We show that the finite size of pixels employed in arrays of the type produced by IMARAD Imaging Systems Ltd. determines the performance of these devices in terms of energy resolution and efficiency. The geometrical considerations are similar to those discussed in the context of small pixels. Based on an understanding of this geometrical effect we show that the performance of the arrays may be optimized for particular device dimensions. In addition we also so that material grown by IMARAD Imaging Systems Ltd. by the modified horizontal Bridgman method is quite uniform and indeed well suited for the fabrication of imaging systems.

Preliminary results of experiments to investigate charge collection in CdZnTe detectors are presented. The experiments support the development of semiconductor- modeling tool for device engineering that will be used to design large volume CdZnTe detectors for gamma ray spectroscopy. Improved diagnostic methods are described, including an automated alpha particle scanner for charge pulse mapping. Semiconductor modeling techniques are presented along with methods to visualize charge transport. Experimental results are compared to a physical model that has been used routinely in research on room temperature devices for gamma ray detection.

We report on the simulation, construction and performance of prototype CZT imaging detectors employing orthogonal coplanar anodes. These detectors employ a novel electrode geometry with non-collecting anode strips in 1D and collecting anode pixels, interconnected in rows, in the orthogonal dimensions. These detectors retain the spectroscopic and detection efficiency advantages of single carried charge sensing devices as well as the principal advantage of conventional strip detectors with orthogonal anode and cathode strips, i.e. an N X N array of imagin pixels are realized with only 2N electronic channels. Charge signals induced on the various electrodes of a prototype detector with 8 X 8 unit cells are in good agreement with the simulations. The position resolution is about 1 mm in the direction perpendicular to the pixel lines while it is of the order of 100 micrometers in the other direction. Energy resolutions of 0.9 percent at 662 keV, 2.6 percent at 122 keV and 5.7 percent at 60 keV have been obtained at room temperature.

Recently, much has appeared in the literature concerning methods to improve the resolution and photopeak efficiency of CdZnTe radiation detectors operating at or around ambient temperature. THese methods generally involve either the use of modified electrode structures or pulse processing techniques, both of which add complexity, and hence cost, to the production and operation of such devices. In this paper, we will report on results obtained with a simpler, modified two-terminal device. The detector structure combines a planar anode with an extended surface cathode, and relies on a standard, single channel preamplifier/shaping amplifier system. The results obtained demonstrate that the charge collection efficiency of the device, as shown by the Peak- to-Valley ratio, is significantly improved when compared to the standard planar geometry, especially at higher photon energies.

We present initial result from our evaluation of a PIN contacted pixelated detector using cadmium zinc telluride substrate produced by IMARAD Imaging Systems. The Horizontal Bridgman (HB) grown crystals from IMARAD have been shown to produce high resolution photopeaks but they are also seen to have large dark current. We seek to test whether the use of PIN contacts could reduce the dark current and thus improve the spectral response especially in the 10-100 keV energy range. We have fabricated PIN and MIN detector with a 4 X 4 array of pixels on 10 X 10 mm substrates. Measurements of the detectors' dark current, spectral response, and internal electric field are presented and compared to IMARAD's ohmic contact detector and PIN detectors made of High Pressure Bridgman material.

The electrical and detection properties of Au/CdZnTe/CdS (m- i-n) gamma-ray and x-ray detectors, fabricated by sputtering technique, have been studied. The CdZnTe crystal was grown by the High Pressure Bridgman method. Current-voltage measurements show that the leakage current is reduced by an order of magnitude when the m-i-n detector is reversed biased. This is due to increased contact barriers, which produced effective blocking contacts. An energy resolution of 3.6 percent for the 59.6 keV line of ²⁴¹Am was obtained in the reverse bias, with almost no energy resolution for the forward bias due to excessive leakage current.

An x-ray and gamma-ray spectrometer (XGRS) is onboard the Near Earth Asteroid Rendezvous (NEAR) spacecraft to determine the elemental composition of the surface of the asteroid 433 Eros. The Eros asteroid is highly non-spherical in physical shape and the development of data management and analysis methodologies are in several areas a divergence from traditional remotely sensed geographical information systems techniques. Field of view and asteroid divergence from traditional remotely sensed geographical information system techniques. Field of view and asteroid surface geometry must be derived virtually and then combined with real measurements of solar, spectral and instrument calibration information to derive meaningful scientific results. Spatial resolution of planned geochemical maps will be improved from the initial conditions of low statistical significance per integration by repeated surface flyovers and regional spectral accumulation. This paper describes the results of a collaborative effort of design and development of the NEAR XGRS instrument ground system undertaken by participants at the Goddard Space Flight Center, University of Arizona, Cornell University, Applied Physics Laboratory, and Max Planck Institute.

An analysis of the variation in size of the best spectroscopy region as a function of the inter-electrode distance for planar CdTe detectors is reported. Four detectors with inter-electrode distance ranging between 1.0 and 2.5 mm, in steps of 0.5 mm, were investigated. The measurements were carried out by scanning the detectors with a narrow beam of 122 keV photons, obtained by using a 20 mm thick tungsten collimator having a 0.2 X 2 mm² collimating channel. The detectors were irradiated in the configuration with the electric field perpendicular to the incoming radiation, Planar Transverse Field. A 'best charge collection region' has been identified, close to the cathode and approximately 0.4 mm wide, independent of the detector thickness in the investigated range, in which the photopeak amplitude and the energy resolution assume the best and sufficiently constant values. When going outside this region, both these parameters become quickly worse, with a trend well described by the Hecht's relationship. These feature appear essentially in the energy range 30 < E_x < 250 keV, a very important one for space astrophysical applications, as well as in medical radiology. At higher energies, changes in the prevalent photon interaction mechanism require use of alternative methods to improve the spectroscopic performance. An appropriate selection of the width of the irradiated region can give the possibility to match the requirements of particular applications.

