This PDF file contains the front matter associated with SPIE Proceedings Volume 6706, including the Title Page, Copyright information, Table of Contents, and the Conference Committee listing.

The excellent room temperature spectral performance of cadmium zinc telluride detectors grown via the Traveling Heater Method (THM) makes this approach suitable for the mass deployment of radiation detectors for applications in homeland security and medical imaging. This paper reports our progress in fabricating thicker and larger area detectors from THM grown CZT. We discuss the performance of such 20x20x10 mm³, and 10x10x10 mm³ monolithic pixellated detectors and virtual Frisch-Grid 4x4x12 mm3 devices, and describe the various physical properties of the materials.

In the past, various virtual Frisch-grid designs have been proposed for cadmium zinc telluride (CZT) and other compound semiconductor detectors. These include three-terminal, semi-spherical, CAPture, Frisch-ring, capacitive Frisch-grid and pixel devices (along with their modifications). Among them, the Frisch-grid design employing a non-contacting ring extended over the entire side surfaces of parallelepiped-shaped CZT crystals is the most promising. The defect-free parallelepiped-shaped crystals with typical dimensions of 5x5x12 mm3 are easy to produce and can be arranged into large arrays used for imaging and gamma-ray spectroscopy. In this paper, we report on further advances of the virtual Frisch-grid detector design for the parallelepiped-shaped CZT crystals. Both the experimental testing and modelling results are described.

100 μm thick layers of CdTe have been grown by Molecular Beam Epitaxy (MBE) on LEC GaAs (001) substrates. The intended application for the CdTe thick films is the fabrication of radiation detectors. As recently reported extensive characterization has been performed. In this paper the results of the previous papers are being summarized, showing the potential of the CdTe films to be used as radiation detector. Furthermore first investigations on the application of the layers as radiation detectors are being presented.

The performance of current long-drift-length Cadmium Zinc Telluride (CZT) detectors principally is determined by the material's quality. Hence, the material's limitations must be better understood and potential solutions identified to grow CZT crystals with the required qualities. Our efforts have focused on developing novel techniques and testing methods that will allow us to explore the correlations between the crystal's defects and the detector's properties. Local stoichiometric variations and other local disordering make it very hard to systematically correlate performance and material defects on a macroscopic scale. Therefore, to delineate the factors limiting the energy resolution of CZT detectors, we directed our efforts towards micron-scale material characterization and assessments of the detectors using the National Synchrotron Light Source (NSLS). The NSLS offers us a highly collimated high-intensity X-ray beam that we employed to undertake detector-performance mapping, and to investigate the association between microscopic defects and fluctuations in collected charge. In this paper, we illustrate our techniques and results.

Low pressure Electro-Dynamic Gradient freeze (EDG) method has been used to grow compensated, high resistivity Cd(1-x)ZnxTe for x and gamma ray detectors. All growths contained excess Tellurium which is added to the growth. Ampoule design and setup to limit vapor transport was determined to be important. Ingots grown in a Pyrolitic Coated Graphite crucible are shown to provide a good response to ionizing radiation at room temperature and can be used multiple times. The highest doping levels of Aluminum are shown to improve mobility lifetime products for electrons and average 8.7x10^-4 cm²/V at 0.5 μsecond shaping fitting the Hecht relation.

In situ characterization methods are being developed at the Idaho National Laboratory that can be used to characterize the atomic lattice structure of materials used for semiconductor and scintillation detectors during the crystal growth and heat treatment processes, which have been shown to be critical for the development of optimized semiconductor and scintillation radiation detectors. Multiple methods for implanting positrons into the material have been developed and integrated with measurement techniques including Doppler broadening, coincidence Doppler broadening and positron lifetime measurement. The INL developed induced positron technique allows positron measurements to be performed at depth up to 10 cm inside crystal boules. Also, a portable measurement system suitable for field use has been developed that is suitable for assessing heat treatments at depths up to 1 cm inside a material in an industrial environment. Results of measurements that address the effects of composition and heatup/melting/cool down on material lattice structures are discussed along with plans for the in situ crystal studies.

We report the results of Differential Aperture X-ray Microscopy (DAXM) measurements near Te precipitates in CdZnTe grown via low-pressure Bridgman. White-beam Laue patterns were acquired with 3-D spatial resolution (with 0.25 μm resolution in the scanning directions and 1 μm resolution in depth) at depths of up to 35 μm deep normal to the surface. We find very little crystal strain (< 10^-3) or rotation (<0.05 degrees) near Te precipitates. We also examine local deformations in the vicinity of a microhardness indent, and find that although significant rotations exist, the spatial extent is limited to a few tens of microns. Furthermore, observed crystal strains are limited to 5 x 10^-3 or less in regions near the microhardness indent.

