower for lower resistivity materials, which is opposite from the
commonly held expectation based upon ohmic contact device behavior. In this paper we present a summary of our
electrical measurements on CdZnTe devices and present results indicating a possible relationship between leakage and
bulk material properties, but the specific material properties and the mechanism responsible for the leakage current
variation have yet to be determined.
Performance measurements of Al/CdTe/Pt pixel diode detectors
Show abstract
We developed Schottky CdTe detectors using Al as an anode electrode and measured their performances. We
first fabricated monolithic detectors with four different thicknesses of 0.5, 0.75, 1.0, and 2.0 mm. An Al anode
electrode was implemented with a guard-ring structure. For the 0.5 mm thick CdTe detector, an energy resolution
of 1.2 keV (FWHM) at 122 keV was achieved at a temperature of −20 °C and a bias voltage of 400 V. Using
the same technology, we next developed 8 × 8 pixel CdTe detectors, again with the four different thicknesses.
The Al anode electrode was pixelated and the Pt cathode was made as a single plate. Signals from all pixels
were successfully obtained and an energy resolution of 1.3 keV and 1.9 keV (FWHM) for 59.5 keV and 122 keV
gamma-rays, was achieved at a temperature of −20 °C and a bias voltage of 400 V using the 0.5 mm thick CdTe
detector. The energy resolution was nearly the same in each pixel.
Other Materials II
Perforated semiconductor neutron detectors for battery operated portable modules
Show abstract
Perforated semiconductor diode detectors have been under development for several years at Kansas State University for
a variety of neutron detection applications. The fundamental device configuration is a pin diode detector fabricated from
high-purity float zone refined Si wafers. Perforations are etched into the diode surface with inductively-coupled plasma
(ICP) reactive ion etching (RIE) and backfilled with 6LiF neutron reactive material. The perforation shapes and depths
can be optimized to yield a flat response to neutrons over a wide variation of angles. The prototype devices delivered
over 3.8% thermal neutron detection efficiency while operating on only 15 volts. The highest efficiency devices thus far
have delivered over 12% thermal neutron detection efficiency. The miniature devices are 5.6 mm in diameter and require
minimal power to operate, ranging from 3.3 volts to 15 volts, depending upon the amplifying electronics. The battery
operated devices have been incorporated into compact modules with a digital readout. Further, the new modules have
incorporated wireless readout technology and can be monitored remotely. The neutron detection modules can be used for
neutron dosimetry and neutron monitoring. When coupled with high-density polyethylene, the detectors can be used to
measure fission neutrons from spontaneous fission sources. Monto Carlo analysis indicates that the devices can be used
in cargo containers as a passive search tool for spontaneous fission sources, such as 240Pu. Measurements with a 252Cf
source are being conducted for verification.
First principles calculation of point defects and mobility degradation in bulk AlSb for radiation detection application
Show abstract
The development of high resolution, room temperature semiconductor radiation detectors requires the introduction of
materials with increased carrier mobility-lifetime (μτ) product, while having a band gap in the 1.4-2.2 eV range. AlSb
is a promising material for this application. However, systematic improvements in the material quality are necessary to
achieve an adequate μτ product. We are using a combination of simulation and experiment to develop a fundamental
understanding of the factors which affect detector material quality. First principles calculations are used to study the
microscopic mechanisms of mobility degradation from point defects and to calculate the intrinsic limit of mobility from
phonon scattering. We use density functional theory (DFT) to calculate the formation energies of native and impurity point
defects, to determine their equilibrium concentrations as a function of temperature and charge state. Perturbation theory
via the Born approximation is coupled with Boltzmann transport theory to calculate the contribution toward mobility
degradation of each type of point defect, using DFT-computed carrier scattering rates. A comparison is made to measured
carrier concentrations and mobilities from AlSb crystals grown in our lab. We find our predictions in good quantitative
agreement with experiment, allowing optimized annealing conditions to be deduced. A major result is the determination
of oxygen impurity as a severe mobility killer, despite the ability of oxygen to compensation dope AlSb and reduce the net
carrier concentration. In this case, increased resistivity is not a good indicator of improved material performance, due to
the concomitant sharp reduction in μτ.
