We are developing 10 mm thick CZT detectors with 3-D readout for ~100 keV to ~1.5 MeV gamma-rays. Multiple-site gamma-ray interactions are fully measured, i.e., the energy and 3-D position of each site are determined. Spatial resolution is 1 mm FWHM. Anode pixel readout with 1 mm pitch is used for x- and y-positions and charge drift times for z-positions. Drift time measurements are triggered by the cathode signal and end when each interaction site's charge cloud reaches an anode pixel. Post-event processing corrects for signal loss due to charge trapping and accurately determines gamma-ray energies, with a goal of 1% energy resolution at 662 keV. Compton kinematic analysis can identify the initial interaction site in most cases as well as constrain the incident gamma-ray direction. Tests were made with a prototype detector, measuring 10 x 10 x 10 mm³ and operated at 1000 V bias. The measured drift time resolution of 25 nsec FWHM at 662 keV and 60 nsec at 122 keV corresponds to z-position resolution of 0.25 and 0.60 mm FWHM, respectively. The technique is described and results of modeling and tests are presented.

The resistivity dependence on Zn concentration had been investigated in semi-insulating (Cd,Zn)Te crystals grown by the vertical Bridgman method. A coorelation between the zinc concentration and the resistivity distribution could be found. The obtained resistivity was in the interval of 2 ×10⁹-10¹⁰ Ω cm as expected from the model of compensation. The main deep compensating levels detected by Photo Induced Current Transient Spectroscopy (PICTS) were at 0.64 ± 0.02 eV and close the middle of the band gap at 0.80 ± 0.02 eV.

We present results of detailed characterization of the spatial properties of large area crossed strip CZT detectors now under development at CASS/UCSD for use in coded aperture astrophysical hard X-ray instrumentation. We address the volumetric uniformity of spectral response for these detectors as determined by charge sharing and loss, diffusion, and electron trapping. Results are presented for our prototype detector having 500 μm pitch and collecting area 32×32 mm²; however, we also explore these characteristics as they effect performance of larger and smaller pitch detectors of similar design.

The Mars Odyssey spacecraft was launched on 7 April 2001 and went into orbit around Mars on 24 October 2001. One of the primary scientific instruments carried on the spacecraft is a germanium gamma-ray spectrometer that will measure the elemental composition of Mars. Cruise measurements taken during July and August 2001 are used to characterize the energy resolution of the detector and to measure and identify the background gamma rays. These gamma rays originate in the detector, in material surrounding the detector, and from the spacecraft. More than 110 gamma rays were observed in the background spectrum. The sources of most of these gamma rays were tentatively identified. Understanding the sources of the background gamma rays is important for the future when using the orbital data to determine the composition of Mars.

A large area (40 cm²) CZT module for space-borne X-ray astronomy applications has been under development at UCSD. This module employs four 32mm x 32mm x 2mm CZT crossed strip detectors with 0.5 mm pitch which are sensitive in the 10-200 keV range. The compact design includes readout and control electronics below the detector plane, which allows for efficient tiling of modules to form large detector planes for wide field of view coded mask imagers, or for efficient packaging within an anticoincidence shield at the focus of a hard X-ray telescope. The module has successfully been shaken on all three axes at 14 g rms to validate the mechanical design for spaceflight applications. Spectral, spatial, and imaging performance is presented.

We have developed germanium detector technologies for use in the Nuclear Compton Telescope (NCT) - a balloon-borne soft γ-ray (0.2-10 MeV) telescope to study astrophysical sources of nuclear line emission and polarization. The heart of NCT is an array of twelve large volume cross strip germanium detectors, designed to provide 3-D positions for each photon interaction with ~1mm resolution while maintaining the high spectral resolution of germanium. Here we discuss the detailed performance of our prototype 19x19 strip detector, including laboratory tests, calibrations, and numerical simulations. In addition to the x and y positions provided by the orthogonal strips, the interaction depth (z-position) in the detector is measured using the relative timing of the anode and cathode charge collection signals. We describe laboratory calibrations of the depth discrimination using collimated sources with different characteristic energies, and compare the measurements to detailed Monte Carlo simulations and charge collection routines tracing electron-hole pairs from the interaction site to the electrodes. We have also investigated the effects of charge sharing and loss between electrodes, and present these in comparison to charge collection simulations. Detailed analysis of strip-to-strip uniformity in both efficiency and spectral resolution are also presented.