We utilized monochromatic high energy synchrotron x-rays to perform transmission diffraction measurements on bulk cadmium zinc telluride crystal used for room temperature radiation detectors. The high-energy measurements assess the crystalline properties throughout the thickness of the structures and we determined - in combination with standard high-resolution diffraction measurements - that most defects propagate through the width of the sample. Maps of both composition and crystalline quality were generated using both the high energy and standard diffraction sources and a clear correlation was observed for both composition variation and for mosaic structure. In some cases, up to twenty individual peaks are observed in a rocking curve with a beam size of 100 X 100 micrometers ² which allows for a determination of the crystallite size. This experiment represents the first time in which transmission high-energy x-ray diffraction techniques have been applied to examine the crystal quality of Cd_1-xZn_xTe used in nuclear radiation detector applications. The technique allows one to measure the bulk structural properties of thick crystals and the technique is particularly useful because these crystal are used as bulk devices. Hence, this enables one to perform a proper analysis of the relationship between the structural defects and the detector performance.

Semi-insulating CdZnTe radiation detectors from five leading crystal growers and universities were characterized by thermally stimulated conductivity (TSC), thermoelectric voltage spectroscopy (TEVS), dark conductivity, current- voltage, and variable temperature time-resolved and spatially-resolved photoluminescence (PL). By TEVS, which is an extension of the hot-probe method, all of the samples were found to have n-type electrical conductivities at room temperature and this implied that the dominant deep level is a donor level. The TSC and TEVS spectra showed that all of the samples had a dominant deep electron trap, a series of shallow electron traps, and a deep hole trap. Some of the samples showed large concentrations of shallow hole traps. A two level model of compensation is proposed which is consistent with the observed resistivities, electrical conductivities at room temperature, observed trap level energies, and observed trapping behavior. It consists of a dominant deep donor level compensating a smaller concentration of a hole acceptor level that may be shallow or deep. The model showed that the electrical conductivity type of the stably compensated materials at RT is determined by the dominant level of the compensation, which is a deep donor level for CdZnTe. Preliminary results from the variable temperature time-resolved and spatially-resolved PL showed that the emission from the traps dominate the photoluminescence spectra from these materials and that there is much spatial variation in the trap concentrations.

One of the critical challenges for large area cadmium zinc telluride (CdZnTe) detector arrays is obtaining material capable of uniform imaging and spectroscopic response. Two complementary nondestructive techniques for characterizing bulk CdZnTe have been developed to identify material with a uniform response. The first technique, IR transmission imaging, allows for rapid visualization of bulk defects. The second technique, x-ray spectral mapping, provides a map of the material spectroscopic response when it is configured as a planar detector. The two techniques have been used to develop a correlation between bulk defect type and detector performance. The correlation allows for the use of IR imaging to rapidly develop wafer mining maps. The mining of material free of detrimental defects has the potential to dramatically increase the yield and quality of large area CdZnTe detector arrays.

Vertical high pressure Bridgman (VHPB) was considered until now to be the most successful crystal growth method to produce CZT, for x- and gamma-ray detector crystals. Recently horizontal Bridgman (HB) CZT crystals produced by IMARAD Co. have also been successfully fabricated into nuclear spectroscopic radiation detectors. In view of our database of many years' study of the electrical properties of VHPB CZT grown and obtained from various sources we also studied the HB CZT crystals in order to compare the defects present in both different kinds of crystals grown by different methods. The VHB-grown samples were examined using thermoelectric emission spectroscopy (TEES), x- and gamma- ray spectroscopy and laser induced transient charge technique (TCT). The surface and the bulk crystalline homogeneity were mapped using triaxial double crystal x-ray diffraction and IR transmission spectroscopy. We have found a correlation between crystallinity, IR transmission microstructure an trapping times. Spectrometer grade VHPB CZT crystal exhibit trapping times of 20 microsecond(s) for electrons and 7 microsecond(s) for holes, however, regions, which were opaque to IR transmission, had trapping times shorter by one order of magnitude. The trapping times of HB CZT for electrons, were 10-15 micrometers . A similar trend has been observed on VHPB CZT crystals with poor crystallinity. The HB CZT crystals that we measured in this study had a crystallinity that was inferior to that of the best spectroscopic grade VHPB crystals.

In this paper, we will give first the description of a versatile material analyzer for x-ray crystallographic and compositional analyses. We will describe how we calculate and elaborate glass capillaries; we will describe some important parameters of the total reflection of x-rays: focusing, transparency, gain and divergence. Our first results concern a compositional study of CdZnTe crystals with a n x-ray beam diameter of 10 micrometers and we will show the variation of Zn on the surface of such a crystal. Another possibility of this device is x-ray microtopography of a film or of a wafer. We show, on one hand the way to find in classical centers by Laue micropatterns on the other hand we could establish a Bragg reflection pattern like those obtained in classical x-ray surface topography. In some heat treatments we can see so-called texture effects. We have also on our device the possibility to see microtexture effects in a way which can be compared to the Seeman-Bohlin experiment. The compositional experiments were also tested by SEM in order to see how these two devices complete each other. We can notice in some studies the advantages of x-rays because photons are less destructive than electrons.

A scintillating fiber detector is currently being studied for the NASA Gamma-Ray Large Area Space Telescope (GLAST) mission. This detector utilizes modules composed of a thin converter sheet followed by an x, y plane of scintillating fibers to examine the shower of particles created by high energy gamma-rays interacting in the converter material. The detector is composed of a tracker with 90 such modular planes and a calorimeter with 36 planes. The two major component of this detector are the scintillating fibers and their associated photodetectors. Here we present current status of development and test result of both of these. The Hamamatsu R5900-00-M64 multianode photomultiplier tube (MAPMT) is the baseline readout device. A characterization of this device has been performed including noise, cross- talk, gain variation, vibration, and thermal/vacuum test. A prototype fiber/MAPMT system has been tested at the Center for Advanced Microstructures and Devices at Louisiana State University with a photon beam and preliminary results are presented.