X-ray detection by epitaxial CVD diamond for medical radiology applications
Show abstract
An epitaxial diamond detector obtained by CVD (Chemical Vapor Deposition) has been used in order to monitor
X-ray pulses from a radiological portable X-ray generator commonly used in hospitals. X-ray maximum energies
varied from 50 to 120 KeV, while (electron anodic current)x(time duration) products were in the range from 20 to
100 mAs. Current pulses were recorded and from their shapes the timing and the collected charge were calculated
and compared with those obtained by standard 6 cm3 ionization chambers and by a silicon diode arrays used in
quality assurance programs for radiological X-ray apparatuses. Both diamond detector and silicon array display a
standard deviation in time recording of 0.3% in the time range from 0.15 up to 2.5 s. The integrated current recorded
from diamond is linear with respect the dose recorded by the ionization chamber from 5 up to 125 mGy, with a
standard deviation on single points of the order of 0.5%. The stability of the detector is very good even without a
priming treatment generally used in order to stabilize diamond dosimeters. Homogeneity of the detector in terms of
its response was tested by means of alpha particles, which indicate an energy resolution of 0.7%, quite close to that
of a standard surface barrier silicon detector. These results indicate that epitaxial diamond could be considered ready
to be used in standard quality control procedures concerning radiological X-ray apparatuses.
Applications II
A new Si/CdTe semiconductor Compton camera developed for high-angular resolution
Show abstract
A semiconductor Compton camera for a balloon borne experiment aiming at observation in high energy astrophysics
is developed. The camera is based on the concept of the Si/CdTe semiconductor Compton Camera,
which features high-energy and high-angular resolution in the energy range from several tens of keV to a few
MeV. It consists of tightly packed double-sided silicon strip detectors (DSSDs) stacked in four layers, and a
total of 32 CdTe pixel detectors surrounding them. The Compton reconstruction was successfully performed and
gamma-ray images were obtained from 511 keV down to 59.5 keV. The Angular Resolution Measure (ARM) at
511 keV is ~ 2.5 degrees, thanks to the high energy resolution in both the DSSD and CdTe parts.
Evaluation of 0.5-mm thick double-sided silicon strip detector for Compton telescope
Show abstract
Double-sided silicon strip detector (DSSD) is a key component to construct the next generation Compton telescope
for the high-sensitivity observation in the energy region from several hundred keV to MeV. The concept of
Compton camera we consider is using DSSD for scatterer, and high-stopping CdTe pixel detector for absorber.
As the scatterer, DSSD has advantages of smaller band gap, higher efficiency of scattering, smaller Doppler
broadening, good response time, and smaller number of readout channels. We have developed and confirmed
that 0.3 mm-thick DSSD has enough performance. As a next step, in order to obtain more efficiency of higher
energy gamma-rays, we developed newly designed DSSD which increase in thickness to 0.5 mm. We measured
the basic properties of 0.5 mm thick DSSD, in terms of leakage current, capacitances, and noise characteristics.
They can be full-depleted around 200 V, and we obtained the energy resolution of 1.3 keV (FWHM) for 60 keV
at -10 °C from one p-side strip. We also set up the newly developed read-out system which is based on technology
of operating ASICs on floating ground, and performed 64 ch read-out on one side.
