In this paper, we present results of experiments to characterize a large-volume (4 cm³), multi-element CdZnTe detector for high-efficiency, gamma-ray spectroscopy. The module includes an array of eight 0.5 cm³ coplanar grid detectors manufactured by eV Products. An eight-channel data acquisition system with list mode output is used to record gamma-ray events for each detector in the array. The list mode data are analyzed to determine the efficiency for coincidence events and to demonstrate different modes of operation (e.g., Compton suppression). The total efficiency of the array is found to match Monte Carlo calculations to within a few percent; however, the full-energy (photopeak) efficiency is significantly lower than predicted by Monte Carlo. The observed difference is probably caused by a combination of electrode design and charge transport properties. Approaches to improve full-energy efficiency are proposed.

Thermoelectric effect spectroscopy (TEES) and thermally stimulated current (TSC) were used to study the deep trapping levels in undoped and Sn doped Cd_1-xZn_xTe crystals. Temperature maximum, varying heating rate and initial rise methods were used to extract activation energies and trapping cross sections of the deep trapping levels in the samples. The concentration of deep levels was estimated from the resistivity data using Neumark model. The pure sample had deep trapping levels with ionization energies of E_V + 0.73 eV and E_V + 0.74 eV. The concentration of the deep levels was estimated at 500 ppb. These deep levels were associated with intrinsic defects due to Cd vacancies and Te antisites. In the Sn doped samples deep levels at E_V + 0.34 eV, E_V + 0.55 eV and E_V + 0.73 eV were observed. The level at E_V + 0.55 eV was associated with Sn, while the E_V + 0.34 eV and E_V + 0.73 eV levels were associated with Cd vacancies. The concentration of these levels was estimated a 10000 ppb. The pulse height measurements on these samples indicated that Sn doping did not improve the detector performance.

An irreversible deterioration of CdTe and CdZnTe detectors after heat treatments in the temperature range of 150 - 200 degrees Celsius was reported by several authors; however, the nature of the processes responsible for the detector degradation and increased dark currents is not fully understood. In this study we have prepared CdTe and CdZnTe detectors equipped with Au contacts. The detectors were tested before and after thermal annealing under vacuum. Using combined measurements of current voltage characteristics, low temperature photoluminescence and nuclear spectroscopic measurements, we have attempted to differentiate between the various possible contributions to the detector degradation and elucidate the defect formation process involved.

Silicon Drift Detectors (SDDs) with integrated readout transistors combine a large sensitive area with a small total readnode capacitance and are therefore well suited for high resolution, high count rate X-ray spectroscopy. The low leakage current level obtained by elaborated process technology makes it possible to operate them at room temperature or with moderate thermo-electric cooling. The monolithic combination of several SDDs to a multichannel drift detector solves the limited of size and allows for the realization of new physics experiments and systems. Up to 3 cm² large SDDs for spectroscopic applications were fabricated and tested. Position sensitive X-ray systems are introduced. The description of the device principle is followed by the introduction of the multichannel drift detector concept. Layout, performance and examples of current and future applications are presented.

A preliminary study of the response of SiC devices to 22 MeV electrons, 6 MV photon beams from a linear accelerator and to (alpha) -particles from a ²⁴¹Am source is presented in this work in view to assess the feasibility of SiC-based detectors and dosimeters. The devices used are 4H-SiC epitaxial n-type layer deposited onto a 4H-SiC n⁺type substrate wafer doped with nitrogen. Schottky contacts have been formed by deposition of a 1000 angstrom gold film on the epitaxial layer. Ti/Pt/Au ohmic contacts have been deposited on the rear side of the detector. The released charge has been observed to increase linearly with the electron dose up to 10 Gy. A linear dependence of the current response of the devices has been also observed as a function of the photon dose-rate in the 2-7 Gy/min range. A preliminary study of the photoconductive response to UV irradiation of semi-insulating 6H-SiC substrates is also reported on samples, with a bulk resistivity of approximately equals 10¹¹ (Omega) cm, produced with a modified Lely technique.