The details for optimizing gamma-ray measurement system for specific applications are not always well understood. The setup and operation of a system plays an important role in performance aspects such as maximizing detector lifetime, stability and minimizing the signal to noise ratio. In addition to system setup and operation, the effects of scintillation detector design and accompanying electronics (PMT) are discussed with respect to both gross counting and spectroscopy measurements in order to obtain reliable results. Data has been taken with various sodium iodide scintillation detectors to study system stability during transient such as power cycling and count rate fluctuations. These fluctuations may introduce substantial measurement uncertainty, and if not accounted for will propagate into an analyses. The above transients can also affect the detector lifetime, and if the system conditions are monitored properly, they can be used as a predictive tool for determining the useful life of a detector. Data is also presented to examine counting statistics in an overlapping spectrum as a function of spectral resolution and count rate. The objective is to determine the optimum counting time for the spectrum to reach a statistically stable shape. The data is reduced by examining the standard deviation of fitted Gaussian curves at ten second intervals. The result is a contour plat showing the time needed to reach stability, which increase with spectral resolution and decrease with a rising count rate.

We have investigated the performance of a position- sensitive, gamma-ray detector based on a CsI(Na) scintillator coupled to a Hamamatsu R3292 Position-Sensitive Photomultiplier Tube. The R3292 has an active area 10.0 cm in diameter. Utilization of the full active area of the photocathode is a goal that has been previously unrealized due to edge effects. Initial measurements with a 0.75 cm thick CsI(Na) crystal indicate that the performance starts to degrade as one reaches a radius of only 3.5 cm, reducing the active area by 60 percent. Measuring the anode wires we have found that this fall off is not solely due to crystal edge effects, but rather is inherent to the tube crystal system. In this paper we describe the results of our measurements and how good performance can be maintained across a full 10cm of the tube face through the use of a few additional electronics channels.

The continuing demand for greater resolution and real-time imaging and the availability of improved, position-sensitive light sensors such as a-Si arrays has pushed the development of small-pixel scintillator arrays. Both linear and 2D arrays of various designs and materials are considered. A discussion of available materials for small pixel arrays is presented, along with practical decision-making selection guidelines. The trade-offs in design parameters for scintillation arrays are discussed including pixel sizes and reflector types. Examples of pitch and pixel tolerances and transmission of arrays are given. The scintillation performance of several BGO and CsI(Tl) arrays on a-Si arrays is shown. References are made to lens-coupled CCD and fiber- optic/PMT readouts. Applications of the arrays discussed include baggage scanning, Computed Tomography imaging, Positron Emission Tomography, Positron Emission Tomography, flash radiography and industrial x-ray inspection.

We report on the growth of high purity n-GaAs using Liquid Phase Epitaxy and on the fabrication of Schottky barrier diodes for use as x-ray detectors using these layers. Our epilayers are grown form an ultra-pure Ga solvent in a graphite boat in a hydrogen atmosphere. Growth is started at a temperature of approximately 800 degrees C; the temperature is ramped down at 2 degrees C/min. to room temperature. Our best epilayers show a net-residual-donor concentration of approximately 2 X 10¹² cm^-3, measured by Hall effect. Electron mobilities as high as 150,000 cm² V^-1 s^-1 at 77K have been obtained. The residual donors have been analyzed by far IR photothermal ionization spectroscopy and found to be sulfur and silicon. Up to approximately 200 micrometers of epitaxial GaAs have been deposited using several sequential growth runs on semi-insulating and n⁺-doped substrates. Schottky barrier diodes have been fabricated using this epitaxial material and have been electrically characterized by current-voltage and capacitance-voltage measurements. The Schottky barriers are formed by electron beam evaporation of Pt films. The ohmic contacts are made by electron beam evaporated and alloyed Ni-Ge-Au films on the backside of the substrate. Several of our diodes exhibit dark currents of the order of 0.3-3.3 nA/mm² at reverse biases depleting approximately 50 micrometers of the epilayer. Electrical characteristics and preliminary performance results of our Schottky diodes using ¹⁰⁹Cd and ²⁴¹Am gamma and x- ray radiation will be discussed.

This paper describes recent advances in radiation-detection grade high purity germanium (HPGe) crystal growth technology and some of the recent significant developments in HPGe detectors for gamma-ray spectroscopy. These important developments were necessary to support the recent increases in the applications, uses and requirements for germanium detectors. Crystal characteristics and the performance of some devices are presented including the developmental work on large coaxial-geometry detectors as a single element, dual element and multi-element closely packed arrays. Also some of the applications of such detectors along with their resulting performance are discussed. Recent development work on monolithic segmentation of large high purity germanium crystals is presented. These detectors are high efficiency, high resolution devices for providing both position and energy information of incident high energy photons. Monolithic segmentation structure of large germanium detector improves energy resolving power by reducing the Doppler broadening effect associated with gamma rays emitted from nuclei moving at high velocity without sacrificing device efficiency. Segmentation provides a powerful tool for doing gamma-ray tracking along with pulse shape analysis, low energy filtration, and low energy background rejection, etc. This paper also reports on a revolutionary monolithic structure which is believed to be the first ever fabricated on large HPGe crystal. Discussions concerning the significance and advantages of this structure along with performance test results of the device are presented.

