The growth and properties of semi-insulating CdZnTe for nuclear radiation detector applications are reviewed. The current state of the high-pressure Bridgman growth and the potentials of the conventional vertical and horizontal Bridgman techniques to grow radiation detector material are discussed. The characteristic macroscopic and microscopic defects of high-pressure Bridgman grown CdZnTe ingots, such as cracks, pipes, inclusions, precipitates, grain boundaries and their effect on the electrical and charge trapping properties of the material are reviewed.

In this paper, the effects of the trapping of electrons and holes on the performance of gallium arsenide x-(gamma) ray detectors are analyzed. Starting from the experimental spectra acquired with state of the art GaAs detectors, it is shown how much the existing modeling of the charge trapping can explain the data and predict the observed excess noise due to the trapping. An analysis made with Monte-Carlo simulations has been carried out and compared with the experiments. The relationship between the fraction of the trapped charge and the energy resolution of the detector has been determined and analyzed.

Undoped CdSe single crystals were investigated by current- voltage measurement and low temperature photoluminescence. Crystal quality and surface morphology were also studied by optical microscopy and scanning electron microscopy. Together with gamma spectroscopy, the results show that the CdSe crystals have high resistivities in the 10¹⁰ (Omega) (DOT)cm range. This indicates their usefulness for room temperature nuclear radiation detection. Small active volume CdSe detector were fabricated and their performance is presented. A comparison of fundamental material properties between CdSe and CdZnTe detectors is also given.

We report a significant improvement of the spectral properties of a cadmium telluride (CdTe) detector. With the use of a high quality CdTe crystal, we formed a high Schottky barrier for the holes on a CdTe surface using a low work-function metal, indium. For a 2 X 2 mm² detector with a thickness of 0.5 mm the leakage current was measured to be 0.7 nA at room temperature (20 degree(s)C) and 10 pA at -20 degree(s)C for a 400 V bias voltage. The low-leakage current of the detector allows us to operate the detector at a higher bias voltage than previous CdTe detectors. The improved charge collection efficiency and the low-leakage current leads to an energy resolution of 1.1 - 2.5 keV FWHM in the energy range 2 keV to 150 keV at 20 degree(s)C without charge loss correction electronics. We confirmed that once a high electric field of several kV/cm is applied, the Schottky CdTe has a very good energy resolution as well as sufficient stability to be used for practical applications.

The electric properties of CdZnTe radiation detectors are largely determined by the electron and hole traps in this material. The traps, in addition to degrading the detector performance, can function as dopants and determine the resistivity of the material. Thermoelectric emission spectroscopy and thermally stimulated conductivity are used to detect these traps in a commercially available spectrometer-grade CdZnTe detector, and the electrical resistivity is measured as a function of temperature. A deep electron trap having an energy of 695 meV and cross section of 8 X 10^-16 cm(superscript 2$ is detected and three hole traps having energies of 70 +/- 20 meV, 105 +/- 30 meV and 694 +/- 162 meV are detected. A simple model based on these traps explains quantitatively all the data, including the electrical properties at room temperature and also their temperature dependence.

The structure and chemistry of grain boundaries in commercial Cd_1-xZn_xTe, prepared by the high- pressure Bridgman technique, have been analyzed using transmission electron microscopy, scanning electron microscopy, infrared-light microscopy and visible-light microscopy. These analyses show that the grain boundaries inside the Cd_1-xZn_xTE materials are decorated with tellurium precipitates. Analysis of a tellurium precipitate at a grain boundary by transmission electron microscopy and selected-area electron diffraction found the precipitate to consist of a single, saucer-shaped grain. Electron diffraction from the precipitate was consistent with the trigonal phase of tellurium (space group P3,21), which is the equilibrium phase at room temperature and atmospheric pressure. This precipitate was found to be aligned with one of the adjacent CZT grains such that the tellurium (0111) planes were nearly parallel to the CZT (111) planes. High-resolution transmission electron microscopy of the Te/Cd_1-xZn_xTe interface showed no tertiary phase at the interface. The structures of the grain boundaries and the Te/Cd_1-xZn_xTe interface are discussed and related to their possible implications on Cd_1-xZn_xTe gamma-ray detector performance.