We report our progress on the development of pixellated imaging CZT detector arrays for our first-generation balloon-borne wide-field hard X-ray (20 - 600 keV) telescope, ProtoEXIST1. Our ProtoEXIST program is a pathfinder for the High Energy Telescope (HET) on the Energetic X-ray Imaging Survey telescope (EXIST), a proposed implementation of the Black Hole Finder Probe. ProtoEXIST1 consists of four independent coded-aperture telescopes with close-tiled (~0.4 mm gaps) CZT detectors that preserve their 2.5mm pixel pitch. Multiple shielding/field-of-view configurations are planned to identify optimal geometry for the HET in EXIST. The primary technical challenge in ProtoEXIST is the development of large area, close-tiled modules of imaging CZT detectors (1000 cm² for ProtoEXIST1), with all readout and control systems for the ASIC readout vertically stacked. We describe the overall telescope configuration of ProtoEXIST1 and review the current development status of the CZT detectors, from individual detector crystal units (DCUs) to a full detector module (DM). We have built the first units of each component for the detector plane and have completed a few Rev2 DCUs (2x2 cm²), which are under a series of tests. Bare DCUs (pre-crystal bonding) show high, uniform ASIC yield (~70%) and ~30% reduction in electronics noise compared to the Rev1 equivalent. A Rev1 DCU already achieved ~1.2% FWHM at 662 keV, and preliminary analysis of the initial radiation tests on a Rev2 DCU shows ~ 4 keV FWHM at 60 keV (vs. 4.7 keV for Rev1). We therefore expect about ≤1% FWHM at 662 keV with the Rev2 detectors.

We proposed that material discriminated X-ray CT with conventional X-ray tube and energy differentiation type 64ch CdTe radiation line sensor. Distribution of Atomic number was obtained by using dual-energy X-ray CT. In this study, problem of conventional X-ray tube was reduced by the collimator and measurement time. So line attenuation coefficient was obtained depend on theory. Atomic number was calculated with two different methods. We could obtain atomic number within about three error margin.

High quality detector grade GaSe and GaTe single crystals have been grown by a modified vertical Bridgman technique using high purity Ga (7N) and in-house zone refined (ZR) precursor materials (Se and Te). A state-of-the-art computer model, MASTRAPP, is used to model heat and mass transfer in the Bridgman growth system and to predict the stress distribution in the as-grown crystals. The model accounts for heat transfer in the multiphase system, convection in the melt, and interface dynamics. The crystals harvested from ingots of 8-10 cm length and 2.5 cm diameter, have been characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), Raman spectroscopy, low temperature photoluminescence (PL), atomic force microscopy (AFM), and optical absorption/transmission measurements. Single element devices up to 1 cm² in area have been fabricated from the crystals and tested as radiation detectors by measuring current-voltage (I-V) characteristics and pulse height spectra using ²⁴¹Am source. The crystals have shown high promise as nuclear detectors with their high dark resistivity (≥10⁹ Ω^.cm), good charge transport properties (μτ_e ~ 1.4x10^-5 cm²/V and μτ_h ~ 1.5x10^-5 cm²/V), and relatively good energy resolution (~4% energy resolution at 60 keV). Details of numerical modeling and simulation, detector fabrication, and testing using a ²⁴¹Am energy source (60 keV) is presented in this paper.

Mercuric iodide (HgI₂) is a well known material for the direct detection of gamma rays; however, the largest volume achievable is limited by the thickness of the detector which needs to be a small fraction of the average trapping length for electrons. We are reporting here preliminary results of using HgI₂ crystals to fabricate photocells used in the readout of various scintillators. The optical spectral response and efficiency of these photocells were measured and will be reported. Preliminary nuclear response from an HgI₂ photocell that was optically matched to a Ce3+:LaBr₃ scintillator will also be presented and discussed. Further improvements will be sought by optimizing the transparent contact technology.

An evolutionary algorithm (ES) for automated deconvolution of γ-ray spectra is described that fits peak shape morphologies typical of spectra acquired from variably radiation damaged γ-ray detectors. Space radiation effects significantly impair semi-conductor γ-ray detector efficiency and induce variable degrees of nuclide peak broadening, distortion in spectra. Mars Odyssey Gamma-ray spectrometer data are used to demonstrate applicability of described algorithms for three degrees of radiation damage. ES methods accurately identify and quantify the discrete set of nuclide peaks in an arbitrary spectrum using a nuclide library. A novel method of constraining peak low energy tails, broadened by detector radiation damage, reduces the peak shape model from six parameters to four yielding a significant minimization of model complexity. Benefits of this approach include the simple implementation of highly specific parameter constraints that appropriately define feasible solution spaces. Methods describe peak low energy tailing descriptors as a continuum of low energy peak tailing curves representing increasing degrees of radiation damage. Curves are addressable by a single real valued parameter. Results illustrate the use of methods to simply describe relative radiation dosimetry using this parameter. Analysis of degraded spectra indicates method sensitivity to low and high levels of space radiation damage prior to and post MO-GRS detector annealings.