CdTe and related compound semiconductors are useful for high energy flux detection at room temperature. We propose a new fabrication technique for CdTe detectors in p-i-n design that is suitable for hard x-ray and gamma-ray spectroscopy. Using a high resistivity single crystal CdTe substrate, an iodine doped n-CdTe layer is grown homoepitaxially on one face of the crystal using the hydrogen plasma-radicals- assisted metalorganic chemical vapor deposition method working at a low substrate temperature of 150 degrees C. An indium electrode is evaporated on the n-CdTe side for an ohmic contact, while a gold electrode is placed on the opposite side which acts as a p-type contact. The p-i-n detector thus fabricated exhibited low leakage current at room temperature operation, below 0.5 nA at an applied bias of 350 V for a 2 X 2 mm² detector of thickness of 1 mm. Leakage current further decreased to 16 pA at 350 V while cooling the detector down to -15 degrees C. Spectral responses of the detector showed improved energy resolution for different radioisotopes of energies in the range form few tens of keV to several hundred keV and stable operation when operated at high applied biases or slightly cooling the detector. Performance of the different detectors thus fabricated will be presented.

Relationship has been studied between preparation conditions of the raw charge, crucible material, growth regimes and structure defectness and electrophysical properties of crystals Cd_1-xZn_xTe. The crystals were grown both from the raw material which had been pre-synthesized in quartz ampoules, and from the raw material synthesized from the elements directly in the growth furnace. It is shown that the best values of electric resistivity (rho) and sensitivity to x-ray and gamma-radiation are obtained for crystals grown in crucibles of highly pure coal graphite material from the pre-synthesized raw charge. Correlation has been established between values of (rho) and crystal defectness: decrease of dislocation density by 10⁴ times led to 10⁷ times higher values of resistivity. Concentration of dislocation etching pits regularly decreased with higher purity of the raw material and optimization of crystal preparation technology.

A CZT Compton Camera (CCC) is being built using two 3D position sensitive CZT detectors. Expected system performance was analyzed by analytical and Monte Carlo approaches. Based on the measurement of detector energy and position resolution, the expected angular resolution is approximately 3 degree and approximately 2 degrees for a +/- 30 degrees FOV for 511 keV and 1 MeV (gamma) -rays, respectively. The intrinsic efficiency for a point source 10 cm from the first detector surface ranges from 1.5 X 10^-4 to 8.8 X 10^-6 for 500 keV to 3 MeV.

A novel x-ray silicon detector for 2D imaging has been recently proposed. The detector, called Controlled-Drift Detector, is operated in integrate-readout mode. Its basic feature is the fast transport of the integrated charge to the output electrode by means of a uniform drift field. The drift time of the charge packet identifies the pixel of incidence. A new architecture to implement the Controlled- Drift Detector concept will be presented. The potential wells for the integration of the signal charge are obtained by means of a suitable pattern of deep n-implants and deep p-implants. During the readout mode the signal electrons are transferred in the drift channel that flanks each column of potential wells where they drift towards the collecting electrode at constant velocity. The first experimental measurements demonstrate the successful integration, transfer and drift of the signal electrons. The low output capacitance of the readout electrode together with the on- chip front-end electronics allows high resolution spectroscopy of the detected photons.

Large area silicon drift detectors (SDD) with areas up to approximately 1 cm² have been fabricated for x-rays. Recent novel designs have produced very low dark current, high electric field, and hence low noise and good charge collection. The developed structures were evaluated with low noise input amplification electronic components on Peltier coolers so that the temperature could be adjusted. Energy resolution of 143 eV FWHM at 5.9 keV was measured with a 50 mm² SDD whose corresponding noise level was 70 eV FWHM.

One of the most challenging and topical problems in modern gamma-ray spectroscopy is the need for thermally stable, high-resolution detectors. Temperature variations affect sensitive performance parameters in existing spectrometers such as leakage current in semiconductor photo-conductors and light output in scintillation-based spectrometers. For this reason, commercial gamma-ray detectors are restricted to operation over a very limited and often inconvenient temperature range. However, many important applications for radiation detectors including well logging, environmental surveillance, and gamma-ray astronomy require detectors that can provide high resolution spectroscopic measurements over a wide operating temperature range. Our research has demonstrated that highly purified and compressed xenon possesses a unique combination of properties that makes it particularly attractive as a thermally stable detection medium for high-resolution gamma-ray spectroscopy. Xenon- based detectors can exhibit energy resolutions of < 2 percent at 662 keV and are relatively insensitive to temperature variations. The state of the art in high- pressure xenon detector development is discussed including detector designs, current applications, existing limitations, and suggestions for future investigations.

The paper describes a gamma-ray spectrometer and reports on its primary parameters. The base of the spectrometer is two cylindrical detectors filled with compressed xenon, whose energy resolution is 14-15 keV at 662 keV. Software developed to control the gamma-ray spectrometer and to process experimental data is also described in brief. Measurement results of gamma-ray radiation from isotopes of plutonium and uranium are given. Measurements were conducted for various time exposures. It is shown that using this equipment, one can detect and identify a single gram of plutonium within several seconds at a distance of 1 m. Possibilities of the gamma-ray spectrometer application in order to accomplish control over displacement of fissile materials are considered.

In image plate technology the understanding of the radiation-induced generation of photostimulable luminescence centers (PSLCs) is still an unsolved problem in commercially utilized x-ray storage phosphors, such as BaFBr: Eu²⁺ and RbBr:Tl⁺. Two different mechanisms have been proposed: the first one assumes a vacancy-free crystal prior to irradiation known as Itoh process originally proposed for alkali halides whereas the second one relies on a vacancy- containing crystal where the vacancies are assumed to be F⁺-centers. In the present study we area addressing this problem through the determination of the relative concentrations of spatially-correlated and non-correlated centers for samples with different activator concentrations. The PSLC-concentration ratio meaning the concentration of the spatially correlated centers to the total center concentration is measured by means of photostimulated luminescence in the temperature range from LHe to room temperature. It is found that the Eu²⁺-concentration is playing a decisive role in the relative occurrence of the two PSLC-types in the sense that for higher Eu²⁺- contents the ratio of correlated to non-correlated centers is increasing. It will be shown that this result clearly favors the Itoh formation mechanism for x-irradiation induced PSLC generation.