Various types of precipitates and grain boundaries have been studied in Cd_1-xZn_xTe (CZT). In this study we used elemental analysis methods such as scanning electron microscopy (SEM), microprobe analysis, inductively coupled plasma mass spectroscopy (ICP/MS) and the new laser ablation ICP/MS methods. Transient charge technique was applied for the first time of CZT crystals for evaluating the electrical transport properties of semiconductors. Another method, IR transmission spectroscopy, enables us to evaluate the microstructure defects and then to correlate this with impurity level and electrical properties in order to have a better understanding of the requirements to improve the yield for large volume CZT spectrometers. We have evaluated crystals from the former Soviet Union, which have high concentration of defects. Precipitates and grain boundaries rich with carbon were observed in CZT crystals. Electrical transport properties such as (mu) (tau) (mobility-lifetime product) were measured and correlated with the chemical physical defects, as observed by IR transmission. On crystals rich with many microstructures, as shown by IR transmission, lifetimes below 1 microsecond(s) were measured, compared with 5 - 15 microsecond(s) on the detector grade materials. SEM and microprobe analysis performed on the precipitates gave high values of carbon. However, using laser ablation ICP/MS, a value in the range of 200 - 800 ppm for carbon was measured.

During the past decade high-pressure Xe (HPXe) detectors have been successfully developed for gamma-ray spectroscopy of the energy range between 0.1 - 2.0 Mev. The feasibility of these detectors has been demonstrated in a number of experiments, which covered areas from gamma-ray astronomy through to various industrial applications. The attraction of HPXe arises from its excellent properties as a detecting medium. It has high stopping power, an intrinsic energy resolution of less than 1%, and with the development of new purification methods, the ability to operate for many years without loss of performance. The recent developments of the HPXe detectors are summarized in this paper. These include: (1) purification techniques, (2) the spectroscopic and scintillation properties of Xe, and (3) some practical aspects of designs of HPXe detectors.

Designs of high pressure xenon gamma-spectrometer with high energy resolution is considered. This device is intended for research of space gamma-radiation and space gamma-bursts. It consists of four independent gamma-detectors which are cylindrical ionization chambers with shielding grids. They are filled with high pressure xenon with density 0.5 g/cm³. Working volume of each chamber is 6000 cm³. Energy resolution of the gamma-spectrometer is 2.5% at energy 662 keV and 1.5% at energy 1170 keV. Its sensitive surface equals to 2500 cm². It is planned to install the developed instrumentation on the scientific power platform of International Space Station.

Highly compressed and purified xenon is emerging as an important detection medium for high resolution, room temperature gamma radiation spectroscopy. Detectors based on compressed xenon offer a unique combination of thermal stability, high energy resolution and large volume. Furthermore, fluid based detectors are not susceptible to radiation damage, and can be constructed in a variety of geometries. However, some important factors have delayed the development of practical xenon detectors for widespread use. These factors include the relatively high operational pressures and voltages and the need to maintain extremely high xenon purity. We have recently developed a 0.7 liter gridded ionization chamber xenon gamma radiation detector in a cylindrical geometry. The detector operates at room temperature and provides an intrinsic energy resolution of 1.8% at 662 keV which is five times greater than scintillation based spectrometers. The detector design and performance variables are discussed in comparison to a previous detector constructed in a planar geometry. Our results indicate that practical xenon detectors can now be developed for a wide variety of applications.