We present preliminary results of X-ray measurements on two small format compound semiconductor arrays. The devices, a 5 X 5 gallium arsenide array and a 3 X 3 cadmium zinc telluride array, were produced specifically to address the material, electronic and technological problems that need to be solved in order to develop mega-pixel, Fano limited spectroscopic arrays. The GaAs device was fabricated on 40 micrometer epitaxial material and has a pixel size of 200 X 200 microns² with pitch 250 micrometer. The CdZnTe array was fabricated on a 5 X 5 X 1.6 mm ³ single crystal of spectroscopic quality. The pixel sizes were 350 X 350 microns² with a pixel pitch of 250 micrometer. Measurements from 5.9 keV to 100 keV were carried out both in our laboratory and at the HASYLAB synchrotron research facility in Hamburg, Germany. The typical FWHM energy resolutions recorded at 5.9 keV by the GaAs and CdZnTe arrays were 394 eV and 900 eV, respectively.

In x-ray and gamma-ray spectroscopy, it is desirable to have detectors with high energy resolution and high absorption efficiency. At LLNL, we have developed superconducting tunnel junction-based single photon x-ray detectors with thin film absorbers that have achieved these goals for photon energies up to 1 keV. However, for energies above 1 keV, the absorption efficiency of these thin-film detectors decreases drastically. We are developing the use of high-purity superconducting bulk materials as microcalorimeter absorbers for high-energy x-rays and gamma rays. The increase in absorber temperature due to incident photons is sensed by a superconducting transition- edge sensor (TES) composed of a Mo/Cu multilayer thin film. Films of Mo and Cu are mutually insoluble and therefore very stable and can be annealed. The multilayer structure allows scaling in thickness to optimize heat capacity and normal state resistance. We measured an energy resolution of 70 eV for 60 keV incident gamma-rays with a 1 X 1 X 0.25 mm³ Sn absorber. We present x-ray and gamma-ray results from this detector design with an Sn absorber. We also propose the use of an active negative feedback voltage bias to improve the performance of our detector and show preliminary results.

An understanding of compensation and trapping in Cd_1-xZn_xTe and HgI₂ is necessary in order to improve the size and spectroscopic performance of radiation detectors fabricated from these materials. Although several electron and hole traps have been identified, very little is currently understood about the effect of specific carrier traps on the mean free path of the charge carriers. Characterization techniques such as Thermally Stimulated Current (TSC) or Thermoelectric Emission Spectroscopy (TEES) have been used for trap identification, while time-of-flight techniques have been employed to determine carrier mobility and lifetime but it has proven difficult to correlate the results of these independent measurements. Furthermore, these characterization methods are complicated by the need to make electrical contacts to the material. Here we report on contactless, thermally stimulated lifetime measurements performed on detector-grade Cd_{1- x}Zn_xTe (x approximately 0.1) and HgI₂ crystals using a microwave cavity perturbation method. The microwave technique is complimentary to contact-based methods and provides both trap identification and lifetime determination in a single measurement. The results provide evidence of lifetime-limiting deep traps in these materials. The trap activation energies and the minimum detrapping times are estimated and the results are compared to previous TSC and TEES investigations.

We present initial results from our evaluation of a gold- contacted pixellated 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 leakage current. Our previous tests with IMARAD CZT showed that the use of indium anodes and gold cathode improved the resistivity compared to the standard indium-contacted detectors. We seek to test whether simple evaporated gold contacts alone could also reduce the leakage current and thus improve the spectral resolution, especially in the 10 - 100 keV energy range. We have fabricated several metal- semiconductor-metal (MSM) detectors with a 4 X 4 array of pixels on 10 X 10 mm substrates. Measurements of the detectors' leakage current, spectral response, and temperature sensitivity are presented and compared to IMARAD's ohmic contact detector and gold contact MSM detectors made of High Pressure Bridgman (HPB) material. Finally, we show preliminary results from a tiled flip-chip pixellated detector made using the IMARAD detectors.