The electronic performance of CZT-based gamma radiation spectrometers is governed by a synergism of bulk and surface properties. Compensation is used to increase the bulk resistivity of Cd_1-xZn_xTe (x~0.1), but the same electronic states that are introduced to increase the material resistivity can also trap charge and reduce the carrier lifetime. Electrical and mechanical surface defects introduced during or subsequent to crystal harvesting are also known to interfere with device performance. Using a contactless, pulsed laser microwave cavity perturbation technique, electronic decay profiles were studied in high pressure Bridgman CZT as a function of temperature. The electronic decay profile was found to depend very strongly on temperature and was modeled using a function consisting of two exponential terms with temperature-dependent amplitudes and time constants. The model was used to relate the observed temperature dependent decay kinetics in CZT to specific trap energies. It was found that, at low temperatures, the electronic decay process is dominated by a deep trap with an energy of approximately 0.69 ± 0.1 eV from the band edge. As the temperature is increased, the charge trapping becomes dominated by a second trap with an energy of approximately 0.60 ± 0.1 eV from the band edge. Surface damage introduces additional charge traps that significantly alter the decay kinetics particularly at low temperatures.

Further progress has been made in the development of the Modified Vertical Bridgman method for the growth of CdZnTe crystals for fabrication of x-ray and gamma-ray detectors to operate at room temperature. Specifically, the diameter of the grown ingots has been increased from 2 to 3 inches. High quality, large volume (up to 6 in³) twin-free single crystals have been produced. Detectors fabricated with this material show sharp energy resolution and good uniformity.

A generation of high performance front-end and read-out ASICs customized for highly segmented CdZnTe sensors is presented. The ASICs, developed in a multi-year effort at Brookhaven National Laboratory, are targeted to a wide range of applications including medical, safeguards/security, industrial, research, and spectroscopy. The front-end multichannel ASICs provide high accuracy low noise preamplification and filtering of signals, with ver-sions for small and large area CdZnTe elements. They implement a high order unipolar or bipolar shaper, an innovative low noise continuous reset system with self-adapting capability to the wide range of detector leakage currents, a new system for stabilizing the output baseline and high output driving capability. The general-purpose versions include pro-grammable gain and peaking time. The read-out multichannel ASICs provide fully data driven high accuracy amplitude and time measurements, multiplexing and time domain derandomization of the shaped pulses. They implement a fast arbitration scheme and an array of innovative two-phase offset-free rail-to-rail analog peak detectors for buffering and absorption of input rate fluctuations, thus greatly relaxing the rate requirement on the external ADC. Pulse amplitude, hit timing, pulse risetime, and channel address per processed pulse are available at the output in correspondence of an exter-nal readout request. Prototype chips have been fabricated in 0.5 and 0.35 μm CMOS and tested. Design concepts and experimental results are discussed.

Recent studies have shown that pixelated HgI₂ detectors with a thickness of 1 cm can yield energy resolutions of 1.4 - 2% FWHM at 662 keV γ-ray energy, which are similar to that obtained on 3-dimensional position sensitive CdZnTe detectors of the same thickness. However, there is a need to better understand the electron transportation and induced charge from the drift of charges within the detector in order to better understand the spectra obtained from HgI₂ detectors. The results of a simple simulation model accounting for electron trapping and various weighting potentials is compared with recent physical spectra obtained from pixelated HgI₂ detectors of 5 mm and 10 mm thickness.

Transient charge transport (TCT) measurements were used to evaluate the electrical conduction properties of HgI₂ single crystals. Some comparative preliminary results for polycrystalline mercuric iodide (poly-HgI₂) thick-film X-ray detectors are also reported. The latter were prepared by physical vapor deposition (PVD). The mobility , trapping time 2, and surface recombination velocity s of electrons or holes were determined by analyses of transient voltages developed across the sample in response to a drift of the corresponding charge carriers created by alpha particle absorption near one of the electrodes. Electron-, and hole mobilities of single crystal HgI₂ were _n = 80 cm²/V•s and p = 4.8 cm²/V•s, respectively. Trapping times were 2n ≅ 22 V and 2_p ≅ 8 V, and surface recombination velocities s_n ≅ 1.1 ×10⁵ cm/s and s_p ≅ 3.6 ×10³ cm/s . Those of the polycrystalline material depend on the deposition technology, and vary between 65 and 88 cm²/V•s for electrons, and between 4.3 and 4.1 cm²/V•s for holes. Bulk trapping-times and surface recombination velocities appear of the same order of magnitude as in the single crystal. An effect of carriers being first generated in near-surface traps and then gradually released is observed for both the single crystal and the polycrystalline material. It is stronger for electrons as compared to holes, and stronger in the polycrystalline material as compared to the single crystal.