XENA is a new Compton telescope concept, designed to image about 50% of the gamma-ray sky with a sensitivity that would significantly surpass CGRO/COMPTEL's multi-year sensitivity with a 2 weeks balloon flight from the Southern Hemisphere. The detector, based on liquid xenon time-projection chambers, is optimized for approximately 0.3 - 10 MeV and combines high efficiency within a 3 sr field-of-view with approximately 1 degree(s) angular resolution and excellent background reduction capability. XENA's primary scientific goal is the discovery and mapping of ⁶⁰Fe radioactivity from the Galaxy, which is pivotal for understanding nucleosynthesis. XENA will detect ⁶⁰Fe even if current predictions are 7X overestimated. At 1.8 MeV, XENA's sensitivity (6 10^-6 cm^-2 s^-1) will significantly refine the COMPTEL ²⁶Al mapping along the Southern Milky Way. Also, XENA would be the first instrument capable to decide whether the 3 - 7 MeV excess seen in Orion is indeed due to nuclear lines from ¹²C and ¹⁶O, and it could discover the predicted lower-energy lines. The scanned sky area includes many continuum (gamma) -ray sources as well, such as pulsars and numerous (gamma) -ray AGNs. Secondary scientific objectives include also supernova remnants, gamma-ray bursts, and solar flares.

In this paper, a novel approach for developing a large area, high spatial resolution x-ray imaging detector is discussed. This approach integrates the flat panel amorphous silicon readout technology with the polycrystalline lead iodide photoconductive x-ray detection technology. This Pbl₂ detector design is promising because it provides high x-ray stopping efficiency, high efficiency conversion of x-ray energy into electronic change, high signal amplitude due to efficient collection of these changes, and high spatial resolution due to electro-static focusing of these changes. We have designed and fabricated prototype 2' X 2' imagers with 200 micrometers pixels (256 X 256 elements) using this approach. The performance of these imagers is characterized by measuring their dark current, x-ray induced signal amplitude, spatial resolution, and uniformity of response. Some basic properties of lead iodide films are also evaluated and presented.

The purpose of this article is to describe a new methodology for analyzing x-ray transmission. This new technique is called the multi-energy method and is a generalized application of traditional dual-energy methods. This method can be used with broad or line spectra, with photons or other particle and on a large energy range e.g. for photons from 20 keV to higher than 10 MeV. The application of this method in the range 20 to 150 keV with Bremsstrahlung spectra is very fast and allows a precision better than 0.5% on the coefficient of attenuation of photons over a large atomic number range (1 to say 40) and over a large thickness range (< 20 g/cm²). This method, described in the article, protected by two international patents, is applied point by point in the explosive device detection system of Heimann Systems, commercially available under the name of XCalibur.

In this paper a new kind of process of ranking x-ray transmission information in the range 20 - 150 keV is described. This ranking method can be applied to other energy ranges and to other particles. Based on a classical statistical method, Principal Component Analysis, it can be applied to continuous transmission measurement as multi- energy method. In this case, the effective Z sensitivity is in the neighboring of 0.001, i.e. 5 times better than the classical dual-energy method. A x-ray imaging application: the explosive device detection systems of Heimann systems is described using both multi-energy model and this ranking process. It provides good quality radiographic images and accurate chemical identification. The entire methodology described in the article is covered by three international patents.

Electrode contacting on semiconductor radiation detectors has been a topic of active interest in many recent investigations. Research activities have focused on the morphology and chemistry of modified surfaces using sophisticated preparation techniques and employing characterization methods that are able to discriminate between surface and bulk effects. From an applied point of view, the detector fabrication technology involves a series of fabrication steps which can be optimized. Results of an ongoing effort to improve the performance of high resolution Cd_xZn_1-xTe spectrometers by addressing wafer surface preparation, electrode deposition and contact passivation are described.

We discuss two simple, computationally efficient methods of estimating pulse height spectra for semiconductor detectors, one based on analytical techniques and the other involving Monte Carlo simulation. The former method gives rapid insight into the impact of major material parameters on detector performance, while the latter yields reasonable realistic spectra incorporating all major effects. We use both techniques in conjunction with a simulated annealing algorithm to extract electron and hole transport parameters from measured spectra.