The radiation detection efficiency and spectral resolution of mercuric iodide detectors can be improved significantly by increasing the volume of the detectors and by using a pixellated anode structure. Detector bodies with a thickness of nominally 10 mm and an active area of approximately 14 mm x 14 mm have been used for these experiments. The detectors were cut from single crystals grown by the physical vapor transport method. The cut surfaces were polished and etched using a string saw and potassium iodide solutions. The Palladium contacts were deposited by magnetron sputtering through stainless steel masks. The cathode contact is continuous; the anode contacts consist of an array of 11 x 11 pixels surrounded by a guard ring. The resistance between a pixel and its surrounding contacts should be larger than 0.25 Gohm. The detector is mounted on a substrate that makes it possible to connect the anode pixels to an ASIC, and is conditioned so that it is stable for all pixels at a bias of -3000 Volts. Under these conditions the spectral resolution for Cs-137 gamma rays (662 keV) is approximately 5% FWHM. When depth sensing correction methods are applied, the resolution improves to about 2% FWHM or better. It is expected that the performance of the devices can be improved by the careful selection of crystal parts that are free of structural defects. Details of the fabrication technologies will be described. The effects of material inhomogeneities and transport properties of the charge carriers will be discussed.

The bulk leakage current in a semiconductor detector is an important parameter that affects the noise level and energy resolution of the detector. For detectors operating with ohmic contacts, the bulk leakage current is determined by the bulk resistivity of the semiconductor material. However, CdZnTe detectors typically utilize Schottky barrier type contacts, in which case the bulk leakage current is expected to depend on the contact behavior and not on the bulk resistivity of the material. We have studied the bulk leakage current and noise of CdZnTe detectors made from materials supplied by different manufacturers of CdZnTe crystals. The results indicate that there is a marked difference in bulk leakage currents among materials from different manufacturers and among different samples from the same manufacturer. In some cases, the bulk leakage current shows no correlation with the bulk resistivity of the materials. In other cases, the bulk leakage currents tend to be lower for lower resistivity materials, which is opposite from the commonly held expectation based upon ohmic contact device behavior. In this paper we present a summary of our electrical measurements on CdZnTe devices and present results indicating a possible relationship between leakage and bulk material properties, but the specific material properties and the mechanism responsible for the leakage current variation have yet to be determined.

We developed Schottky CdTe detectors using Al as an anode electrode and measured their performances. We first fabricated monolithic detectors with four different thicknesses of 0.5, 0.75, 1.0, and 2.0 mm. An Al anode electrode was implemented with a guard-ring structure. For the 0.5 mm thick CdTe detector, an energy resolution of 1.2 keV (FWHM) at 122 keV was achieved at a temperature of −20 °C and a bias voltage of 400 V. Using the same technology, we next developed 8 × 8 pixel CdTe detectors, again with the four different thicknesses. The Al anode electrode was pixelated and the Pt cathode was made as a single plate. Signals from all pixels were successfully obtained and an energy resolution of 1.3 keV and 1.9 keV (FWHM) for 59.5 keV and 122 keV gamma-rays, was achieved at a temperature of −20 °C and a bias voltage of 400 V using the 0.5 mm thick CdTe detector. The energy resolution was nearly the same in each pixel.

Perforated semiconductor diode detectors have been under development for several years at Kansas State University for a variety of neutron detection applications. The fundamental device configuration is a pin diode detector fabricated from high-purity float zone refined Si wafers. Perforations are etched into the diode surface with inductively-coupled plasma (ICP) reactive ion etching (RIE) and backfilled with ⁶LiF neutron reactive material. The perforation shapes and depths can be optimized to yield a flat response to neutrons over a wide variation of angles. The prototype devices delivered over 3.8% thermal neutron detection efficiency while operating on only 15 volts. The highest efficiency devices thus far have delivered over 12% thermal neutron detection efficiency. The miniature devices are 5.6 mm in diameter and require minimal power to operate, ranging from 3.3 volts to 15 volts, depending upon the amplifying electronics. The battery operated devices have been incorporated into compact modules with a digital readout. Further, the new modules have incorporated wireless readout technology and can be monitored remotely. The neutron detection modules can be used for neutron dosimetry and neutron monitoring. When coupled with high-density polyethylene, the detectors can be used to measure fission neutrons from spontaneous fission sources. Monto Carlo analysis indicates that the devices can be used in cargo containers as a passive search tool for spontaneous fission sources, such as ²⁴⁰Pu. Measurements with a ²⁵²Cf source are being conducted for verification.