Many expectations are haven to the INTEGRAL satellite and the development and plan of devices in the next generation, which will exceed existing devices in a future, are also in progress. As a detector technology for the next generation, we have developed a liquid Xe Time Prjection Chamber(TPC) for MeV gamma-ray observations. It is designed to image gamma-rays in the energy range of 200keV to 10 MeV. The chamber consists of two sensitive volumes of 20cm 20cm 5cm. Two anode systems are at the center of effective area and made with a print substrate that has strip type anodes and induction electrodes.

We investigated a red to yellow color change observed in α-HgI₂ when cooled below 150 K. A phase transformation to β-HgI₂, which has a yellow color, was ruled out by a variable temperature x-ray diffraction study. Instead the color change at cryogenic temperatures is caused by a shift in the transmission edge to shorter wavelengths which we attribute to a widening band gap at low temperatures. Using optical transmission spectroscopy the width of the band gap was measured between 10 and 330 K. At room temperature the gap was 2.11 ± 0.03 eV which is significantly smaller than the most recently published values of ~ 2.3 eV. This smaller band gap was further verified by measuring the thermoelectric current at elevated temperatures.

The goal of the present work has been to develop a method for the efficient and reliable production of gadolinium- and boron-containing solid scintillator. Polyvinyl toluene, silicone rubber, and other materials were investigated. Gadolinium in a solid overcomes the limitations of the physical form of a liquid, and silicone rubber as a carrier for either gadolinium or boron overcomes the thermal limitations of plastics. Silicone rubbers also introduce some interesting and useful properties (such as flexibility) of their own. We report here the fabrication of solid organic scintillators loaded with as much as 2% gadolinium, and more than 5% boron. A gadolinium-containing compound, soluble in vinyl toluene monomer and not inhibiting polymerization, was found. The same compound was also found to be soluble in phenyl-substituted silicone fluids that subsequently could be polymerized. In addition, a class of boron compounds also soluble in silicone fluids and not inhibiting polymerization was found. In the absence of phosphors, the resulting boron- and gadolinium-loaded disks were clear and colorless, or only slightly yellow. The disks were compatible with UV-, blue-, blue-green, and green-emitting phosphors and a variety of colors were realized. In addition, it was found in the case of gadolinium loading, the discrete spectrum due to atomic x-rays and conversion was observable if the scintillator sample was small.

Semiconductor based thermal neutron detectors provide a compact technology for neutron detection and imaging. Such devices can be produced by externally coating semiconductor charged particle detectors with neutron reactive films that convert free neutrons into charged-particle reaction products. Commonly used films for such devices utilize the ¹⁰B(n,a)⁷Li reaction or the ⁶Li(n,a)³H reaction, which are attractive due to the relatively high energies imparted to the reaction products. Unfortunately, thin film or "foil" type thermal neutron detectors suffer from self-absorption effects that ultimately limit neutron detection efficiency. Design considerations that maximize the efficiency and performance of such devices are discussed. Linear arrays fabricated from thin-film-coated pixel detectors are presented with results.

Thermal neutrons passing through air have scattering lengths of about 20 meters. At further distances, the majority of neutrons emanating from a moderated source will scatter multiple times in the air before being detected, and will not retain information about the location of the source, except that their density will fall off somewhat faster than 1/r². However, there remains a significant fraction of the neutrons that will travel 20 meters or more without scattering and can be used to create an image of the source. A few years ago, a proof-of-principle "camera" was demonstrated that could produce images of a scene containing sources of thermalized neutrons and could locate a source comparable in strength with an improvised nuclear device at ranges over 60 meters. The instrument makes use of a coded aperture with a uniformly redundant array of openings, analogous to those used in x-ray and gamma cameras. The detector is a position-sensitive He-3 proportional chamber, originally used for neutron diffraction. A neutron camera has many features in common with those designed for non-focusable photons, as well as some important differences. Potential applications include detecting nuclear smuggling, locating non-metallic land mines, assaying nuclear waste, and surveying for health physics purposes.