Silicon photodetectors have been successfully employed for scintillation detection in gamma-ray spectroscopy and imaging applications. When compared to photomultiplier tubes (PMTs), silicon photodetectors are characterized by higher quantum efficiency to the scintillation light. Moreover, these devices are more compact, immune to magnetic fields and can be easily integrated in monolithic arrays of units whose size could range from few mm² up to some cm². New gamma-ray detection systems based on scintillators coupled to silicon photodetectors have been recently developed for astrophysics experiments as well as for imaging systems in nuclear medicine. Among silicon detectors, conventional silicon PN photodiodes (PDs), avalanche photodiodes (APDs) and silicon drift detectors (SDDs) have been used with scintillators both as single units and as monolithic arrays. In the paper, the main features of silicon photodetectors used with scintillators for gamma detection are shortly described and the more recent achievements in their development are overviewed. Finally, a comparison of the achievable performances with PDs, APDs, and SDDs is reported.

Cerium-doped lutetium oxyorthosilicate (LSO:Ce) is an excellent scintillator due to its fast decay time (approximately 40 ns), excellent brightness (> 3X bismuth germanate), and high density (7.4 gm/cm³). However, the luminescence process in this technologically important scintillator is not well understood. Elucidation of electronic traps and their role in scintillator afterglow is lacking and fundamental ion-lattice coupling parameters have not been established. From thermally stimulated luminescence and emission spectra data on several oxyorthosilicates we show the traps to be intrinsic and uniquely associated with the C2/c crystal structure. Temperature dependent optical absorption measurements reveal prominent Gaussian bands at 3.432 eV (peak a), 3.502 eV (peak b), 4.236 eV (peak c) and 4.746 eV (peak d). The second moments are well described by the usual linear coupling model yielding the Huang-Rhys parameter and vibrational quantum energies for each peak. Oscillator strengths of the 4f yields 5d transitions are calculated from Smakula's formula and knowledge of the cerium distribution between the two crystallographically inequivalent sites. From the known correlation between average Ce-ion- ligand distance and oscillator strength we conclude that peak a is correlated with the seven-oxygen-coordinated site, and peaks b, c and d are associated with the six-oxygen- coordinated site.

Sodium iodide doped with thallium is the lowest cost crystalline scintillator material available in large sizes. A PC-based scanning system consisting of two independently programmable x-y translators is used to characterize the response of large area (60 cm X 90 cm) X-ray detector plates by measuring the response amplitude and energy resolution generated by X-ray energy excitation. A variety of energy sources and scanning of the source are employed in addition to scanning of the photomultiplier photon detector. Initial scan data and analysis from a 40 cm X 40 cm detector plate are presented.

Oxide glasses containing Ce were prepared by the conventional melting method under reducing atmosphere. Spectroscopic properties of the glasses were studied by steady-state luminescence spectroscopy. In the present paper we made a report to summarize the characteristics of excitation and emission spectra in difference oxide hosts.

Over the last four years we have been developing imaging Cadmium Zinc Telluride pixel detectors optimized for astrophysical focusing hard X-ray telescopes. This application requires sensors with modest area (approximately 2 cm X 2 cm), relatively small (approximately less than 500 micrometer) pixels and sub-keV energy resolution. For experiments operating in satellite orbits, low energy thresholds of approximately 1 - 2 keV are also desirable. In this paper we describe the desired detector performance characteristics, and report on the status of our development effort. In particular, we present results from a prototype sensor with a custom low- noise VLSI readout designed to achieve excellent spectral resolution and good imaging performance in the 2 - 100 keV band

Gamma-ray imaging with position-sensitive germanium detectors offers the advantages of excellent energy resolution, high detection efficiency, and potentially good spatial resolution. The development of the amorphous-semiconductor electrical contact technology for germanium detectors has simplified the production of these position-sensitive detectors and has made possible the use of unique detection schemes and detector geometries. We have fabricated prototype orthogonal-strip detectors for gamma-ray imaging studies using this contact technology. With these detectors, we demonstrate that a gamma- ray interaction event in the detector can be located in three dimensions. This more accurate determination of the interaction event position should ultimately lead to better image resolution. We have also taken advantage of the bipolar blocking nature of the amorphous-semiconductor contacts in order to investigate the use of field-shaping electrodes. The addition of such electrodes is shown to improve the spectroscopic performance of the detectors by substantially eliminating charge collection to the inter-electrode surfaces. In addition, we demonstrate that this incomplete charge collection process can also be reduced by adjusting the properties of the amorphous-semiconductor layer. In this paper, we summarize the development of these position- sensitive detectors and present the results of our studies with the detectors.