The development of high resolution, room temperature semiconductor radiation detectors requires the introduction of materials with increased carrier mobility-lifetime (μτ) product, while having a band gap in the 1.4-2.2 eV range. AlSb is a promising material for this application. However, systematic improvements in the material quality are necessary to achieve an adequate μτ product. We are using a combination of simulation and experiment to develop a fundamental understanding of the factors which affect detector material quality. First principles calculations are used to study the microscopic mechanisms of mobility degradation from point defects and to calculate the intrinsic limit of mobility from phonon scattering. We use density functional theory (DFT) to calculate the formation energies of native and impurity point defects, to determine their equilibrium concentrations as a function of temperature and charge state. Perturbation theory via the Born approximation is coupled with Boltzmann transport theory to calculate the contribution toward mobility degradation of each type of point defect, using DFT-computed carrier scattering rates. A comparison is made to measured carrier concentrations and mobilities from AlSb crystals grown in our lab. We find our predictions in good quantitative agreement with experiment, allowing optimized annealing conditions to be deduced. A major result is the determination of oxygen impurity as a severe mobility killer, despite the ability of oxygen to compensation dope AlSb and reduce the net carrier concentration. In this case, increased resistivity is not a good indicator of improved material performance, due to the concomitant sharp reduction in μτ.

An epitaxial diamond detector obtained by CVD (Chemical Vapor Deposition) has been used in order to monitor X-ray pulses from a radiological portable X-ray generator commonly used in hospitals. X-ray maximum energies varied from 50 to 120 KeV, while (electron anodic current)x(time duration) products were in the range from 20 to 100 mAs. Current pulses were recorded and from their shapes the timing and the collected charge were calculated and compared with those obtained by standard 6 cm³ ionization chambers and by a silicon diode arrays used in quality assurance programs for radiological X-ray apparatuses. Both diamond detector and silicon array display a standard deviation in time recording of 0.3% in the time range from 0.15 up to 2.5 s. The integrated current recorded from diamond is linear with respect the dose recorded by the ionization chamber from 5 up to 125 mGy, with a standard deviation on single points of the order of 0.5%. The stability of the detector is very good even without a priming treatment generally used in order to stabilize diamond dosimeters. Homogeneity of the detector in terms of its response was tested by means of alpha particles, which indicate an energy resolution of 0.7%, quite close to that of a standard surface barrier silicon detector. These results indicate that epitaxial diamond could be considered ready to be used in standard quality control procedures concerning radiological X-ray apparatuses.

A semiconductor Compton camera for a balloon borne experiment aiming at observation in high energy astrophysics is developed. The camera is based on the concept of the Si/CdTe semiconductor Compton Camera, which features high-energy and high-angular resolution in the energy range from several tens of keV to a few MeV. It consists of tightly packed double-sided silicon strip detectors (DSSDs) stacked in four layers, and a total of 32 CdTe pixel detectors surrounding them. The Compton reconstruction was successfully performed and gamma-ray images were obtained from 511 keV down to 59.5 keV. The Angular Resolution Measure (ARM) at 511 keV is ~ 2.5 degrees, thanks to the high energy resolution in both the DSSD and CdTe parts.

Double-sided silicon strip detector (DSSD) is a key component to construct the next generation Compton telescope for the high-sensitivity observation in the energy region from several hundred keV to MeV. The concept of Compton camera we consider is using DSSD for scatterer, and high-stopping CdTe pixel detector for absorber. As the scatterer, DSSD has advantages of smaller band gap, higher efficiency of scattering, smaller Doppler broadening, good response time, and smaller number of readout channels. We have developed and confirmed that 0.3 mm-thick DSSD has enough performance. As a next step, in order to obtain more efficiency of higher energy gamma-rays, we developed newly designed DSSD which increase in thickness to 0.5 mm. We measured the basic properties of 0.5 mm thick DSSD, in terms of leakage current, capacitances, and noise characteristics. They can be full-depleted around 200 V, and we obtained the energy resolution of 1.3 keV (FWHM) for 60 keV at -10 °C from one p-side strip. We also set up the newly developed read-out system which is based on technology of operating ASICs on floating ground, and performed 64 ch read-out on one side.

A Mixed-Mode Pixel Array Detector has been developed to measure protein crystallographic diffraction patterns. X-rays are stopped in a 500 μm thick layer of silicon diodes, and collected charge is processed by an attached ASIC. Goals of the project are high flux (10⁸ x-rays/s/pixel) capability and fast readout (< 0.5 ms dead time between frames). "Mixed-Mode" refers to a readout method whereby integrated signal accumulating in each pixel is compared against a threshold value. When the threshold is reached, a digital count is added to an 18-bit in-pixel counter and a set quantity of charge is removed from integrator. At the end of the x-ray exposure, analog signal left in the integrator is separately processed. Thus, one obtains mixed digital and analog data where the counter bits are a high order word and the analog residual provides higher precision. Typically, each count is equivalent to 100 10 keV x-rays, for a well-depth >10⁷ 10 keV x-rays/ pixel. The analog residual is digitized to 9-bit precision allowing measurement of the residual charge to better than a quarter of the charge from single 10 keV x-rays. Measurements are presented on x-ray tests at the Cornell High Energy Synchrotron Source (CHESS). Dynamic range, linearity, point-spread function and noise properties are shown. Status will be is reported on five different approaches for ASIC-diode hybridization. Progress toward bonding of a 128 x 512 pixel device is also presented.