Polarized transmission optical profiles were employed to characterize the CdZnTe (CZT) room-temperature radiation detectors. 2D images reflecting the internal electric field intensity changes were obtained utilizing the Pockels electro-optic effect. Varieties of different types of CZT detectors, i.e., planar and P-I-N detectors, were investigated under different operating bias voltages, respectively. Single crystal and polycrystalline CZT detectors were also studied and compared. Nonuniform internal electric field distributions throughout the detector volumes were observed and analyzed. The grain- boundary effects to the internal electric fields will be presented and discussed, along with a theoretical simulation. A semiconductor energy band model associated with depletion layer width will be emphasized and discussed.

Novel electrode configurations, such as coplanar grids, have been successful in mitigating the effects of poor hole transport in CdZnTe gamma-ray detectors. However, poor material uniformity remains a major problem preventing the widespread application of such detectors in gamma-ray spectroscopy. Uniform electron transport is critical for achieving good gamma-ray detection performance in the coplanar-grid configuration. We have investigated the use of alpha-particle response as a quick and simple electron transport uniformity screening technique for material selection, and as a method to study other spectral broadening mechanisms in coplanar-grid detectors. The method consists of uniformly illuminating, with an alpha-particle source, the cathode side of the CdZnTe crystal in either a planar or a coplanar-grid detector configuration. In the planar geometry, the variation in the measured pulse height is dictated in large part by the uniformity of the electron transport. An alpha-particle spectrum that has a single sharp peak with little background indicates uniform electron transport and, consequently, that the CdZnTe crystal should result in a coplanar-grid detector with good gamma-ray detection performance. In the coplanar geometry, the measured pulse-height variation provides information on additional sources of spectral broadening. In this paper we present the results of our study to measure the correlation between these simple alpha-particle measurements and the coplanar-grid gamma-ray detector response.

Coded mask imagers for future high energy x-ray astronomy missions will require detector planes with areas of hundreds to thousands of cm² and position resolutions < 1 mm. Such detectors will enable coded mask imagers to discover and study thousands of high energy x-ray sources. The UCSD/WU/UCR/NOVA collaboration has been developing CZT detector systems with crossed-strip readout to meet these requirements. We report progress on a compact detector module with 41 cm² area and 0.5 mm spatial resolution. The design includes the bias network and ASIC readout electronics, and allows modules to be combined in large area arrays with very high live-area factors. Results from laboratory and balloon flight tests are presented.

High-resolution, gamma- and x-ray spectrometry are used routinely in nuclear materials safeguards verification measurements. These measurements are mostly performed with high-purity germanium detectors, which require cooling at liquid-nitrogen temperatures, thus limiting their utility in field and unattended safeguards measurement applications. Sodium iodide (NaI) scintillation detectors do not require cooling but their energy resolution (10% at 122 keV) is insufficient for many verification measurements. Semiconductor detectors that operate at room temperatures, such as cadmium-zinc-telluride (CZT) detectors, with energy resolution performance reaching 2.0% at 122 keV may be used for certain safeguards verification applications. We have developed hardware to utilize CZT detectors in x- and gamma- ray measurement systems and software to apply such a system in measuring ²³⁵U enrichment for safeguards verification purposes. The paper reports on the CZT detector-based measurement system and measurement results obtained with it. The paper also discusses work on additional improvements to broaden the applications of the system.

We report on the design and construction of a solar flare spectrometer that will employ four 15 X 15 X 15 mm CZT detectors with coplanar electrodes. The compact spectrometer, which will provide measurements in the 40 - 800 keV energy range, is being constructed for flight on the Joint Air Force Academy/Weber State University Satellite (JAWSAT). JAWSAT is scheduled for a Minuteman II launch into a 650 km sun-synchronous orbit early during the next solar maximum. The terminator orbit will allow our Room Temperature Semiconductor Spectrometer (RTeSS) to monitor solar hard x-ray and gamma-ray emission nearly continuously during the 23rd solar sunspot maximum. The primary scientific objective of the RTeSS is measurement of the broad line complex near 450 keV that is generated in solar flare spectra by the interaction of energetic alpha particles with helium in the solar atmosphere. The RTeSS spectral resolution of approximately 4% FWHM at 662 keV is well matched to the line splitting expected for likely alpha particle angular distributions and should allow us to place important constraints on the acceleration and the transport of flare ions. The primary technological objective of the RTeSS program is to test the feasibility of using CZT detectors for the construction of solar flare spectrometers that are capable of high resolution observations when operating at room temperature. If successful in low earth orbit, CZT solar flare spectrometers could lead to significant savings in mass, volume, and power consumption over conventional high-resolution spectrometers.