We report on recent three-dimensional imaging performance and detection efficiency measurements obtained with 5 mm thick prototype CdZnTe detectors fabricated with orthogonal coplanar anode strips. In previous work, we have shown that detectors fabricated using this design achieve both very good energy resolution and sub-millimeter spatial resolution with fewer electronic channels than are required for pixel detectors. As electron-only devices, like pixel detectors, coplanar anode strip detectors can be fabricated in the thickness required to be effective imagers for photons with energies in excess of 500 keV. Unlike conventional double-sided strip detectors, the coplanar anode strip detectors require segmented contacts and signal processing electronics on only one surface. The signals can be processed to measure the total energy deposit and the photon interaction location in three dimensions. The measurements reported here provide a quantitative assessment of the detection capabilities of orthogonal coplanar anode strip detectors.

The Liquid Xenon Gamma-Ray Imaging Telescope (LXeGRIT) is a balloon born experiment sensitive to (gamma) -rays in the energy band of 0.2 - 20 MeV. The main detector is a time projection chamber filled with high purity liquid xenon (LXeTPC), in which the three-dimensional location and energy deposit of individual (gamma) -ray interactions are accurately measured in one homogeneous volume. To determine the (gamma) - ray initial direction (Compton imaging), as well as to reject background, the correct sequence of interactions has to be determined. Here we report the development and optimization of an algorithm to reconstruct the Compton scattering sequence and show its performance on Monte Carlo events and LXeGRIT data.

Ion Beam Induced Charge Collection (IBICC) is a proven albeit relatively new method to measure the electronic transport properties of room temperature radiation detectors. Using an ion microbeam, the charge collection efficiency of CZT detectors can be mapped with submicron resolution and maps of the electron mobility and lifetime can be calculated. The nuclear microprobe can be used not only for characterizing detectors but also with the use of Time Resolved IBICC (TRIBICC) and lateral IBICC/TRIBICC we can deduce information about the electron and hole mobility and lifetime profiles, and about the variation of electric field along the detectors' axes. The Sandia Nuclear Microprobe has been and is being used routinely to characterize CZT detectors and measure their electronic transport properties. In this paper we will present the results of these measurements for different detectors. Furthermore the damage effects caused by the probing beam will be discussed and a simple model will be presented to explain the characteristic charge collection efficiency pattern observed after high dose irradiation.

Various types of Cd_1-xZn_xTe (0.04 < X < 0.24) detector crystals grown by vertical high pressure Bridgman (VHPB), low pressure Bridgman (LPB) i.e. vertical ambient pressure Bridgman (VB), horizontal ambient pressure Bridgman (HB) and vapor grown crystals have been evaluated and compared. We have used the following methods in order to evaluate the CZT: (1) Triaxial crystal x-ray diffraction (TAD) for determination of the surface crystalline homogeneity, (2) Nuclear spectroscopic response of detectors and (3) Sensitivity to radiation from high flux x-rays for investigations of the suitability for x-ray digital imaging. Finally a comparison between the various methods of CZT crystal growth will be given.

As the demand for high quality, low cost X-ray, (gamma) -ray and imaging detector devices increases, there is a need to improve the quality and production yield of semiconductor materials used in these devices. One effective strategy for improving semiconductor device yield is through the use of better device characterization tools that can rapidly and nondestructively identify defects at early stages in the fabrication process. Early screening helps to elucidate the underlying causes of defects and to reduce downstream costs associated with processing defect laden materials that are ultimately scrapped. We report here a method for characterizing tellurium inclusion defects in cadmium zinc telluride semiconductor materials based on near infrared imaging. With this approach, large area wafers are inspected rapidly and non-destructively in two and three spatial dimensions by collecting NIR image frames at multiple regions of interest throughout the wafer using an automated NIR imaging system. The NIR image frames are subjected to image processing algorithms including background correction and image binarization. Particle analysis is performed on the binarized images to reveal tellurium inclusion statistics, sufficient to pass or fail wafers. In addition, data visualization software is used to view the tellurium inclusions in two and three spatial dimensions.