A 4π direction-sensitive gamma imager is presented, using a 1 cm³ 3D CZT detector from Yinnel Tech and the RENA-3 readout ASIC from NOVA R&D. The measured readout system electronic noise is around 4-5 keV FWHM for all anode channels. The measured timing resolution between two channels within a single ASIC is around 10 ns and the resolution is 30 ns between two separate ASIC chips. After 3D material non-uniformity and charge trapping corrections, the measured single-pixel-event energy resolution is around 1% for Cs-137 at 662 keV using a 1 cm³ CZT detector from Yinnel Tech with an 8 x 8 anode pixel array at 1.15 mm pitch. The energy resolution for two pixel events is 2.9%. A 10 uCi Cs-137 point source was moved around the detector to test the image reconstruction algorithms and demonstrate the source direction detection capability. Accurate source locations were reconstructed with around 200 two-pixel events within a total energy window ±10 keV around the 662 keV full energy peak. The angular resolution FWHM at four of the five positions tested was between 0.05-0.07 steradians.

Recently, some of the authors showed that it is possible to grow CZT crystals by the boron oxide encapsulated vertical Bridgman method. The most important feature of the technique is that the crystal, during the growth, is fully encapsulated by a thin layer of liquid boron oxide, so that the crystal-crucible contact is prevented. The stress of the crucible to the crystal is strongly reduced also during the cooling, because the boron oxide layer is molten down to about 500°C. A number of detectors have been prepared out of these crystals. The transport properties (μτ product) have been studied by photoconductivity measurements as well as by determining the response to hard X-ray irradiation. The transport properties have been studied as a function of the indium content and of the position of the wafer which the detector was cut out.

A simple x-ray computed tomography (CT) system utilizing a cadmium telluride detector and its application to enhanced iodine K-edge angiography are described. The CT system is of the first generation type and consists of an x-ray generator, a turn table, a translation unit, a motor drive unit, a cadmium telluride detector, an interface unit for the detector, and a personal computer (PC). Tomography was performed by the repetition of the translation and rotation. Narrow-photon-energy bremsstrahlung x-rays with a peak photon energy of approximately 35 keV is very useful for performing enhanced K-edge angiography because these rays are absorbed effectively by iodine-based contrast media with a K-edge of 33.2 keV. The tube voltage, the tube current, and the aluminum filter thickness were 60 kV, 1.5 mA, and 3.0 mm, respectively. Holes filled with iodine media in phantoms are visible with high contrasts, and the CT system can be applied to photon-counting and fluorescent x-ray CT systems.

This paper reports on the use of a seeded vapour phase technique to grow bulk crystals of CdTe onto commercially available 50 mm diameter (211)B GaAs substrates. High quality crystals, several mm in thickness were grown on the GaAs at linear growth rates of ~ 120 μm/h. Characterisation by double and triple axis XRD showed the best crystals to have θ-2θ FWHM_θ values of ~ 24 arc sec corresponding to low strain dispersion (< 2×10^-4). Rocking curve scans included two to three sharp peaks, indicative of some small mosaicity. When mapped across a the surface of the crystal, the FWHM was uniform and < 93 arc sec. Contactless resistivity showed a similar degree of uniformity with a mean value of 4.4 × 10⁹ ± 1.6 × 10⁹ Ω cm. Infrared microscopy showed that within the resolution of the microscope (~ 5 μm) there were very few Te inclusions.

A preliminary experiment for producing narrow-photon-energy cone-beam x-rays using a silicon single crystal is described. In order to produce low-photon-energy x-rays, a 100-µm-focus x-ray generator in conjunction with a (111) plane silicon crystal is employed. The x-ray beams from the source are confined by an x-y diaphragm, and monochromatic cone beams are formed by the crystal and three lead plates. The x-ray generator consists of a main controller and a unit with a high-voltage circuit and a 100-µm-focus x-ray tube. In this experiment, the maximum tube voltage and current were 35 kV and 0.50 mA, respectively, and the x-ray intensity of the microfocus generator was 343 μGy/s at 1.0 m from the source with a tube voltage of 30 kV and a current of 0.50 mA. The effective photon energy is determined by Bragg's angle, and the photon-energy width is regulated by the angle delta. Using this generator in conjunction with a computed radiography system, quasi-monochromatic radiography was performed using a cone beam with an effective energy of approximately 15.5 keV.