First results from the 128x128 pixel mixed-mode Si x-ray detector chip
Show abstract
A Mixed-Mode Pixel Array Detector has been developed to measure protein crystallographic diffraction patterns. X-rays
are stopped in a 500 μm thick layer of silicon diodes, and collected charge is processed by an attached ASIC. Goals
of the project are high flux (108 x-rays/s/pixel) capability and fast readout (< 0.5 ms dead time between frames). "Mixed-Mode" refers to a readout method whereby integrated signal accumulating in each pixel is compared against a threshold
value. When the threshold is reached, a digital count is added to an 18-bit in-pixel counter and a set quantity of charge is
removed from integrator. At the end of the x-ray exposure, analog signal left in the integrator is separately processed.
Thus, one obtains mixed digital and analog data where the counter bits are a high order word and the analog residual
provides higher precision. Typically, each count is equivalent to 100 10 keV x-rays, for a well-depth >107 10 keV x-rays/
pixel. The analog residual is digitized to 9-bit precision allowing measurement of the residual charge to better than
a quarter of the charge from single 10 keV x-rays. Measurements are presented on x-ray tests at the Cornell High Energy
Synchrotron Source (CHESS). Dynamic range, linearity, point-spread function and noise properties are shown. Status
will be is reported on five different approaches for ASIC-diode hybridization. Progress toward bonding of a 128 x 512
pixel device is also presented.
4 pi direction sensitive gamma imager with RENA-3 readout ASIC
Show abstract
A 4π direction-sensitive gamma imager is presented, using a 1 cm3 3D CZT detector from Yinnel
Tech and the RENA-3 readout ASIC from NOVA R&D. The measured readout system electronic noise is
around 4-5 keV FWHM for all anode channels. The measured timing resolution between two channels
within a single ASIC is around 10 ns and the resolution is 30 ns between two separate ASIC chips. After
3D material non-uniformity and charge trapping corrections, the measured single-pixel-event energy
resolution is around 1% for Cs-137 at 662 keV using a 1 cm3 CZT detector from Yinnel Tech with an 8 x 8
anode pixel array at 1.15 mm pitch. The energy resolution for two pixel events is 2.9%. A 10 uCi Cs-137
point source was moved around the detector to test the image reconstruction algorithms and demonstrate
the source direction detection capability. Accurate source locations were reconstructed with around 200
two-pixel events within a total energy window ±10 keV around the 662 keV full energy peak. The angular
resolution FWHM at four of the five positions tested was between 0.05-0.07 steradians.
CZT IV and CdTe
CZT X-ray detectors obtained by the boron encapsulated vertical Bridgman method
Show abstract
Recently, some of the authors showed that it is possible to grow CZT crystals by the boron oxide encapsulated vertical
Bridgman method. The most important feature of the technique is that the crystal, during the growth, is fully
encapsulated by a thin layer of liquid boron oxide, so that the crystal-crucible contact is prevented. The stress of the
crucible to the crystal is strongly reduced also during the cooling, because the boron oxide layer is molten down to about
500°C. A number of detectors have been prepared out of these crystals. The transport properties (μτ product) have been
studied by photoconductivity measurements as well as by determining the response to hard X-ray irradiation. The
transport properties have been studied as a function of the indium content and of the position of the wafer which the
detector was cut out.
Simple x-ray computed tomography system utilizing a cadmium telluride detector
Show abstract
A simple x-ray computed tomography (CT) system utilizing a cadmium telluride detector and its application to
enhanced iodine K-edge angiography are described. The CT system is of the first generation type and consists of an
x-ray generator, a turn table, a translation unit, a motor drive unit, a cadmium telluride detector, an interface unit for
the detector, and a personal computer (PC). Tomography was performed by the repetition of the translation and
rotation. Narrow-photon-energy bremsstrahlung x-rays with a peak photon energy of approximately 35 keV is very
useful for performing enhanced K-edge angiography because these rays are absorbed effectively by iodine-based
contrast media with a K-edge of 33.2 keV. The tube voltage, the tube current, and the aluminum filter thickness were
60 kV, 1.5 mA, and 3.0 mm, respectively. Holes filled with iodine media in phantoms are visible with high contrasts,
and the CT system can be applied to photon-counting and fluorescent x-ray CT systems.