We report on the growth of thick CdTe layers on ZnTe/(100) GaAs hybrid substrates by the novel H₂ transport vapor phase epitaxy (H₂T-VPE) method. High crystalline quality (100)-oriented CdTe single crystal epilayers can be fabricated under atmospheric pressure and at growth temperatures (T_D) in the 600 - 800 degree Celsius interval. Double crystal X-ray diffraction measurements performed on epilayers thicker than 30 micrometer show CdTe (400) peaks with FWHM < 59 arcsec. Samples grown under optimized conditions exhibit mirror-like surfaces. Nominally undoped epilayers grown < 650 degrees Celsius are p-type and low resistive, but they turn n-type above 650 degrees Celsius, as a result of donor (likely Ga) diffusion from the substrate. RT resistivities ((rho) ) approximately 10⁶ (Omega) (DOT)cm are obtained for 675 degrees Celsius < T_D < 700 degrees Celsius, but (rho) decreases for higher temperatures and thinner samples. Layers grown under these conditions show RT electron concentrations in the 10¹⁴ - 10¹¹ cm^-3 range. The detection capability of H₂T-VPE grown CdTe is demonstrated by time- of-flight measurements performed at RT on Au/n-CdTe/n⁺- GaAs diode structures under reverse bias conditions. The present results show the potentials of H₂T-VPE for the growth of detector-grade CdTe.

Thin CdTe detectors (3 X 5 mm² electrode area and 0.5 mm thick), mounted in back-to-back configuration with common anode, have been characterized, aiming at determining the actual detection region with good spectroscopy performance. This geometry allows one to use devices with 1.0 mm useful absorbing thickness when irradiated in the classical Planar Parallel Field (PPF) configuration, and a large active area (3 X 1 mm²) and high absorbing thickness (5 mm) when irradiated in the Planar Transverse Field (PTF) configuration, while the charge collection takes place on maximum distances of 0.5 mm. These dimensions have been chosen on the basis of previous studies performed on various detectors irradiated in the PTF configuration which have shown that the 'best spectroscopy region' width assumes a constant value of about 0.4 mm in the thickness range 1.0 - 2.5 mm, while it is largely reduced when using smaller electrode area (2 X 2 mm²) 0.5 mm thick devices. The tests were performed by using a well collimated (0.1 X 1 mm², 20 mm thick collimator) photon beam in the 10 - 300 keV energy range. The results concerning the spectroscopic behavior of both the coupled detectors and of single detectors (for comparison) have shown that the edge effects observed in smaller detectors with the same configurations are significantly reduced, giving better energy resolutions at Ex equals 122 keV, but no wider good spectroscopy regions; at low energies the effects of larger electrodes (increased electric capacitance and leakage current) result in a higher noise and increased detection threshold.

The effects of two intrinsic deep levels on electrical compensation and effects of dislocations on carrier mobility in semi-insulating CdTe and CdZnTe radiation detector crystals are reported here. These levels were found in samples grown by conventional Bridgman and high-pressure Bridgman techniques. The levels were observed with thermoelectric effect spectroscopy at distinct temperatures corresponding to thermal ionization energies of E_s1 equals 0.27 +/- 0.07 eV, E_d1 equals E_v + 0.735 +/- 0.005 eV and E_d2 equals E_v + 0.743 +/- 0.005 eV. The shallow level (E_s1) is associated with dislocations. The first deep level (E_d1) is associated with the doubly ionized Cd vacancy acceptor and the second deep level (E_d2) is associated with the Te-antisite (Te_Cd). The second deep level (Te_Cd) was found to electrically compensate the material to produce high resistivity CdTe and CdZnTe, provided that the Cd vacancy concentration is sufficiently reduced during crystal growth or by post-growth thermal processing. The dislocations were found to affect the mobility of the carriers in the CdTe and CdZnTe crystals.