Recently, the Department of Homeland Security (DHS) has integrated all nuclear detection research, development, testing, evaluation, acquisition, and operational support into a single office: the Domestic Nuclear Detection Office (DNDO). The DNDO has specific requirements set for all commercial off-the-shelf and government off-the-shelf radiation detection equipment and data acquisition systems. This article would investigate several recent developments in field deployable gamma radiation detectors that are attempting to meet the DNDO specifications. Commercially available, transportable, handheld radio isotope identification devices (RIID) are inadequate for DHS' requirements in terms of sensitivity, resolution, response time, and reach-back capability. The leading commercial vendor manufacturing handheld gamma spectrometer in the United States is Thermo Electron Corporation. Thermo Electron's identiFINDER^TM, which primarily uses sodium iodide crystals (3.18 x 2.54cm cylinders) as gamma detectors, has a Full-Width-at-Half-Maximum energy resolution of 7 percent at 662 keV. Thermo Electron has just recently come up with a reach-back capability patented as RadReachBack^TM that enables emergency personnel to obtain real-time technical analysis of radiation samples they find in the field¹. The current project has the goal to build a prototype handheld gamma spectrometer, equipped with a digital camera and an embedded cell phone to be used as an RIID with higher sensitivity, better resolution, and faster response time (able to detect the presence of gamma-emitting radio isotopes within 5 seconds of approach), which will make it useful as a field deployable tool. The handheld equipment continuously monitors the ambient gamma radiation, and, if it comes across any radiation anomalies with higher than normal gamma gross counts, it sets an alarm condition. When a substantial alarm level is reached, the system automatically triggers the saving of relevant spectral data and software-triggers the digital camera to take a snapshot. The spectral data including in situ analysis and the imagery data will be packaged in a suitable format and sent to a command post using an imbedded cell phone.

The Advanced GAmma Tracking Array (AGATA) is a European project that is aiming to construct a complete 4π High Purity Germanium (HPGe) gamma-ray spectrometer for nuclear structure studies at future Radioactive Ion Beam (RIB) Facilities. The proposed array will utilise digital electronics, Pulse Shape Analysis (PSA) and Gamma-Ray Tracking (GRT) algorithms, to overcome the limited efficiencies encountered by current Escape Suppressed Spectrometers (ESS), whilst maintaining the high Peak-to-Total ratio. Two AGATA symmetrical segmented Canberra Eurisys (CE) prototype HPGe detectors have been tested at the University of Liverpool. A highly collimated Cs-137 (662keV) beam was raster scanned across each detector and data were collected in both singles and coincidence modes. The charge sensitive preamplifier output pulse shapes from all 37 channels (one for each of the 36 segments and one for the centre contact) were digitised and stored for offline analysis. The shapes of the real charge and image charge pulses have been studied to give detailed information on the position dependent response of each detector. 1mm position sensitivity has been achieved with the parameterisation of average pulse shapes, calculated from data collected with each of the detectors. The coincidence data has also been utilised to validate the electric field simulation code Multi Geometry Simulation (MGS). The precisely determined 3D interaction positions allow the comparison of experimental pulse shapes from single site interactions with those generated by the simulation. It is intended that the validated software will be used to calculate a basis data set of pulse shapes for the array, from which any interaction site can be determined through a χ² minimisation of the digitized pulse with linear combinations of basis pulseshapes. The results from this partial validation, along with those from the investigation into the position sensitivity of each detector are presented.

The Modulation Transfer Function (MTF) is a quantitative function based on frequency resolution that characterizes imaging system performance. In this study, a new MTF methodology is investigated for application to Radiography by Selective Detection (RSD), an enhanced single-side x-ray Compton backscatter imaging (CBI) technique which detects selected scatter components. The RSD imaging modality is a unique type of real-time radiography that uses a set of fin and sleeve collimators to preferentially select different components of the x-ray backscattered field. Radiography by selective detection has performed successfully in different non-destructive evaluation (NDE) applications. A customized RSD imaging system was built at the University of Florida for inspection of the space shuttle external tank spray-on foam insulation (SOFI). The x-ray backscatter RSD imaging system has been successfully used for crack and corrosion detection in a variety of materials. The conventional transmission x-ray image quality characterization tools do not apply for RSD because of the different physical process involved. Thus, the main objective of this project is to provide an adapted tool for dynamic evaluation of RSD system image quality. For this purpose, an analytical model of the RSD imaging system response is developed and supported. Two approaches are taken for the MTF calculations: one using the Fourier Transform of a line spread function and the other one using a sine function pattern. Calibration and test targets are then designed according to this proposed model. A customized Matlab code using image contrast and digital curve recognition is developed to support the experimental data and provide the Modulation Transfer Functions for RSD.

We find that the high-Z crystal Barium Iodide is readily growable by the Bridgman growth technique and is less prone to crack compared to Lanthanum Halides. We have grown Barium Iodide crystals: undoped, doped with Ce³⁺, and doped with Eu²⁺. Radioluminescence spectra and time-resolved decay were measured. BaI₂(Eu) exhibits luminescence from both Eu²⁺ at 420 nm (~450 ns decay), and a broad band at 550 nm (~3 μs decay) that we assign to a trapped exciton. The 550 nm luminescence decreases relative to the Eu²⁺ luminescence when the Barium Iodide is zone refined prior to crystal growth. We also describe the performance of BaI₂(Eu) crystals in experimental scintillator detectors.

Ceramic and single crystal Lutetium Aluminum Garnet scintillators exhibit energy resolution with bialkali photomultiplier tube detection as good as 8.6% at 662 keV. Ceramic fabrication allows production of garnets that cannot easily be grown as single crystals, such as Gadolinium Aluminum Garnet and Terbium Aluminum Garnet. Measured scintillation light yields of Cerium-doped ceramic garnets indicate prospects for high energy resolution.