Neutron imagin techniques are known since a long time as a possible tool for non-destructive testings. However, they found a reduced number of applications out of the inspection of the nuclear fuel so that their diffusion was limited to some centers. This is particularly true in Europe because industries and the army are not used to carry out extensive programs for R and D in the field of NDT. A new European- wide cooperative action was agreed in order to solve this problem. This action is currently running under the COST framework of the European Commission and is comprised of excellence universities, nuclear centers and industries from 10 European countries. Main purpose of this action is the application of neutron imaging - non limiting to transmission radiography and tomography - to the development of advanced materials to be used in the fields of aerospace, civil engineering and mechanics. The initial cumulative results of this action will be presented; furthermore the possibilities for future enlargement of this activity to other European and non-European countries will be explained as well.

A theological model is presented which analyses the sensitivity of composite detectors to a flux of x-rays emerging form a radiological x-ray generator. The model describes the many factor which influenced the x-ray response, for the case where the detector is composed of several layers of crystallites separated by a polymeric glue as is the case of composite HgI₂ detectors fabricated by the screen print method. The model also describes the variation of the sensitivity with grain size and dielectric constant, taking into account the dielectric constant of the binder showing also the experimental result. Finally, the experimental result of the sensitivity vs. the voltage is shown for single crystal and composite HgI₂ detectors and these results are compared with polycrystalline PbI₂ and a-Se, which are the main material candidates for medical digital radiology.

Radiographic imaging continues to be a key diagnostic in many areas at Los Alamos National Laboratory. Radiographic recording systems have taken on many forms, from high repetition-rate, gated systems to film recording and storage phosphors. Some systems are designed for synchronization to an accelerator while others may be single shot or may record a frame sequence in adynamic radiographic experiment. While film recording remains a reliable standby in the radiographic community, there is growing interest in investigating electronic recording for many applications. The advantages of real time access to remote data acquisition are highly attractive. Cooled charge-coupled (CCD) camera systems are capable of providing greater sensitivity with improved signal-to-noise ratio. This paper begins with a review of performance characteristics of the Bechtel Nevada large format imaging systems, a gated system capable of viewing scintillators up to 300 mm in diameter. We then examine configuration alternatives in lens coupled and fiber optically coupled electro-optical recording systems. Areas of investigation include tradeoffs between fiber optic and lens coupling, methods of image magnification, and spectral matching from scintillator to CCD camera. Key performance features discussed include field of view, resolution, sensitivity, dynamic range, and system noise characteristics.

Single-element CdZnTe detectors are limited in size, and therefore efficiency, by the poor hole transport, even with a coplanar grid. We are investigating the possibility of a 27-element array using 15 mm X 15 mm X 15 mm elements for gamma-ray energies to 10 MeV for NASA planetary missions. We present experimental results for combinations of various size coplanar grid detectors using NIM electronics and energies to 6.1 MeV. Summation of the signals after linear gating and requiring coincidence produces only a small increase in the energy resolution. Our results indicate that good efficiency and spectrum not complicated by a large Compton continuum can be achieved by simply summing the spectra from 15 X 15 X 15 mm³ detectors for gamma-ray energies below about 2 MeV. Above 2 MeV, 2-fold coincidence might be required, depending on the spectrum, to suppress the Compton continuum and escape peaks. We use a Monte Carlo calculation to predict the performance of the 27-elements array for a lunar highlands spectrum. Such ambient-temperature, high-efficiency, good- resolution arrays will facilitate new NASA mission to determine elemental composition of planetary bodies and terrestrial applications requiring high-efficiency, good- resolution portable instruments.

A new method of engineering the internal electric field of CdZnTe (CZT) radiation detectors will be introduced. The internal electric field distribution within a CZT detector is engineered via an IR beam with a special photon energy and characterized by a separate polarized optical transmission profile beam utilizing the Pockels electro- optic effect. A theoretical model and calculation will be presented to understand the internal electric field engineering we have performed in our work. 2D images reflecting the internal electrical field intensity changes will be shown and the application of this field engineering method to improve the radiation detectors will be discussed.

This paper describes the development of cylindrical coplanar grid CdZnTe detectors for gamma ray spectroscopy. Cylindrical detector offer a number of advantages over established designs. For example, grid structures for cylindrical detectors are simpler than those for rectangular designs. The goal of our work is to design a cylindrical coplanar grid detector with excellent resolution at low- and high-energy. Information on detector design and manufacturing is presented. Six detectors are characterized. The pulse height resolution of the best detector is 13.5 keV full width at half maximum at 662 keV and 5.5 keV FWHM at 122 keV.

We have used room temperature photoluminescence to correlate radiation detector performance with Cadmium Zinc Telluride (CdZnTe) material quality in previous work. Enhancements in the experimental apparatus and the analysis software now allow us to achieve higher spatial resolution along with PL lineshape analysis. We have examined several CdZnTe crystals previously characterized as radiation detectors with the new apparatus and report on the results of this analysis method are presented along with experimental results.