The fabrication methods and performances of CdTe radiation detectors in a p-i-n structure which helps to reduce the leakage current are reported. Two different types of doping methods were studied in order to attain heavily doped CdTe layers. One is the hydrogen plasma- radical-assisted metalorganic chemical vapor deposition (HPRA-MOCVD) and the other is the excimer laser processing, both of which are carried out at a low temperature. Using the HPRA-MOCVD, iodine doped n-type CdTe layers with carrier concentration around 10¹⁸ cm^-3 and electron mobility of 100 cm²/V-s were grown epitaxially on the GaAs substrates at a low temperature of 150 degrees Celsius. As the other method, excimer laser was irradiated on the high resistivity CdTe crystals (resistivity: 10⁹ (Omega) cm) by depositing a thin layer of dopant materials like Sb, Na₂Te, or In on the crystals surfaces. For the Sb and Na₂Te dopant, heavily doped p⁺-type thin layer was formed on the CdTe crystal, whereas In dopant exhibited n⁺-type layer. Carrier concentrations in those laser doped layers were in the order of 10¹⁹ cm^-3. These low temperature processes can be adapted for the fabrication of CdTe and CdZnTe detectors.

We have studied charge splitting among the anodes of a CdZnTe (CZT) strip detector using a 30 micrometer collimated X-ray beam. The results indicate that charge splitting is a consequence of photoelectron range and diffusion. We also studied charge loss to the inter-anode regions. We showed that the charge loss increases as depth of interaction increases for interactions occurring above the inter-anode region.

We are developing CdZnTe pixel detectors for use as focal plane sensors in astronomical hard X-ray telescopes. To optimize the spectral response and imaging performance, we are investigating the effect of contact geometry on charge collection. Specifically, we have studied contact designs with orthogonal thin strips placed between pixel contacts. We apply a negative bias on the grid with respect to the pixel potential to force charge to drift toward the contacts. The grid bias is selected to be just sufficient to avoid charge collection on the grid, while increasing the transverse electric field on the surface between contacts. In contrast to focusing electrodes designed to force field lines to terminate on the pixel contact, our approach allows us to overcome the effects of charge loss between the pixels without significant increase of the leakage current, improving the overall energy resolution of the detector. In this paper we describe the performance of a CdZnTe pixel detector containing a grid electrode, bonded to a custom low-noise VLSI readout. We discuss the advantages of this type of detector for high spectral resolution applications.

A high-density, very low power (<<500 (mu) W/channel) readout system for the Hamamatsu R5900-00-M64 and similar multianode photomultiplier tubes (MAPMTs) is under development at NOVA R&D. It is intended for space-flight instruments involving a large number of channels, and provides for efficient readout of such a system at low to moderate event rates in the presence of a sparse hit pattern. Data is buffered within the readout ASIC while prompt summary information is made available to the instrument trigger system, which can then cause the data to be read from the buffer only when necessary. A prototype of the analog front- end has been designed and fabricated, and a prototype of the complementary digital functions has been designed and implemented in a field-programmable gate array (FPGA). We report here on these designs and on test results of the prototype readout system with the Hamamatsu MAPMT. Performance parameters such as gain, noise, and trigger threshold have been studied, and pulse height spectra with a single- photoelectron source and with a Sr⁹⁰ source and scintillating fiber have been obtained.

Amorphous silicon (a-Si:H) technology has created a successful manufacturing business for large area active matrix arrays, of which liquid crystal displays (AMLCD) are the best known, and image sensors are an emerging technology for medical x-ray imaging. The large area, flat plate, format is the key feature of the technology that sets it apart from other digital imaging approaches. The principal requirements for medical imaging are sensitivity and high dynamic range. A-Si:H detectors have already proved to perform at least as well as x-ray film for radiographic applications and comparable to image intensifiers for fluoroscopy. There are several approaches to improving the performance of the image sensors is order to achieve higher sensitivity and higher spatial resolution. This paper describes some of these approaches.