We report initial results from the design and evaluation of two pixellated PIN Cadmium Zinc Telluride detectors and an ASIC-based readout system. The prototype imaging PIN detectors consist of 4 X 4 1.5 mm square indium anode contacts with 0.2 mm spacing and a solid cathode plane on 10 X 10 mm CdZnTe substrates of thickness 2 mm and 5 mm. The detector readout system, based on low noise preamplifier ASICs, allows for parallel readout of all channels upon cathode trigger. This prototype is under development for use in future astrophysical hard X-ray imagers with 10 - 600 keV energy response. Measurements of the detector uniformity, spatial resolution, and spectral resolution will be discussed and compared with a similar pixellated MSM detector. Finally, a prototype design for a large imaging array is outlined.

Recently developed CdTe detectors with a Schottky anode contact are well suited for (gamma) -ray imaging applications in astrophysics and nuclear medicine. Since the response of the detectors to (gamma) -rays is homogeneous throughout their volume the detectors can be oriented to let the radiation enter perpendicular to the electric field. Thus, large area arrays with 10 mm thickness can be assembled. The leakage current and capacitance of the detectors is sufficiently low to connect more than a hundred together in a row-column read out reducing the number of electronic channels.

The analysis of experimental statistics, related to perfect CsI(Tl) crystals without impurities of oxygen radicals, shows that irradiation-induced variations in scintillation light output are mainly originated by the production of lattice defects capable of trapping electronic excitations. Below the nature of such irradiation-produced defects considered. It is shown that in the dose range of the order of 10 krad, where the known mechanisms of irradiation- induced changes are inefficient, the latter are originated by small pores growing under irradiation. Micropores affect scintillation yield even at low concentrations (approximately 10^-7) due to a large section of exciton trapping exceeding the geometric section. The proposed model of growing micropores permits one to explain the observed does dependences of conversion efficiency and to use them for the control over the concentration of incipient micropores--a structural characteristic important for radiation stability.

An exciting possibility for the GLAST main instrument is a scintillating fiber system where the properties of both a tracker and a calorimeter are combined in one type of detector module. This instrument provides all the detector capabilities required to achieve the science goals of the GLAST mission, at a substantially reduced cost compared to the baseline technology, and with the benefit of increased effective area and superior low energy angular resolution.

The payload of the European Space Agency mission INTEGRAL has been designed to study simultaneously gamma-ray sources in a wide field of view over many decades in energy (around 2 eV + 4 keV - 20 MeV) and thus make a major contribution to short time-scale high-energy astrophysics. The Optical Monitoring Camera (OMC) will contribute with observations of the optical emission of the gamma-ray targets, while an X- ray monitor will provide measurements in the hard X-ray range. This multiwavelength capability will provide invaluable diagnostic information on the nature and the physics of the studied sources. The main scientific objectives of the OMC are: (1) to monitor the optical emission from the sources observed by the gamma- and X-ray instruments, measuring this time and intensity structure of the optical emission for comparison with variability at high energies, and (2) to provide the brightness and position of the optical counterpart of any gamma- or X-ray transient taking place within its field of view. In addition, because of the wide field of view of the OMC, a large number of serendipitous optical variable and transient sources will be monitored. The OMC is based on a refractive optics with an aperture of 50 mm focused onto a large format CCD (1024 X 2048 pixels) working in frame transfer mode. With a field of view of 5 degree(s) X 5 degree(s) it will be able to monitor sources down to 19 V magnitudes. Typical exposure will consist of 10 integrations of 100 seconds each.