We have investigated the applicability of phosphate glasses as host systems for the formation of rare-earth-activated gamma- and x-ray scintillators. Glass scintillators have generally suffered from low light yields, usually attributed to inefficient energy transfer from the glass matrix to the luminescent center. Our research on these phosphate glasses has shown that their structural properties can be readily varied and controlled by compositional alterations. The melting and pouring temperature of ~1050°C for these phosphate glasses is significantly lower than the processing temperatures generally associated with the formation of silicate glass scintillators. The calcium-sodium phosphate glasses will tolerate relatively high cerium concentrations based on the initial melt compositions, and the light yield for gamma-ray excitation at 662 keV was determined as a function of cerium concentration up to the saturation level. The rare-earth-activated Ca-Na phosphate glass primary-component decay time was in the range of 32 to 42 nsec for various Ce concentrations with the contribution of the light output of the primary component ranging from 80 to 90%. Studies of the effects of co-doing with both Ce and Gd were also carried out in the case of the Ca-Na phosphate glass hosts. The effects of post-synthesis thermochemical treatments in a variety of atmospheres and at various processing temperatures were also investigated for the Ce-activated Ca-Na phosphate scintillators.

Ceramic materials show significant promise for the production of reasonably priced, large-size scintillators. Ceramics have recently received a great deal of attention in the field of materials for laser applications, and the technology for fabricating high-optical-quality polycrystalline ceramics of cubic materials has been well developed. The formation of transparent ceramics of non-cubic materials is, however, much more difficult as a result of birefringence effects in differently oriented grains. Here, we will describe the performance of a few new ceramics developed for the detection of gamma- and x-ray radiation. Results are presented for ceramic analogs of three crystalline materials - cubic Lu₂O₃, and non-cubic LaBr₃, and Lu₂SiO₅ or LSO (hexagonal, and monoclinic structures, respectively). The impact of various sintering, hot-pressing and post-formation annealing procedures on the light yield, transparency, and other parameters, will be discussed. The study of LaBr₃:Ce shows that fairly translucent ceramics of rare-earth halides can be fabricated and they can reach relatively high light yield values. Despite the fact that no evidence for texturing has been found in our LSO:Ce ceramic microstructures, the material demonstrates a surprisingly high level of translucency or transparency. While the scintillation of LSO:Ce ceramic reaches a light yield level of about 86 % of that of a good LSO:Ce single crystal, its decay time is even faster, and the long term afterglow is lower than in LSO single crystals.

Nanophosphor LaF₃:Ce has been synthesized and incorporated into a matrix to form a nanocomposite scintillator suitable for application to γ-ray detection. Owing to the small nanocrystallite size (sub-10 nm), optical emission from the γ / nanophosphor interaction is only weakly Rayleigh scattered (optical attenuation length exceeds 1 cm for 5-nm crystallites), thus yielding a transparent scintillator. The measured energy resolution is ca. 16% for ¹³⁷Cs γ rays, which may be improved by utilizing brighter nanophosphors. Synthesis of the nanophosphor is achieved via a solution-precipitation method that is inexpensive, amenable to routine processing, and readily scalable to large volumes. These results demonstrate nanocomposite scintillator proof-of- principle and provide a framework for further research in this nascent field of scintillator research.

Cd_1-xZn_xTe nanopowder with the average particle size 10 nm was produced through vapor deposition. Dense ceramic material was compacted from the nanopowder at room temperature. The effect of annealing on grain growth, phase transitions and some physical properties was studied.

Multi-Pixel Photon Counter (MPPC) is a Hamamatsu's new Si avalanche photodiode. The main features of the MPPC are very high gain (10⁵ to10⁶) and very fast operation. The MPPC offers considerable advantages over widely used photomultiplier tubes (PMT) for scintillation detection, especially due to their high quantum efficiency, low cost, compactness, low operating voltage, mechanical robustness and insensitivity to magnetic fields. One of the most attractive applications of MPPCs is positron emission tomography (PET). In the present study, the MPPCs were coupled to inorganic scintillators including LSO, BGO and YAP in order to evaluate the scintillator/MPPC devices in terms of energy and timing resolutions for PET applications. The scintillation detector consisted of an LSO scintillator (2 mm × 2 mm × 4 mm) coupled to an MPPC exhibited energy resolutions of 196 keV FWHM and 189 keV FWHM for 511 keV and 662 keV gamma-rays, respectively. The MPPC was operated at a bias of 71.4 V and at room temperature. In order to investigate timing properties of the LSO/MPPC device, coincidence timing spectra between a reference scintillation detector which consisted of a BaF₂ crystal coupled to a PMT and the LSO/MPPC device were measured with 511 keV positron annihilation gamma-rays from a ²²Na source. A coincidence timing resolution of 3.2 ns FWHM was obtained with the LSO/MPPC system.