A linear CdZnTe pad detector array with approximately 1 mm² pad area has been developed. The detector has a wide energy range from about 20 to 200 keV. To read out these detector arrays, a fast, low-noise monolithic mixed signal ASIC chip has been developed. A prototype x-ray imaging system consisting of the CdZnTe detector array and the monolithic ASIC chip has been fabricated and tested. In this system, the detectors are abutted against each other to form an approximately 1 m long linear array. The system has been used to take preliminary scanned images of complex objects at various energies. New results from this system will be presented.

Rapid and spatially resolved imaging of neutron fields is not a well-developed technology relative to photon- and electron- based imaging techniques, However, glass-based microchannel plate (MCP) technology is relatively mature, enabling its application to neutron imaging to be straightforward. NOVA's approach to improved neutron detection efficiency and spatial resolution is derived from the suggestion of Fraser and Pearson, where ¹⁰B is directly incorporated into the glass matrix of the MCP structure. The ¹⁰B(n,(alpha) )⁷Li capture conversion stimulates the emission of secondary electrons into the adjacent microchannel structure. An electron cascade ensues, amplifying the detection event into a subnanosecond electron pulse emitted from the microchannel. The pulse can be electronically registered by a radiation-hard readout device and processed as a digital image. The image corresponds to spatial variations in the neutron flux striking the MCP input face. NOVA has constructed and tested a number of prototype neutron imaging detectors using cold and thermal neutron beams at the NIST Center for Neutron Research (NCNR) in Gaithersburg, MD. Features having a spatial separation of 30 micrometer are clearly resolvable in the raw images. Software processing further improves the image resolution. Conversion efficiency for thermal neutrons of an initial iteration of the specially modified MCPs is approximately 20%, with negligible gamma ray-induced background. In-progress modifications to the MCP base material and structure should produce imaging detectors with neutron detection efficiencies that exceed 50%.

The properties of high rare-earth-containing germanate glass have been investigated to assess the potential for using this material to construct electromagnetic calorimeters for particle physics. We report here on measurements of scintillation yield, transmission and decay time, on large blocks of Ce³⁺-doped Gd₂O₃-based glasses, the samples were excited by a high energy X-ray beam and the associated scintillation yield and decay time was measured. The optical transmission of the samples was measured. It was observed that scintillation yield of present scintillation glass is 20 - 30% of BGO scintillation yield, decay time is in range of 60 - 90 ns, glass density is 5.75 g/cm³. It was concluded that higher density and availability and low cost make this glass become promising candidate for cerium doped dense scintillator.

The properties of high rare-earth-containing borosilicate glasses have been investigated to assess the potential for using these materials to construct electromagnetic calorimeters for particle physics. We report here on measurements of scintillation yield, transmission and decay time, on large blocks of Ce³⁺-doped Gd₂O₃-based glasses. The samples were excited by a high energy X-ray beam and the associated scintillation yield and decay time was measured. The optical transmission of the samples was measured. It was observed that scintillation yield of present scintillation glass is 20% of BGO scintillation yield, decay time is in range of 50 - 80 ns, glass density is 5.50 g/cm³. It was concluded that higher light yield and density make this glass become promising candidate for cerium doped dense glass scintillator.

Various passivating agents that reduce the surface leakage current of CZT crystals have been previously reported. In none of the studies, NH₄F/H₂O₂ was identified as a promising passivation agent for CZT. We now present a study that includes the effect of NH₄F/H₂O₂ treatment on the surface properties and detector performance. An elemental depth profile was obtained via Auger Electron Spectroscopy. Furthermore, X-ray Photoelectron Spectroscopy acquired at different processing times to identify the chemical states of the elemental species that composed the dielectric layer. It was found that the NH₄F/H₂O₂ surface passivation significantly improved the sensitivity and energy resolution of CZT detectors. Furthermore, the NH₄F/H₂O₂ treatment did not attack the Au electrodes, which eliminated the need to protect the contacts in the detector fabrication process.