Description of the gamma-neutron measurement complex consisting of High Pressure Xenon gamma-spectrometer and multilayer neutron detectors based on ³He counters is presented. Main characteristics and calibration results of the measurement complex are considered. Proceeding data and their interpretation principles are discussed, applications of gamma-neutron complex for radiation control of fissile materials are considered.

Development of monolithic arrays of multiplexed, high-gain avalanche photodiodes suitable for use in a spectroscopic radiation-imaging device is underway at RMD. To dramatically reduce the electronics required to support a large array of discrete pixels, we have utilized a unique property of avalanche photodiodes and the method in which they are produced to develop a relatively simple readout scheme using row-column addressing. By adding a step to the avalanche photodiode creation, it is possible to place two, separate diode contacts onto the back of each photodiode in the array. These isolation diodes allow the readout of an entire row or column of photodiodes through a common readout line. A data-decoding matrix uniquely determines the position in the array while simultaneously supporting the goal of reducing the number of signal readout lines and support electronics. This approach reduces the number of pre-amplifiers, pulse-shaping circuits, and sample-and-hold stages from n² to 2n (n pixels on a side) per array. Recent research has been carried out with 14 × 14 pixel, planar-processed avalanche photodiode array having pixels 2.00 mm on a side with 2.25-mm pitch. These arrays will be paneled to form the photodetector of a radiation imager of approximately 100-cm² photosensitive area. To reduce the contribution of noise from each pixel to the common readout lines, research is being carried out to develop a discriminator with an adjustable threshold for each avalanche photodiode readout connection. Initial performance results from multiplexed arrays, a discussion of the active discrimination contacts, and the current status of the imager research project are given.

Integrated CMOS Active Pixel Sensor (APS) arrays have been fabricated and tested using X-ray and electron sources. The 128 by 128 pixel arrays, designed in a standard 0.25 micron process, use a ~10 micron epitaxial silicon layer as a deep detection region. The epitaxial layer has a much greater thickness than the surface features used by standard CMOS APS, leading to stronger signals and potentially better signal-to-noise ratio (SNR). On the other hand, minority carriers confined within the epitaxial region may diffuse to neighboring pixels, blur images and reduce peak signal intensity. But for low-rate, sparse-event images, centroid analysis of this diffusion may be used to increase position resolution. Careful trade-offs involving pixel size and sense-node area verses capacitance must be made to optimize overall performance. The prototype sensor arrays, therefore, include a range of different pixel designs, including different APS circuits and a range of different epitaxial layer contact structures. The fabricated arrays were tested with 1.5 GeV electrons and Fe-55 X-ray sources, yielding a measured noise of 13 electrons RMS and an SNR for single Fe-55 X-rays of greater than 38.

To conserve volume and power, photodiode/scintillator combinations are strong candidates for gamma-ray detection in space applications. High sensitivity to MeV gamma rays necessitates large-volume scintillators, which are most effectively read out with large-area photodiodes. However, because photodiodes have unity gain, the electronic noise limits resolution, and therefore small-area photodiodes that minimize capacitance are preferred. Thus, optimization of resolution involves maximizing light production and transport in the scintillator and light collection in the photodiode, while minimizing photodiode area. Measurements of performance are reported for 1×1×1cm³/10×10mm², 80cm³/18×18mm², and 85cm³/10×10mm² CsI(Tl)/photodiode combinations. Each large scintillator was a single crystal, machined to a geometry that comprised a 40mm diameter × 50mm height cylindrical section that was extended through a 20°conical section to a square face that matched the respective photodiode sensitive surface. Absolute scales were estimated for the light output by measuring the photodiode responses to ²⁴¹Am (59.54keV), ⁵⁷Co (122.06 and 136.47keV), and ¹³³Ba (80.99keV) and assuming a value of 3.67eV/electron-hole pair. The photodiode quantum efficiencies for the CsI(Tl) emission spectrum, corrected for Si reflection back into the scintillator, was taken to be 0.835. We obtained values of 58.2, 46.7, and 34.6 photons/keV for the combined light production and transport into the CsI for the 1cm³, ~80cm³, and ~85cm³ detectors, respectively. The best measured resolutions at 662keVfor the detectors were 5.9%, 7.2%, and 7.4% FWHM, respectively.

