This PDF file contains the front matter associated with SPIE Proceedings Volume 7807, including the Title Page, Copyright Information, Table of Contents, Introduction, and the Conference Committee listing.

In our previous design of virtual Frisch-grid CdZnTe (CZT) detectors, the charge drift-lines can be terminated at the side surfaces before the carriers reach the collecting anode; this results in a loss of signal from the interacting events near the detector's edges. Here, we describe our new design for the anode contact that reduces these edge effects by focusing the electric field towards the detectors' central axes. Four detectors were fabricated with the new hybrid anode contact, and their performances were evaluated and compared to those from the previous design for our virtual Frisch-grid detectors. The results obtained for all four showed similar improvement: therefore, we illustrate them with the findings from one detector.

CdZnTe (CZT) semiconducting crystals are of interest for use as room temperature X- and γ-ray spectrometers. Several studies have focused on understanding the various electronic properties of these materials, such as the surface and bulk resistivities and the distribution of the electric field within the crystal. Specifically of interest is how these properties are influenced by a variety of factors including structural heterogeneities, such as secondary phases (SPs) and line defects as well as environmental effects. Herein, we report the bulk current, surface current, electric field distribution and performance of a spectrometer-grade CZT crystal exposed to above band-gap energy illumination.

CdZnTe (CZT) crystals used for nuclear-radiation detectors often contain high concentrations of subgrain boundaries and networks of poligonized dislocations that can significantly degrade the performance of semiconductor devices. These defects exist in all commercial CZT materials, regardless of their growth techniques and their vendor. We describe our new results from examining such detectors using IR transmission microscopy and white X-ray beam diffraction topography. We emphasize the roles on the devices' performances of networks of subgrain boundaries with low dislocation densities, such as poligonized dislocations and mosaic structures. Specifically, we evaluated their effects on the gamma-ray responses of thick, >10 mm, CZT detectors. Our findings set the lower limit on the energy resolution of CZT detectors containing dense networks of subgrain boundaries and walls of dislocations.

CdTe is a promising material for X-ray imaging since it has high stopping power for X-rays. However defects in the material, non ideal charge transport and long range X-ray fluorescence deteriorates the image quality. We have investigated the response of a CdTe sensor with very small pixels using an X-ray microbeam entering the sensor at a small incident angle. Effects of defects as well as depth of interaction can be measured by this method. Both electron and hole collection mode has been tested. The results show distorted electrical field around defects in the material and also shows the small pixel effect. It is also shown that charge summing can be used to get correct spectral information.

Pulse laser was due to investigate characteristics of Schottky-diode type CdTe detector. The pulse laser was irritated to cleaved surface of the CdTe detector to control incident position and time accurately. Even though one-photon energy of the plse laser is different from that of γ-ray mainly detected by the CdTe detectors, amount of pulse energy of the laser can be adjustable to the γ-ray energy by controlling incident power and pulse duration time. Incident position dependence of electric signal through preamplifier was measured and the dependence was changed by charging bias voltage applied to the CdTe detector. This dependence can be explained by distribution of depletion layer at the Schottky interface.

Room temperature In/CdTe/Au diode detectors have been developed with record energy resolution (0.7% FWHM at the 662 keV peak of ¹³⁷Cs) and electrical and detection properties of the detectors have been investigated. The detectors were fabricated using laser-induced doping of a thin surface layer of semi-insulating CdTe crystals with In. High resistivity p-like CdTe(111) crystals pre-coated with an In dopant film were irradiated with nanosecond laser pulses in a water environment that made it possible to introduce and activate In dopant atoms with high concentration and form a steep p-n junction in the surface region of the crystals. Multiple laser irradiation of the samples from the In-coated side increased forward current of the diodes and decreased reverse one. The special surface processing was used, including chemical and thermal procedures, to modify the surface state of the CdTe crystals before deposition of an In dopant film and electrodes. Finally, the room temperature CdTe-based X- and γ-ray radiation detectors with a p-n junction have been obtained with extremely high energy resolution.