Heteroepitaxial growth and properties of crystals of CdTe on GaAs substrates
Show abstract
This paper reports on the use of a seeded vapour phase technique to grow bulk crystals of CdTe onto
commercially available 50 mm diameter (211)B GaAs substrates. High quality crystals, several mm in
thickness were grown on the GaAs at linear growth rates of ~ 120 μm/h. Characterisation by double
and triple axis XRD showed the best crystals to have θ-2θ FWHMθ values of ~ 24 arc sec
corresponding to low strain dispersion (< 2×10-4). Rocking curve scans included two to three sharp
peaks, indicative of some small mosaicity. When mapped across a the surface of the crystal, the
FWHM was uniform and < 93 arc sec. Contactless resistivity showed a similar degree of uniformity
with a mean value of 4.4 × 109 ± 1.6 × 109 Ω cm. Infrared microscopy showed that within the resolution
of the microscope (~ 5 μm) there were very few Te inclusions.
Tunable narrow-photon-energy x-ray source using a silicon single crystal
Show abstract
A preliminary experiment for producing narrow-photon-energy cone-beam x-rays using a silicon single crystal is
described. In order to produce low-photon-energy x-rays, a 100-µm-focus x-ray generator in conjunction with a (111)
plane silicon crystal is employed. The x-ray beams from the source are confined by an x-y diaphragm, and
monochromatic cone beams are formed by the crystal and three lead plates. The x-ray generator consists of a main
controller and a unit with a high-voltage circuit and a 100-µm-focus x-ray tube. In this experiment, the maximum tube
voltage and current were 35 kV and 0.50 mA, respectively, and the x-ray intensity of the microfocus generator was
343 μGy/s at 1.0 m from the source with a tube voltage of 30 kV and a current of 0.50 mA. The effective photon
energy is determined by Bragg's angle, and the photon-energy width is regulated by the angle delta. Using this
generator in conjunction with a computed radiography system, quasi-monochromatic radiography was performed
using a cone beam with an effective energy of approximately 15.5 keV.
Applications III
Field-deployable gamma-radiation detectors for DHS use
Show abstract
Recently, the Department of Homeland Security (DHS) has integrated all nuclear detection research,
development, testing, evaluation, acquisition, and operational support into a single office: the Domestic Nuclear
Detection Office (DNDO). The DNDO has specific requirements set for all commercial off-the-shelf and
government off-the-shelf radiation detection equipment and data acquisition systems. This article would
investigate several recent developments in field deployable gamma radiation detectors that are attempting to meet
the DNDO specifications. Commercially available, transportable, handheld radio isotope identification devices
(RIID) are inadequate for DHS' requirements in terms of sensitivity, resolution, response time, and reach-back
capability. The leading commercial vendor manufacturing handheld gamma spectrometer in the United States is
Thermo Electron Corporation. Thermo Electron's identiFINDERTM, which primarily uses sodium iodide crystals
(3.18 x 2.54cm cylinders) as gamma detectors, has a Full-Width-at-Half-Maximum energy resolution of 7 percent
at 662 keV. Thermo Electron has just recently come up with a reach-back capability patented as
RadReachBackTM that enables emergency personnel to obtain real-time technical analysis of radiation samples
they find in the field1. The current project has the goal to build a prototype handheld gamma spectrometer,
equipped with a digital camera and an embedded cell phone to be used as an RIID with higher sensitivity, better
resolution, and faster response time (able to detect the presence of gamma-emitting radio isotopes within 5
seconds of approach), which will make it useful as a field deployable tool. The handheld equipment continuously
monitors the ambient gamma radiation, and, if it comes across any radiation anomalies with higher than normal
gamma gross counts, it sets an alarm condition. When a substantial alarm level is reached, the system
automatically triggers the saving of relevant spectral data and software-triggers the digital camera to take a
snapshot. The spectral data including in situ analysis and the imagery data will be packaged in a suitable format
and sent to a command post using an imbedded cell phone.