The main goal of this paper is the optimization of X-View, a turn-key detection system for high resolution and real-time X-ray non-destructive testing. X-View consists of an microfocus X-ray generator and an acquisition detection system. Two large area detection systems have been developed based on amorphous (a-Si:H) and new CMOS technologies. The first one consists of an X-ray scintillator converter, arrays of amorphous silicon thin film transistors (TFT) and photodiodes (pitch down to 100 μm). The second one, based on CMOS technology, used in high resolution applications, consists of a scintillator and arrays of CMOS photodiodes (pitch of 50 μm). Both are equipped with a fast real-time electronic system for readout and digitization of images and appropriate computer tools for control, real-time image treatment data representation and off-line analysis. Images quality have been improved using a microfocus X-ray generator (focus of 50 μm). Decreasing the spot size of the generator improves the X-ray image quality. The geometric blurring is reduced, and object magnifications are possible. Our study presents the main characteristics of both detection systems (wide dynamic range, lack of blooming, high frame rate), quantitative and qualitative analysis X-ray inspection applications (electronics, various industries, medical, pharmaceutical, etc).

Spectrometer grade, room-temperature radiation detectors have been produced on Cd_0.90Zn_0.10Te grown by the low-pressure Bridgman technique. Small amount of indium has been used to compensate the uncompensated Cd vacancies for the crystals to be semi-insulating. The properties of the detectors are critically dependent on the amount of excess Te introduced into the growth melts of the Cd_0.90Zn_0.10Te crystals and the best detectors are fabricated from crystals grown with 1.5% excess Te. Detector resolution ⁵⁷Co and ²⁴¹Am radiation peaks are observed on all detectors expect the ones produced on Cd_0.90Zn_0.10Te grown from the melt in the stoichiometric condition. The lack of resolution of these stoichiometric grown detectors is explained by a p/n conduction-type inhomogeneity model.

We present laboratory results from our compound semiconductor program designed to produce X- and gamma-ray detectors with both high spectral and spatial resolution and with high quantum efficiencies over the energy range 1 to 500 keV. A number of materials are presently under study, including GaAs, InP, CdZnTe, HgI₂ and TlBr. Extensive measurements on simple monolithic detectors and small format arrays have been carried out both in our laboratory and at the ESRF, HASYLAB and BESSY II synchrotron radiation facilities. The results have been used in conjunction with a material science program ultimately intended to produce near Fano limited, monolithic detectors and large area pixelated arrays for the next generation X-ray astrophysics and planetary space missions.

A 128-pixel gamma-ray imaging detector unit, which has high-energy resolution with room temperature operation, was fabricated by using diode-type CdTe detector. The diode structure was prepared by indium-doped n-type CdTe thin layer formed by excimer laser doping on one-side of high resistivity p-like single crystal CdTe wafer, and gold electrode as a Shottkey electrode evaporated on opposite side of the wafer. This diode-detectors showed good diode I-V characteristics with low leakage current. This CdTe detectors were pixelized in the 2mm × 2mm, and the 128 chips (32 × 4 chips) were mounted on the ceramic printed circuit boards at 3mm interval with 1mm gap. The printed circuit boards are directly connected with the MCSA-EX1 ASIC chip and 128 ch radiation spectrum analyzer systems. When using the Am-241 and the Co-57 as radioisotopes, the spectral response from all pixels had within 4.4 keV of FWHM at 122 keV peak of Co-57 for radiation performed at room temperature. The intensities of the peak from pixels were also uniform.

TlBr is relatively unique amongst semiconductors, in view of its very high density coupled with its large band-gap. In fact, its density (7.5 g cm^-3) is comparable to bismuth germanate and thus it has excellent absorbing power for hard X- and gamma rays. For example, a 1 mm thick detector, has a usable efficiency of ~10% at 500 keV. Its lowest energy of operation is set by leakage currents at typically 1 keV. Such a wide dynamic range is ideally suited for a range of applications, such as gamma-ray astronomy, isotopic measurements for environmental redemption and particularly nuclear medicine, where small size and large stopping power are highly desirable attributes. Additionally, its band-gap energy of 2.7 eV is large enough to ensure low noise performance at room or even elevated temperatures. In this paper we assess the suitability of using TlBr arrays for X- and gamma-spectroscopy and specifically for nuclear medicine applications. For example, it is shown that the attributes of TlBr make it an ideal material for the production of intra-operative, radio-guided surgical probes, which operate in the hard X- and low energy gamma-ray regions. By using a segmented or pixelated anode design and tailoring the geometry to promote the near-field effect, most of the negative material attributes normally associated with TlBr (e.g. poor hole transport) can be largely negated.