Position-sensitive CZT detectors for research in astrophysics in the five - several hundred keV range are being developed by several groups. These are very promising for large area detector arrays in coded mask imagers and small-area focal plane detectors for focusing x-ray telescopes. We have developed detectors with crossed-strip readout and optimized strip widths and gaps to improve energy resolution. A 'steering electrode' is employed between the anode strips to improve charge collection. A model of charge drift in the detectors and charge induction on the electrodes has been developed to allow us to better understand these types of detectors and improve their design. The model presently accounts for the electric field within the detector, the charges' trajectories, mobility and trapping of holes and electrons, and charge induction on all electrodes including their time dependence. Additional effects are being added. The model is described and its predictions are compared with laboratory measurements. Results include (1) the dependence of anode, cathode and steering electrode signal on interaction depth, transverse position, electron and hole trapping, strip width and gap, and bias, (2) trajectories of charges for various anode and steering electrode bias voltages, (3) a method to improve energy resolution by sign depth of interaction information, and (4) an electrode geometry and bias optimized for the improved energy resolution. In general, the model provides good agreement with the measurements.

Gamma photon absorbed in a semiconductor detector generates holes and electrons that flow towards the opposite contacts. Poisson and the continuity equations require that additional charge will come out of ohmic contacts and flow towards the gamma generated charge. The fast electron flow, from the negative contact towards the holes, overcomes holes trapping and transforms the detector into a practically single carrier device. The detector operation is optimized by trade off between the leakage current and the lifetime of the gamma generated electrons, that is, proper positioning of the Fermi level within the semiconductor forbidden band. Crystals that grow by standard methods, and contain too may traps for conventional detector operation, are applied to fabrication of uniform ohmic detector arrays.

The electronic structures of Cd vacancy and vacancy-Cl pair in CdTe and Cd_1-xZn_xTe are studied with both an ab initio pseudopotential method and an empirical tight- binding method. Ad hoc model potentials were added to simulate the many-body self energy correction to the band structure in order to obtain the correct band gaps at high symmetry points. The local geometries of defects are first determined by the ab initio method with the use of an eight- atom supercell. The band structures of the supercell system are also calculated via an empirical tight-binding model with interaction parameters among atoms surrounding the defect site adjusted to fit the corresponding results obtained from the ab initio method. The tight-binding method is then used to calculate the defect levels for a much larger supercell consisting of CdTe or CdZnTe. The resulting defect levels are in fair agreement with available experimental data.

We have developed a Monte Carlo simulator for semi- insulating GaAs detectors which gives the energy spectra of x-ray radiations. The simulated spectra are analyzed in terms of: shaping time, trapping properties of the material, and applied reverse voltage. The main features of the spectra as well as the associated charge collection efficiency and the energy resolution of the photoelectric peak are interpreted in physical terms for the whole range of applied voltages covering under- and over-depleted conditions. The results of the simulations provide a general interpretation scheme which is satisfactorily tested with experimental results.

The performance of CZT pixel detectors, with dedicated ICs and electronic processors, have been demonstrated. These nuclear imaging modules, developed primarily for the medical market, may be utilized for other applications, such as large area nuclear spectrometers. An improved crystal growth technique ensures a practical supply of wafers of which high performance detectors are fabricated. We believe that the high spectroscopic quality of these detectors stems from their effective Ohmic behavior, coupled with the geometrical, 'small pixel' effect. The Ohmic operation of these detectors has been described in a schematic way only, where the detailed non-equilibrium mechanism, responsible for it, still remains to be explained in detail. The IMARAD detector type 2, with contacts which strongly limit the dark current, exhibit even improved spectroscopic behavior, due to a dynamic Ohmic behavior of these contacts.

In this paper we analyze in detail a peculiar behavior of CdTe detectors, namely the fact that in conjunction with a typical resistive electrical characteristic in which the current is proportional to the externally applied biasing voltage through an equivalent detector resistance, the detector shows a noise level well above the corresponding Johnson noise and close to the shot noise of the standing current. By using a correlation spectrum analyzer, a careful and extensive experimental analysis of the current noise behavior of CdTe detectors for X and (gamma) ray has been performed. The current noise spectra have been measured over a wide range of frequencies, from below 1Hz up to 100kHz and operating points from 0V up to 150V. In addition to a strong 1/f component, a white noise is present at the level of the shot noise of the standing current and extending in frequency for a limited range related to the carriers transit time across the detector.

In several fields of the nuclear industry, high fluxes of gamma radiations have to be measured. The small size of semiconductor detectors would be a good approach; however, in general degraded by the radiations. To overcome these difficulties, highly radiation-resistant silicon detectors have been developed for the measurements of high fluxes of gamma-rays, as encountered for example during in situ measurements in reactors. The detector is mounted in a waterproof 8.0 mm diameter probe, incorporating also all the electronics. The system can operate in two modes: either as counter of individual pulses generated by the radiations, or direct measurement of the induced d.c. current. In the first situation, the sensor operates as photodiode, whereas in the second case, photovoltaics mode is selected, i.e. without any applied bias voltage. When operating in the counter mode, a linear response over eight orders of magnitude of the flux can be measured, in the photovoltaics mode, the dark current limits the lower flux to values of about 5- 10.10^-2 Gyh^-1.

Recent results on the radiation hardness of silicon detectors fabricated on gloat zone bulk silicon enriched by carbon and oxygen are reported. The results indicate that the radiation hardness of silicon detectors can be determined by the concentration of oxygen and carbon atoms in the bulk material. The study has been carried out in the framework of the RD48 collaboration, which is studying the radiation hardening of silicon detectors.

In recent years, room temperature semiconductor detectors like CdTe and CdZnTe have been proposed for several scientific, industrial and medical applications. In some cases, these applications require the capability to operate for a long time in intense, sometimes mixed, radiation fields while retaining full spectroscopic performances. In spite of its importance, a detailed characterization of the effects of the radiation damage has not been competed yet for these materials. However, preliminary results carried out by irradiating CdTe detectors with ⁶⁰Co gamma rays demonstrated that the fatal photon dose is on the order of some tens of thousands Gy, while the detectors are rather insensitive to doses up to few thousands Gy. A wider activity is now being performed by the authors studying both the CdTe and the CdZnTe when involving several types of irradiating sources and different techniques to quantify the induced damaging. This includes the detailed quantitative analysis of retained spectroscopic performances at low and medium energies, the measurement of the dark current and the characterization of the defects and modifications induced in the structure of the crystal by the irradiation process.