Results from the characterisation of Advanced GAmma Tracking Array prototype detectors and their consequences for the next-generation nuclear physics spectrometer
Show abstract
The Advanced GAmma Tracking Array (AGATA) is a European project that is aiming to construct a complete 4π High
Purity Germanium (HPGe) gamma-ray spectrometer for nuclear structure studies at future Radioactive Ion Beam (RIB)
Facilities. The proposed array will utilise digital electronics, Pulse Shape Analysis (PSA) and Gamma-Ray Tracking
(GRT) algorithms, to overcome the limited efficiencies encountered by current Escape Suppressed Spectrometers (ESS),
whilst maintaining the high Peak-to-Total ratio.
Two AGATA symmetrical segmented Canberra Eurisys (CE) prototype HPGe detectors have been tested at the
University of Liverpool. A highly collimated Cs-137 (662keV) beam was raster scanned across each detector and data
were collected in both singles and coincidence modes. The charge sensitive preamplifier output pulse shapes from all 37
channels (one for each of the 36 segments and one for the centre contact) were digitised and stored for offline analysis.
The shapes of the real charge and image charge pulses have been studied to give detailed information on the position
dependent response of each detector. 1mm position sensitivity has been achieved with the parameterisation of average
pulse shapes, calculated from data collected with each of the detectors. The coincidence data has also been utilised to
validate the electric field simulation code Multi Geometry Simulation (MGS). The precisely determined 3D interaction
positions allow the comparison of experimental pulse shapes from single site interactions with those generated by the
simulation. It is intended that the validated software will be used to calculate a basis data set of pulse shapes for the
array, from which any interaction site can be determined through a χ2 minimisation of the digitized pulse with linear
combinations of basis pulseshapes. The results from this partial validation, along with those from the investigation into
the position sensitivity of each detector are presented.
An adapted modulation transfer function for x-ray backscatter radiography by selective detection
Show abstract
The Modulation Transfer Function (MTF) is a quantitative function based on frequency resolution that characterizes
imaging system performance. In this study, a new MTF methodology is investigated for application to Radiography by
Selective Detection (RSD), an enhanced single-side x-ray Compton backscatter imaging (CBI) technique which detects
selected scatter components. The RSD imaging modality is a unique type of real-time radiography that uses a set of fin
and sleeve collimators to preferentially select different components of the x-ray backscattered field. Radiography by
selective detection has performed successfully in different non-destructive evaluation (NDE) applications. A customized
RSD imaging system was built at the University of Florida for inspection of the space shuttle external tank spray-on
foam insulation (SOFI). The x-ray backscatter RSD imaging system has been successfully used for crack and corrosion
detection in a variety of materials. The conventional transmission x-ray image quality characterization tools do not apply
for RSD because of the different physical process involved. Thus, the main objective of this project is to provide an
adapted tool for dynamic evaluation of RSD system image quality. For this purpose, an analytical model of the RSD
imaging system response is developed and supported. Two approaches are taken for the MTF calculations: one using the
Fourier Transform of a line spread function and the other one using a sine function pattern. Calibration and test targets
are then designed according to this proposed model. A customized Matlab code using image contrast and digital curve
recognition is developed to support the experimental data and provide the Modulation Transfer Functions for RSD.
Scintillators
Barium iodide single-crystal scintillator detectors
Show abstract
We find that the high-Z crystal Barium Iodide is readily growable by the Bridgman growth technique and is less
prone to crack compared to Lanthanum Halides. We have grown Barium Iodide crystals: undoped, doped with
Ce3+, and doped with Eu2+. Radioluminescence spectra and time-resolved decay were measured. BaI2(Eu)
exhibits luminescence from both Eu2+ at 420 nm (~450 ns decay), and a broad band at 550 nm (~3 μs decay)
that we assign to a trapped exciton. The 550 nm luminescence decreases relative to the Eu2+ luminescence
when the Barium Iodide is zone refined prior to crystal growth. We also describe the performance of BaI2(Eu)
crystals in experimental scintillator detectors.