The Large Area Telescope (LAT) on the Gamma-ray Large-Area Space Telescope (GLAST) mission is designed to provide unprecedented sensitivity in the exploration of the gamma-ray sky. Gamma rays with energy above 10 MeV are detected via the pair conversion process, using a precision silicon tracker-converter and a hodoscopic CsI calorimeter. Charged cosmic rays are rejected by a tiled plastic-scintillator anti-coincidence detector. We report here on the design, prototyping, testing and expected performance of the silicon tracker-converter, which will be the largest silicon detector system in space after the GLAST launch in 2006. Specifically, we discuss the electronics system, the mechanical system, results from beam tests and a balloon flight, assembly procedures and prototyping experience, and expected performance of the tracker-converter.

Photoconductive polycrystalline mercuric iodide coated on amorphous silicon flat panel thin film transistor (TFT) arrays is the best candidate for direct digital X-ray detectors for radiographic and fluoroscopic applications in medical imaging. The mercuric iodide is vacuum deposited by Physical Vapor Deposition (PVD). This coating technology is capable of being scaled up to sizes required in common medical imaging applications. Coatings were deposited on 2”×2” and 4”×4” TFT arrays for imaging performance evaluation and also on conductive-coated glass substrates for measurements of X-ray sensitivity and dark current. TFT arrays used included pixel pitch dimensions of 100, 127 and 139 microns. Coating thickness between 150 microns and 250 microns were tested with beam energy between 25 kVP and 100kVP utilizing exposure ranges typical for both fluoroscopic, and radiographic imaging. X-ray sensitivities measured for the mercuric iodide samples and coated TFT detectors were superior to any published results for competitive materials (up to 7100 ke/mR/pixel for 100 micron pixels). It is believed that this higher sensitivity can result in fluoroscopic imaging signal levels high enough to overshadow electronic noise. Diagnostic quality of radiographic and fluoroscopic images of up to 15 pulses per second were demonstrated. Image lag characteristics appear adequate for fluoroscopic rates. Resolution tests on resolution target phantoms showed that resolution is limited to the TFT array Nyquist frequency including detectors with pixel size of 139 microns resolution ~3.6 lp/mm) and 127 microns (resolution~3.9 lp/mm). The ability to operate at low voltages (~0.5 volt/micron) gives adequate dark currents for most applications and allows low voltage electronics designs.

We have investigated the radiation damage effects on a CCD to be employed in the Japanese X-ray astronomy mission including the Monitor of All-sky X-ray Image (MAXI) onboard the International Space Station (ISS). The X-ray CCD camera, ACIS, onboard Chandra have been seriously damaged by low energy protons having energy of ~150 keV since low energy protons release their energy mainly at the charge transfer channel, resulting a decrease of the charge transfer efficiency. We thus focused on the low energy protons in our experiments. We measured the degradation of the charge transfer efficiency and the dark current as a function of incremental fluence. We have also developed the different device architectures to minimize the radiation damage in orbit. We thus compared the differences of performance after proton irradiation. We then investigated the spatial distribution of the low energy protons in the orbit of the ISS. We found that their density has a peak around l~20° and b~-55° independent of the attitude. The peak value is roughly two orders of magnitude larger than that at the South Atlantic Anomaly. Taking into account the new anomaly and orbit of the ISS, we estimated the charge transfer inefficiency of MAXI CCDs to be 1.1 × 10^-5 per each transfer after two years of mission life in the worse case analysis if the highest radiation-tolerant device is employed. This value is well within the requirement and we have confirmed the high radiation-tolerance of MAXI CCDs.