The literature of radiation damage measurements on cadmium zinc telluride (CZT), cadmium telluride (CT), and mercuric iodide is reviewed and supplemented in the case of CZT by new alpha particle data. CZT strip detectors exposed to intermediate energy proton fluences exhibit increased interstrip leakage after 10¹⁰ p/cm² and significant bulk leakage after 10¹² p/cm². CZT exposed to 200 MeV protons shows a two-fold loss in energy resolution after a fluence of 5 X 10⁹ p/cm² in thick planar devices but little effect in 2 mm devices. No energy resolution effects were noted from moderated fission spectrum neutrons after fluences up to 10¹⁰ n/cm², although activation was evident. Exposures of CZT to 5 MeV alpha particle at fluences up to 1.5 X 10¹⁰ (alpha) /cm² produced a near linear decrease in peak position with fluence and increases in FWHM beginning at about 7.5 X 10⁹ (alpha) /cm².

The on-board flight software for the Near Earth Asteroid Rendezvous (NEAR) spacecraft was modified to produce continuous 1-sec sampled rate information from the shield of the x-ray and gamma ray spectrometer (XGRS) instrument. Since the XGRS shield can also detect gamma ray bursts (GRB), this rate information can be used in combination with the GRB detections by the Ulysses and near-Earth GRB instruments as part of the interplanetary network (IPN) to triangulate the source direction of GRBs. It is the long baseline of NEAR combined with the Ulysses baseline that makes small error box locations possible. We have developed an automated system to analyze the periodic telemetry dumps from the NEAR spacecraft. It extracts this new data type, scans the ate information for increases which are plausibly of GRB origin, and combines these with the GRB detections from the others spacecraft. Because the processing is automated, the time delay to produce the triangulated positions is kept to a minimum, up to 48 hours. This automated processing and distribution of the GRB locations is done within the GRB Coordinates Network system. About 60 locations per year with errors ranging from a few to tens of arcminutes are expected. These rapid precise localizations may provide about 10 times the rate currently provided by the WFC and NFI instruments on BeppoSAX.

In recent years, considerable effort has been expended in producing semiconductor based x-ray detectors for x-ray astrophysics with high spectral and high spatial resolution. In practical terms, this means producing pixelated detectors, comprising over 10³ pixels each less than 100 microns in size, with spectral resolving powers, E/(Delta) E > 20 at 10 keV. While progress at soft x-rays wavelengths has been spectacular, largely due to the introduction of x- ray sensitive CCD's, progress at higher energies has been slow. This is because traditional high resolution detectors either suffer from poor detection efficiencies above 10 keV, as in the case of Si based technology, or are very constrained by cryogenic and fabrication problems as in the case of Ge based detectors. Recent developments in the material science of wide-gap semiconductors, and in particular GaAs and CdZnTe, have shown that it may now be possible to construct efficient hard x-ray detector with near Fano limited energy resolution. In this paper, we report on hard x-ray measurements with two prototype deep depletion epitaxial GaAs detectors of active areas 2.22 mm² and thicknesses 40 and 400 microns at the ESRF and HASYLAB synchrotron research facilities. The results show that charge collection efficiencies must be in excess of 98 percent and that the material used to produce them is of extremely high purity, with impurity concentrations < 10¹³ cm^-3.

Properties are reported of new types of active module 32- channel scintielectronic detectors of 'scintillator- photodiode' type with step h equals 0,8 mm for low-energy x-ray introscopic systems. As scintillator, 'fast' modification of crystal ZnSe(Te) was used in three variants: individual single crystal elements for each channel, a single crystal plate common for all channels, and a small crystalline composite plate with grain size d equals 0,25 divided by 0,5 h. Active detectors based on ZnSe(Te) scintillators and active 32-channel photoarrays with combined pattern of preliminary transformation and sequential sampling ensures x-ray sensitivity of 0,8 divided by 2.

New dense oxide glass scintillator has been discovered in the system: B₂O₃-SiO₂-Al₂O₃- Gd₂O₃ doped with trivalence cerium. This scintillator has a unique combination of properties including high scintillating light output, high density and desirable emission wavelength, large radiation hardness, short radiation length, and easy to produce in large size and low price. Here we compare the properties with those of the two recently developed dense scintillators: Ce³⁺-doped fluorophosphate glass and Ce³⁺-doped fluorohafnate glass, and with those of the three most widely used scintillating crystals Ce:YAP,CeF₃ and BGO.

Cadmium zinc telluride (CdZnTe) is being developed for room- temperature x-ray and gamma ray detectors. Identification and control of point defects and charge compensators are currently important issues. We have used electron paramagnetic resonance (EPR) and photo-induced EPR to evalute shallow-donor defects in CdZnTe crystal grown by two different techniques. Samples grown by the high-pressure Bridgman technique and a crystal grown by horizontal Bridgman at IMARAD and doped with indium were included in this study. Prior to the EPR investigations, we performed liquid-helium photoluminescence (PL) in order to examine the radiation recombination paths and identify the presence of other defects in these crystals. Spectra were obtained showing sharp excitonic lines, shallow and deep DAP emission bands, and a deeper 1.1 eV emission. The PL data help define the optical excitation range used in photo-EPR measurements. The photo-EPR data obtained from our samples is used to determine the concentration of isolated donor centers, while the EPR signal present under no illumination gives a measure of the net compensation. We also report the excitation wavelength dependence of the isotropic EPR signal from the shallow donors.