Ce-doped single crystal and ceramic garnet for gamma-ray detection
Show abstract
Ceramic and single crystal Lutetium Aluminum Garnet scintillators exhibit energy resolution with bialkali
photomultiplier tube detection as good as 8.6% at 662 keV. Ceramic fabrication allows production of garnets that
cannot easily be grown as single crystals, such as Gadolinium Aluminum Garnet and Terbium Aluminum Garnet.
Measured scintillation light yields of Cerium-doped ceramic garnets indicate prospects for high energy resolution.
New rare-earth-activated phosphate glass scintillators
Show abstract
We have investigated the applicability of phosphate glasses as host systems for the formation of rare-earth-activated
gamma- and x-ray scintillators. Glass scintillators have generally suffered from low light yields, usually attributed to
inefficient energy transfer from the glass matrix to the luminescent center. Our research on these phosphate glasses has
shown that their structural properties can be readily varied and controlled by compositional alterations. The melting and
pouring temperature of ~1050°C for these phosphate glasses is significantly lower than the processing temperatures
generally associated with the formation of silicate glass scintillators. The calcium-sodium phosphate glasses will
tolerate relatively high cerium concentrations based on the initial melt compositions, and the light yield for gamma-ray
excitation at 662 keV was determined as a function of cerium concentration up to the saturation level. The rare-earth-activated
Ca-Na phosphate glass primary-component decay time was in the range of 32 to 42 nsec for various Ce
concentrations with the contribution of the light output of the primary component ranging from 80 to 90%. Studies of the
effects of co-doing with both Ce and Gd were also carried out in the case of the Ca-Na phosphate glass hosts. The
effects of post-synthesis thermochemical treatments in a variety of atmospheres and at various processing temperatures
were also investigated for the Ce-activated Ca-Na phosphate scintillators.
Performance of new ceramic scintillators for gamma- and x-ray detection
Show abstract
Ceramic materials show significant promise for the production of reasonably priced, large-size scintillators. Ceramics
have recently received a great deal of attention in the field of materials for laser applications, and the technology for
fabricating high-optical-quality polycrystalline ceramics of cubic materials has been well developed. The formation of
transparent ceramics of non-cubic materials is, however, much more difficult as a result of birefringence effects in
differently oriented grains. Here, we will describe the performance of a few new ceramics developed for the detection of
gamma- and x-ray radiation. Results are presented for ceramic analogs of three crystalline materials - cubic Lu2O3, and
non-cubic LaBr3, and Lu2SiO5 or LSO (hexagonal, and monoclinic structures, respectively). The impact of various
sintering, hot-pressing and post-formation annealing procedures on the light yield, transparency, and other parameters,
will be discussed. The study of LaBr3:Ce shows that fairly translucent ceramics of rare-earth halides can be fabricated
and they can reach relatively high light yield values. Despite the fact that no evidence for texturing has been found in our
LSO:Ce ceramic microstructures, the material demonstrates a surprisingly high level of translucency or transparency.
While the scintillation of LSO:Ce ceramic reaches a light yield level of about 86 % of that of a good LSO:Ce single
crystal, its decay time is even faster, and the long term afterglow is lower than in LSO single crystals.
LaF3:Ce nanocomposite scintillator for gamma-ray detection
Show abstract
Nanophosphor LaF3:Ce has been synthesized and incorporated into a matrix to form a nanocomposite
scintillator suitable for application to γ-ray detection. Owing to the small nanocrystallite size (sub-10 nm),
optical emission from the γ / nanophosphor interaction is only weakly Rayleigh scattered (optical attenuation
length exceeds 1 cm for 5-nm crystallites), thus yielding a transparent scintillator. The measured energy
resolution is ca. 16% for 137Cs γ rays, which may be improved by utilizing brighter nanophosphors. Synthesis of
the nanophosphor is achieved via a solution-precipitation method that is inexpensive, amenable to routine
processing, and readily scalable to large volumes. These results demonstrate nanocomposite scintillator proof-of-
principle and provide a framework for further research in this nascent field of scintillator research.