We have employed the mesh experiment for the front-illuminated (FI)CCD having small pixel size of 8μm and for the back-illuminated(BI) CCD having pixel size of 24μm. BI CCDs possess the same structure as the FI CCDs. Since X-ray photons enter from the back surface of the CCD, the primary charge cloud is formed far from the electrodes. The primary charge cloud expands through diffusion process until they reach the potential well which is just below the electrodes. Therefore, the diffusion time for the charge cloud produced by X-rays is longer than those in the FI CCD, resulting the larger charge cloud shape to be expected. The mesh experiment enables us to specify the X-ray point of interaction with a subpixel resolution. We then have measured a charge cloud shape produced in the FI CCD as well as the BI CCD. We found that there are two components of the charge cloud shape having different size: a narrow component and a broad component for both CCDs. The size of narrow component obtained with the FI CCD is 0.6-1.4μm in unit of a standard deviation which is consistent with the previous experiments with FI CCD whole pixel size is 24μm. For the BI CCD, the size of the narrow component is 2.8-5.7μm and strongly depends on the attenuation length in Si of incident X-rays. The shorter the attenuation length of X-rays is, the larger the charge cloud becomes. This result is qualitatively consistent with a diffusion model inside the CCD. On the other hand, the size of the broad component is roughly constant of ≈ 13μm and does not depend on X-ray energies. Judging from the design value of the CCD and the fraction of each component, we conclude that the narrow component is originated in the depletion region whereas the broad component is in the field-free region. Taking into account the charge cloud shape obtained, we calculated the X-ray point of interaction for all X-ray events. We estimated the uncertainty of the position resolution to compare it with the location of the mesh hole. We then obtained the position resolution of 1 μm for both CCDs which is similar value of the previous results whereas the fraction of split pixel event becomes roughly three times and an order of magnitude larger than previous results. We can thus develop the X-ray spectroscopic detector having a micron order position resolution with a high throughput.

A new large area detector of high-energy x-ray and b-radiation has been designed and studied. The design includes wedge-shaped light transducers. A composite material based on small crystalline ZnSe(Te) was applied onto the wide surface of light transducer. This design ensures optimum light collection from large sensitive surface onto the output window of much smaller size. An experimental specimen has been prepared, which showed β-sensitivity C_β=5.5 cm². The spectrograms of a ⁹⁰Sr+⁹⁰Y β-source obtained with the specimen under study make it possible to evaluate the age of the source by the ratio of low- and high-energy regions of the spectrum. Other designs are proposed for application of large-area detectors possessing wedge-shaped light transducers as elements of assembled constructions for high efficiency detectors operating under flow conditions.

A novel sealed gaseous PPAC detector is described which is significantly less prone to discharges and can consequently achieve high gas gains at high counting rates. The detector has demonstrated stable gains greater than 10⁴ at counting rates in excess of 10⁷ counts/mm²-sec.

We are developing pixel detectors for macromolecular crystallography, in which diffracted X-rays are directly absorbed by high-resistivity, crystalline silicon that has been micro-machined by inductively-coupled plasma etching. Arrays of 64 × 64 holes at 150 μm pitch are first formed by etching through the entire silicon bulk, then backfilled with polysilicon that is doped to create conducting p and n type columnar electrodes. When reverse biased, these electrodes generate electric fields that define the individual pixels. By forming conducting polysilicon on the sides of the sensors, which are cut-out of the silicon wafer by plasma etching, the entire surface of the detector may be made active. CMOS readout integrated circuits are conductively bump bonded behind each 3D detector, providing a direct connection to every pixel. A large array will be assembled with no insensitive bands along the edges by overlapping these sensors, each of area 0.96cm². This detector will measure X-ray signal intensities of up to 10⁵ events/pixel/sec without any pile-up loss, by using an integration method that retains the benefits of discrete photon counting. The detector sensitivity will be highly uniform, it will not exhibit any dark signal or spurious noise, and no geometric distortion will occur within each sensor.

We have prototyped and characterized a very large format X-ray detector for macromolecular crystallography. The X-ray field strength is converted to visible light in a phosphor film. Light from the phosphor is focused onto a CCD imager by a lens specially designed for this detector, that has a very high numerical aperture. The CCD is very large (61 mm, 4,096 × 4,096 pixels), and employs a very low-noise on-chip preamplifier. Lens coupling between phosphor film and CCD avoids many of the optical imperfections of fiber optic coupling, but it remains a challenge to make a lens system with optical transfer efficiency matching or exceeding that of fiber optical systems. We have met this challenge by enhancing system gain in our detector through implementation of modern lens technologies and imaginative CCD design. At this point the system gain equals that of conventional CCD-based X-ray crystallography detectors, which couple the CCD to the phosphor through a fiber optic taper. Although many of our technical developments could also be used in fiber optic detectors, the overriding virtues of the lens-couple detector are simplicity, optical perfection, and cost.