We have studied the ability of different oxidizing agents, other than H₂O₂ to reduce the surface leakage current of CdZnTe devices. All chemical treatments were performed in aqueous solutions, at room temperature, with weight percent concentrations of 2.5g/25ml. Before and after I-V curves were obtained. It was found that by increasing the basicity of the chemical treatment, greater reduction in surface leakage current occurred. The result show that these alternative chemical treatments reduced the surface leakage current as well as or better than H₂O₂ chemical treatment.

The technological process of fabrication of surface barrier silicon detectors by using ultra-high purity P-type silicon crystals with extra-high resistivity is described. An investigation on the reduction of the leakage current of this device type has been carried out. Indeed, several surface barrier silicon detectors have been realized and each of them has undergone a specific surface treatment. The best result has been obtained with K₂Cr₂O₇- solution treatment.

New type of polystyrene-based scintillators UPS98GC were tested regarding long term stability, radiation hardness and light yield uniformity for different doses and dose-rate levels of gamma radiation. They were compared to SCSN-81 produced by Kuraray Co. which has often used in high-energy physics experiments. The dependence of scintillator properties on radiation dose rates as well on total dose values is studied. It is shown that for relatively small dose rate, closed to those expected during scintillator lifetime, our UPS98GC does not yield to SCSN-81.

We report the results of recent calibration data analysis of a prototype scintillating fiber tracking detector system designed to perform imaging, spectroscopy and particle identification on 20 to 250 MeV neutrons and protons. We present the neutron imaging concept and briefly review the detection principle and the prototype description. The prototype detector system records ionization track data on an event-by-event basis allowing event selection criteria to be used in the off-line analysis. Images of acrylic phantoms from the analysis of recent proton beam calibrations are presented as demonstrations of the particle identification, imaging and energy measurement capabilities. The measured position resolution is < 500 micrometers . The measured energy resolution is 14.2 percent at 35 MeV. The detection techniques employed can be applied to measurements in a variety of disciplines including solar and atmospheric physics, radiation therapy and nuclear materials monitoring. These applications are discussed briefly as are alternative detector configurations and future development plans.

Evaluation was made of changes in scintillation and dynamic characteristics of detection blocks of 'scintillator- photodiode' type based on crystals ZnSe(Te) and CsI(Tl) after long-term storage of unprotected crystals under normal climatic conditions, as well as after action of water vapor. It is shown that changes of energy resolution of large CsI(Tl) crystal are not significant; only the radiation noise threshold is changed due to the phosphorescence of the disturbed surface layer of the crystal. For single crystals ZnSe(Te), which are used for spectrometry of charged particles and short-range gamma-radiation as thin plates, both dynamic noise threshold and spectrometric characteristics are changed. Possibilities to restore the initial crystal properties are discussed, and ways to protect the crystal surface from environmental effects are considered.

The difference is sped of fast neutrons and gamma rays has been investigated as a mechanism for distinguishing neutron and gamma interactions in plastic scintillators. Successive scattering of neutrons in a scintillator results in pulses that are broader than those from gamma rays because of the longer time interval between scatterings. However, the experimentally observed pulse widths for these neutrons appear to be insufficiently different from those of gamma rays to permit discrimination on a pulse-by-pulse basis. An alternate technique using scintillators separated by an air gap has also been investigated using time-of-flight measurements. For this configuration the efficiency for neutron detection has been measured along with the interference from gamma rays when sources emitting both neutrons and gamma rays are present.

In irradiated dielectrics a high electrical field strength and heating arise, the electrothermal breakdown appears therefore. In this work on base of experimental and theoretical data development of thermofluctuation theory of dielectrics fatigue is formulated.

This paper describes the preliminary results obtained from our study of optical and electrical properties of BiI₃ crystals. The bismuth iodine polycrystals were grown using commercial starting material by vertical Bridgman method. For our measurements we used only single crystal samples that were cut out from grown crystals perpendicular to C₆-axis.

The charge transport properties and radiation detector performance of semi-insulating CdTe single crystal grown by the conventional vertical Bridgman technique in this paper. The measured room-temperature electrical resistivity of the crystals is below the theoretical maximum allowed by the band gap of CdTe indicating incomplete electrical compensation of the material. The crystals show excellent spectroscopic performance in the 15 keV - 662 keV energy range, with reduced low-energy tailing in the photopeaks. The energy resolution of the best detector was 2.7 keV full width half maximum (FWHM) at 59.5 keV, 4.5 keV FWHM at 122 keV, and 20.1 keV FWHM at 662 keV. This improved performance is attributed to the improved hole transport over the typical HPB CdZnTe. The measured mobility-lifetime product of holes, (mu) (tau) _h approximately equals 2.3 X 10^-4 cm²/V, is significantly higher than that typical for HPB CdZnTe crystals. The measured electron (mu) (tau) _e approximately equals 1.6 X 10^-3 cm²/V of these CdTe crystals suggest somewhat poorer electron transport than in a spectroscopic grade HPB material.

We have investigated, both experimentally and theoretically, how to reconstruct in 3D the interaction positions for (gamma) -rays penetrating into a double-sized Ge cross trip detector. We found that when a suitable geometry is used, the 3D-reconstruction problem can be reduced to three 1D ones, which greatly simplifies the task. We report measurements on a 10mm thick detector with 2mm strip pitch, showing that at least 2mm position resolution can easily be achieved perpendicular to the detector plane. While the in- plane resolution is presently limited to the strip pitch we present work on progress in developing algorithms to improve this. This includes in particular the expected effects of the electronics and the interstrip capacitance on the signal shapes. Finally, we present captured waveforms that indicate the possibility of reconstructing more complex events such as Compton scattering.