Poster Session
Recrystallization of ceramic material fabricated from Cd1-xZnxTe
Show abstract
Cd1-xZnxTe nanopowder with the average particle size 10 nm was produced through vapor
deposition. Dense ceramic material was compacted from the nanopowder at room temperature.
The effect of annealing on grain growth, phase transitions and some physical properties was
studied.
Evaluation of the multi-pixel photon counters with inorganic scintillators
Show abstract
Multi-Pixel Photon Counter (MPPC) is a Hamamatsu's new Si avalanche photodiode. The main features of the MPPC
are very high gain (105 to106) and very fast operation. The MPPC offers considerable advantages over widely used
photomultiplier tubes (PMT) for scintillation detection, especially due to their high quantum efficiency, low cost,
compactness, low operating voltage, mechanical robustness and insensitivity to magnetic fields. One of the most
attractive applications of MPPCs is positron emission tomography (PET). In the present study, the MPPCs were coupled
to inorganic scintillators including LSO, BGO and YAP in order to evaluate the scintillator/MPPC devices in terms of
energy and timing resolutions for PET applications. The scintillation detector consisted of an LSO scintillator (2 mm × 2
mm × 4 mm) coupled to an MPPC exhibited energy resolutions of 196 keV FWHM and 189 keV FWHM for 511 keV
and 662 keV gamma-rays, respectively. The MPPC was operated at a bias of 71.4 V and at room temperature. In order to
investigate timing properties of the LSO/MPPC device, coincidence timing spectra between a reference scintillation
detector which consisted of a BaF2 crystal coupled to a PMT and the LSO/MPPC device were measured with 511 keV
positron annihilation gamma-rays from a 22Na source. A coincidence timing resolution of 3.2 ns FWHM was obtained
with the LSO/MPPC system.
Study of radiation detectors based on semi-insulating GaAs and InP: aspects of material and electrode technology
Show abstract
In this work, the most important aspects of semi-insulating (SI) GaAs and SI InP-based radiation detectors will be listed.
Based on that, the material and technology requirements will be identified. Further, the status of development of X- and
gamma-ray detectors based on the bulk SI GaAs and SI InP will be reviewed. The emphasis will be concentrated on the
following important aspects: (i) basic material characteristics, (ii) role of the electrodes (blocking contacts, ohmic
contacts) in the overall performances of detectors. The fabrication technology and the performances of detectors and
their application in the first quantum X-ray digital scanner recently developed will also be illustrated along with some
conclusions about the material/application relations: (i) radiation detectors based on bulk SI GaAs may readily find
applications in X-ray digital radiology imaging systems, whilst (ii) SI InP-based detectors are very promising but need
further development to reach performances suitable for the application.
Investigation of TlBr detector response under high-flux x-rays
Show abstract
Thallium bromide (TlBr) is a compound semiconductor with high density, high atomic numbers and wide
bandgap. In addition, recent results indicate that the mobility-lifetime product of electrons can be quite high,
approaching the values for CdTe and CZT. These properties make TlBr a very promising material for nuclear radiation
detector at room temperature. In this paper we report on our investigation of the performance of planar TlBr detector
under high flux x-rays irradiation. This study proposes an alternate contact method that reduces the polarization effects,
and the afterglow for a wide range of high flux applications.