We present the requirements, design, and performance of an analog circuit for processing the non-collecting anode strip signals from a cadmium zinc telluride (CZT) strip detector with orthogonal coplanar anodes. Detector signal simulations and measurements with a prototype are used to define the range of signal characteristics as a function of location of the gamma interaction in the detector. The signals from the non- collecting anode strip electrodes are used to define two of the three spatial coordinates including the depth of interaction, the z dimension. Analog signal processing options are discussed. A circuit to process the signals from the non- collecting anode strips and extract from them the depth of interaction is described. The circuit employs a time-over- threshold (TOT) measurement. The performance of the detector prototype with a preliminary version of this circuit is presented, and future development work is outlined.

Radiation monitoring systems are desired in many places where radioactive materials are utilized. In this paper, a color gamma camera system developed in Tsinghua University (P.C. China) is reported. The system consist of a compact X - (gamma) ray detector system, a single hole collimator, the scanning mechanism and computer system. The MLEM method is implemented for image reconstruction, which enables one to generate images of high resolution with relatively big aperture. With the associated software, several scanning modes, which work with different speeds and resolutions, are provided and can be selected in the operations. In addition, the system can detect radioactive sources emitting rays of different energies and display them with color images. Experiments were made using Am-241 (59.5 KeV) and Na-22 (511 KeV) to test the performance of the system. The results are presented which show that the resolution of this system can be as high as 1.5 degrees. Furthermore, simulations using Matlab were made to examine the capability of imaging point sources with a small number of counts and imaging distributed sources. Promising results were obtained and are reported. Discussions about camera design and further improvements are given at the end.

We have developed a model of the statistical variations in the electrical charge transport in CdxZn1-xTe(CZT) detectors. The model includes a simulation of the charge carrier generation for each photon interaction, using a calculated absorption coefficient of the photoelectric absorption. Next, we simulate the induced signal as carriers drift towards the collecting electrode under trapping conditions with negligible detrapping. Finally, a pulse height histogram is composed simulating the spectral response of the detector and incorporating the electronic noise component. A comparison between experimental and calculated CZT spectra was performed. These results and the potential for using the model in detector design will be discussed and presented.

Diamond has the highest radiation-damage level among radiation-detector semiconductor materials. Besides, low carbon nucleus charge, Z equals 6, provides tissue equivalence of diamond detectors. However, essential restrictions are imposed on production of natural-diamond detectors by extremely low final yield of selection procedure and corresponding expensiveness of high-quality type IIa natural diamonds. The solution of this problem could be found through the development of single-crystal synthetic-diamond detectors. Basic radiation-response properties of high-pressure high- temperature (HPHT) single-crystal synthetic-diamond (SD) detectors and natural-diamond (ND) detectors made of extremely low nitrogen content (type IIa) material were comparatively studied under hard X-ray, UV, and alpha-particle irradiation. Four orders of magnitude higher steady-state responsivity to radiation has been observed for SD detectors. The gain evaluated under UV irradiation exceeded 6000 (the corresponding value of responsivity was above 1000 A/W). The study of alpha-particle-induced electromotive force (EMF) polarity has revealed the opposite type of surface bending of energy bands in synthetic and natural diamonds. The difference in detector performance could be explained in terms of presented model of charge carrier injection and transport in diamond.

We report on device fabrication and testing of CZT grown by the Modified Vertical Bridgman (MVB) method. Several samples of single-crystal MVB grown CZT were obtained from Yinnel Tech. Both single element devices and 2-dimensional arrays were fabricated. Resistivity and electron mobility-lifetime product were measured, and pulse height spectra were recorded for various isotopic sources. Arrays 5 mm thick and an array 1.13-cm thick were evaluated.

Measurements with 60 keV gamma-rays from a ²⁴¹Am check source show that most of the volume of a trapezoidal Frisch grid device is active and operates as a detector. Measurements were performed on a trapezoidal device constructed from 'counter grade' CdZnTe material, which despite the lower quality of the starting material still demonstrated excellent energy resolution at relatively high gamma ray energies. The energy resolution was comprised at low energies, most likely due to surface effects and material non-uniformities. Models indicate that performance can be improved with very straightforward and simple design changes.