In this work, the most important aspects of semi-insulating (SI) GaAs and SI InP-based radiation detectors will be listed. Based on that, the material and technology requirements will be identified. Further, the status of development of X- and gamma-ray detectors based on the bulk SI GaAs and SI InP will be reviewed. The emphasis will be concentrated on the following important aspects: (i) basic material characteristics, (ii) role of the electrodes (blocking contacts, ohmic contacts) in the overall performances of detectors. The fabrication technology and the performances of detectors and their application in the first quantum X-ray digital scanner recently developed will also be illustrated along with some conclusions about the material/application relations: (i) radiation detectors based on bulk SI GaAs may readily find applications in X-ray digital radiology imaging systems, whilst (ii) SI InP-based detectors are very promising but need further development to reach performances suitable for the application.

Thallium bromide (TlBr) is a compound semiconductor with high density, high atomic numbers and wide bandgap. In addition, recent results indicate that the mobility-lifetime product of electrons can be quite high, approaching the values for CdTe and CZT. These properties make TlBr a very promising material for nuclear radiation detector at room temperature. In this paper we report on our investigation of the performance of planar TlBr detector under high flux x-rays irradiation. This study proposes an alternate contact method that reduces the polarization effects, and the afterglow for a wide range of high flux applications.

Intra Operative Radiation Therapy (IORT) is a technique based on delivery of a high dose of ionising radiation to the cancer tissue, after tumour ablation, during surgery, while reducing the exposure of normal surrounding tissue. Novac7 and Liac are new linear accelerators expressly conceived to perform in the operating room. These accelerators supply electron beams with high dose rate. Because of this peculiar characteristic, classical dosimetric techniques are not able to give at once a real-time response and an extensive measure of the absorbed dose. In past years the authors realized a prototype for IORT dosimetry able to give the real time bi-dimensional image of dose distribution on a single layer. In the framework of a research project funded by the INFN (Italian National Institute of Nuclear Physics), a collaboration between the Physics Department of Bologna, Italy, the Physics Department of Cosenza and the Medicine Department of Catanzaro, Italy, has studied a new system composed of six layers. Each layer includes two orthogonal bundles of scintillating optical fibres. The fibres are optically coupled with four arrays of photodiodes as read-out system. This new system will be able to characterize completely the electron beam in energy, intensity and spatial distribution. In real time it will be able to measure the 3D dose distribution, providing a full check of quality assurance for IORT. The various phases of design, development and characterization of the instrument will be illustrated, as well as some experimental tests performed with the prototype. We verified that the system is able to give a real time response, which is linear versus dose and not affected by the high dose rate. The conclusions confirm the capability of the instrument to overcome problems encountered with classic dosimetry, showing that the obtained results strongly encourage the continuation of this research.

During the last decades, low temperature detectors have undergone a considerable growth and are now widely acknowledged as useful instruments in many fundamental physics experiments. In this field, the phonon mediated particle detectors known as bolometers are remarkable and are successfully used in various branches of physics research for their good sensitivity, energy resolution and flexibility in the choice of the constituting materials. Bolometers have proved to be powerful devices for radiation detection; in particular, they are able to detect Gamma-rays with resolutions comparable to those obtained with the best Ge diodes. They are also suited for applications in the area of nuclear and particle physics, like the study of rare events or dark matter. Although an effective technique, the use of bolometers in the specific field of the search for neutrinoless double beta decay is affected by the lack of spatial resolution. This results in the expected signal of this rare decay hidden under an indistinguishable background due to possible surface radioactive contaminations in the materials facing the detectors. An approach to this problem is to make bolometers surface sensitive by applying ultra-pure crystalline foils on the main detector through direct thermal contact and by operating them as active shields. In this contribution we present for the first time surface sensitivity achieved with large mass TeO₂ bolometers (~800 g) operated underground at ~10 mK, dedicated to the detection of neutrinoless double beta decay of ¹³⁰Te. Our encouraging measurements suggest that this could be a viable method for the discrimination of background events.

The effects of crystal geometry and aspect ratio on a CdZnTe Frisch collar device were investigated. A 19.08x19.34x4.95mm³ device fabricated from CdZnTe grown by Redlen Technologies was used as the starting material. The crystal was re-fabricated many times to achieve several aspect ratios while the device length was held constant at 4.75±0.15mm. The following aspect ratios were successfully fabricated from the initial device: 0.26, 0.52, 0.71, 0.96, 1.19, 1.36, 1.59 and 1.92. The energy spectra of ²⁴¹Am and ¹³⁷Cs were recorded for all devices in both planar and Frisch collar configurations. The current-voltage (I-V) characteristic curve was also determined for planar configurations. It was observed that the Frisch collar effect begins to occur for devices with an aspect ratio of approximately 1.0. Device performance continued to improve as the aspect ratio was further increased and was noted to significantly improve the energy resolution of the device when the aspect ratio was greater than 1.5. The CdZnTe devices were also theoretically modeled to support the experimental conclusion.