Measurements of quantum efficiencies are presented for three epitaxial gallium arsenide detectors with nominal depletion depths of 40, 325, and 400 μm. Attempts to measure the depletion as a function of bias indicated that the apparent depletion depth was much less than the intrinsic layer thickness. Expectation that the of the detection efficiency could be increased using a larger intrinsic layer could not be met. The largest value of a depletion depth measured by X-rays was determined at about 100 μm for the 400 μm device.

High-pressure Bridgman (HPVB) and vertical zone melting (HPVZM) growth has been applied for manufacturing Cd_1-xZn_xTe (x = 0.04 - 0.2), CdSe and ZnSe crystal tapes with sizes up to 120×120×12 mm. The influence of the technological parameters of the growth process on the crystal quality and some properties is discussed. The dependence of the inclusion (bubbles) content on deviation from the melt stoichiometry is determined. The method for growing plates with low content of the inclusions is described. High-resistivity crystal tapes of undoped CdZnTe (10¹⁰ Ohm×cm), CdSe (10¹¹ Ohm×cm) and ZnSe (>10¹¹ Ohm×cm) were prepared. Possibility of the tape growth on the oriented seed is shown on example of CdSe. The difference between HPVB and HPVZM results is described. Main HPVZM advantage for II-VI compound crystal growth is possibility of obtaining crystals with stoichiometric composition or with controlled deviation from stoichiometry. Hence HPVZM is preferable for growing high-resistivity II-VI crystals with low inclusion content.

Thermodynamic conditions for a post growth annealing to prepare near stoichiometric semi-insulating (SI) of CdTe with a minimized concentration of point defects are looked for in undoped and Sn-doped single crystals. The high temperature (200-1000°C) in-situ conductivity σ and Hall effect measurements are used to control the native defect density and to find out the Cd pressure P_Cd at which shallow defects are compensated. We show, that contrary to the undoped samples, where the change of the type of conductivity by variations of P_Cd is easy, the Sn-doped samples exhibit due to the Sn self-compensation much more stable behavior. The temperature near 500°C is reported to be optimum for the real-time annealing of bulk samples. The chemical diffusion is sufficiently fast at this temperature, simultaneously the lower temperature is preferred because the native defect density can be tuned gently by changing P_Cd. The measurement of temperature dependencies of σ in annealed samples below 500°C is used to establish the position of Fermi level and to characterize the structure of both shallow and deep levels detected in the sample. The quasichemical formalism is used for evaluation of defect density and for analysis of nature of deep levels.

High quality CdTe crystals with resistivities higher than 10⁸ Ω cm were grown by the 'contactless' PVT technique. Group III elements In and Al, and the transition metal Sc were introduced at the nominal level of about 6 ppm to the source material. Low-temperature photoluminescence (PL) has been employed to identify the origins of PL emissions of the crystals. It was found that the emission peaks at 1.584 eV and 1.581 eV exist only in the In-doped crystal. The result suggests that the luminescence line at 1.584 eV is associated with Cd-vacancy/indium complex. The intensity of the broadband centered at 1.43 eV decreases dramatically with introduction of Sc.

Selective operation of laser doping process and laser ablation process using KrF excimer laser irradiation (wavelength 248nm, pulse duration 20ns) was carried out that is used for fabricating an integrated gamma-ray imaging detector. At high vacuumed condition, laser irradiated surface region is heated up to ablated. At high-pressure condition, ablation is suppressed and impurity on the surface was melt and diffuse into the CdTe substrate. We have fabricated an integrated imaging detector using above combination process. The detector showed low leakage current at room temperature and good gamma-ray detection property.

Recently developed semiconducting HgI₂ polymeric composite x-ray detectors are very promising for digital radiography and non-destructive testing. We present a new model that explains well the variation of the detector's sensitivity with applied bias and crystallite shape and size, based on charge transport between grain boundaries through the polymeric layer gaps. A previously published model, which was developed for a different polymer/semiconductor composite failed to account for many details of our experimental results. The new polymeric binder presented in the present paper showed non-linear dark current versus voltage dependence, and a higher sensitivity. At the low voltage range, it showed a higher sensitivity for a reduced grain size. The new model addresses the grain geometry as an important factor, and accounts well to most sensitivity characteristics of our detectors. The model excludes itself from a debate in the literature concerning specifics of charge transport mechanisms in composites containing electrically conducting particles. However, certain charge transport between neighbor crystallites is necessary in order to explain the experimental results.