Study and realization of real-time in-depth dosimetry system for IORT (intra operative radiation therapy)
Show abstract
Intra Operative Radiation Therapy (IORT) is a technique based on delivery of a high dose of ionising radiation to the
cancer tissue, after tumour ablation, during surgery, while reducing the exposure of normal surrounding tissue. Novac7
and Liac are new linear accelerators expressly conceived to perform in the operating room. These accelerators supply
electron beams with high dose rate. Because of this peculiar characteristic, classical dosimetric techniques are not able to
give at once a real-time response and an extensive measure of the absorbed dose.
In past years the authors realized a prototype for IORT dosimetry able to give the real time bi-dimensional image of dose
distribution on a single layer. In the framework of a research project funded by the INFN (Italian National Institute of
Nuclear Physics), a collaboration between the Physics Department of Bologna, Italy, the Physics Department of Cosenza
and the Medicine Department of Catanzaro, Italy, has studied a new system composed of six layers. Each layer includes
two orthogonal bundles of scintillating optical fibres. The fibres are optically coupled with four arrays of photodiodes as
read-out system. This new system will be able to characterize completely the electron beam in energy, intensity and
spatial distribution. In real time it will be able to measure the 3D dose distribution, providing a full check of quality
assurance for IORT.
The various phases of design, development and characterization of the instrument will be illustrated, as well as some
experimental tests performed with the prototype. We verified that the system is able to give a real time response, which
is linear versus dose and not affected by the high dose rate.
The conclusions confirm the capability of the instrument to overcome problems encountered with classic dosimetry,
showing that the obtained results strongly encourage the continuation of this research.
Surface sensitivity in large mass bolometers: discrimination of the origin of events
Show abstract
During the last decades, low temperature detectors have undergone a considerable growth and are now widely
acknowledged as useful instruments in many fundamental physics experiments. In this field, the phonon mediated
particle detectors known as bolometers are remarkable and are successfully used in various branches of physics
research for their good sensitivity, energy resolution and flexibility in the choice of the constituting materials.
Bolometers have proved to be powerful devices for radiation detection; in particular, they are able to detect
Gamma-rays with resolutions comparable to those obtained with the best Ge diodes. They are also suited for
applications in the area of nuclear and particle physics, like the study of rare events or dark matter. Although
an effective technique, the use of bolometers in the specific field of the search for neutrinoless double beta
decay is affected by the lack of spatial resolution. This results in the expected signal of this rare decay hidden
under an indistinguishable background due to possible surface radioactive contaminations in the materials facing
the detectors. An approach to this problem is to make bolometers surface sensitive by applying ultra-pure
crystalline foils on the main detector through direct thermal contact and by operating them as active shields.
In this contribution we present for the first time surface sensitivity achieved with large mass TeO2 bolometers
(~800 g) operated underground at ~10 mK, dedicated to the detection of neutrinoless double beta decay of
130Te. Our encouraging measurements suggest that this could be a viable method for the discrimination of
background events.
The crystal geometry and the aspect ratio effects on spectral performance of CdZnTe Frisch collar device
Show abstract
The effects of crystal geometry and aspect ratio on a CdZnTe Frisch collar device were investigated. A
19.08x19.34x4.95mm3 device fabricated from CdZnTe grown by Redlen Technologies was used as the starting material.
The crystal was re-fabricated many times to achieve several aspect ratios while the device length was held constant at
4.75±0.15mm. The following aspect ratios were successfully fabricated from the initial device: 0.26, 0.52, 0.71, 0.96,
1.19, 1.36, 1.59 and 1.92. The energy spectra of 241Am and 137Cs were recorded for all devices in both planar and Frisch
collar configurations. The current-voltage (I-V) characteristic curve was also determined for planar configurations. It was
observed that the Frisch collar effect begins to occur for devices with an aspect ratio of approximately 1.0. Device
performance continued to improve as the aspect ratio was further increased and was noted to significantly improve the
energy resolution of the device when the aspect ratio was greater than 1.5. The CdZnTe devices were also theoretically
modeled to support the experimental conclusion.