Cadmium Zinc Telluride (CZT) is the detector material of choice for the detection of X-rays in the 10 keV-1MeV energy band with excellent spatial and energy resolutions and without cryogenic cooling. In this contribution, we report on recent results of the CZT detector development program and several astrophysical experiments which make use of CZT detectors. In the first part of the paper, we discuss the performance of pixel and cross-strip CZT detectors read out with an ASIC developed at the Brookhaven National Laboratory. Our pixel detectors achieve some of the best energy resolutions reported in the literature. Cross-strip detectors are found to give an inferior performance and we investigate the reason for this performance difference. We also present results from a precision measurement of the effect of a steering grid on multi-pixel events obtained with a 200 micrometer collimator. In the second part of the paper, we describe the design and performance of the hard X-ray polarimeter X-Calibur. The polarimeter uses a 14 cm long scintillator scatterer, surrounded by an assembly of 32 2-5 mm thick CZT detectors. We discuss the sensitivity of the polarimeter to measure the linear polarization of 10 keV-80 keV X-rays on short and long balloon flights and results from testing the polarimeter in the laboratory.

GammaTracker is a handheld radioisotope identification device in development at Pacific Northwest National Laboratory that uses eighteen pixelated Cadmium-Zinc Telluride (CZT) crystals to provide energy resolution approaching that of high-purity germanium without the need for cryogenic cooling. Additionally, these crystals can be used to provide directional and imaging capabilities that cannot be found in other handheld detectors. A significant number of CZT crystals have been procured during the development of the GammaTracker system; the majority of these were procured with the same set of specifications. Each of these detectors has been characterized in terms of key parameters, including current-voltage response and pixel-by-pixel energy resolution. The results of this testing indicate that the overall quality of CZT crystals is improving over time.

Radiation detectors have been fabricated using bulk semi-insulating (SI) 4H-SiC crystals and SI and n-type 4H-SiC epitaxial layers grown by chemical vapor deposition (CVD) on highly doped (0001) 4H-SiC substrates. The devices have been characterized by optical microscopy, current-voltage (I-V) measurements, thermally stimulated current (TSC) spectroscopy (94K - 650 K), Hall effect, van der Pauw measurements, and electron beam induced current (EBIC) technique. Both epitaxial layers exhibited relatively shallow levels related to Al, B, L- and D- centers. Deep level centers in the n-type epitaxial layer peaked at ~ 400 K (E_a ~ 1.1 eV) and ~ 470 K were correlated with IL2 defect and 1.1 eV center in high purity bulk SI 4H-SiC. The SI epitaxial layer exhibited peak at ~ 290 K (E_a = 0.82 - 0.87 eV) that was attributed to IL1 center and 3C inclusions, and at ~ 525 K that was related to intrinsic defects and their complexes with energy levels close to the middle of the band gap. The TSC spectra of the SI epitaxial layer exhibited peaks with different current polarity which we attributed to the built-in electric field reversal. Results of EBIC and optical microscopy characterization showed segregation of threading dislocations around comet tail defects in the n-type epitaxial layers and presence of stacking faults and 3C-SiC inclusions in both epitaxial layers. The I-V characteristics of the devices on SI epi obtained in wide temperature range (94K - 650 K) exhibited steps at ~ 1 V and ~ 70 V corresponding to the ultimate trap filling of deep centers peaked at > 500 K and at ~ 250 K (E_a ~ 0.57 eV), & ~ 300 K (E_a ~ 0.85 eV) respectively. Slow processes of the injected carrier capture on traps resulted in the I-V characteristic with negative differential resistance (NDR). The high temperature resistivity measurements of bulk SI 4H-SiC sample revealed resistivity hysteresis that was attributed to the filling of the deep level electron trap centers.

Phosphors are key components of x-ray framing cameras. On implosion experiments at the National Ignition Facility, the x-ray framing cameras must operate in a harsh neutron induced ionizing radiation. One promising approach of neutron induced background reduction is separation of the neutron background with using difference of x-ray and neutron time-of- flight. To complete x-ray imaging before arrival of the neutron induced radiation to the detector, it is crucial to find a phosphor which has high efficiency and fast decay time. We tested various phosphor materials to optimize design of framing cameras for implosion experiments.

TlBr is a material of interest for use in room temperature gamma ray detector applications due to is wide bandgap 2.7 eV and high average atomic number (Tl 81, Br 35). Researchers have achieved energy resolutions of 1.3 % at 662 keV, demonstrating the potential of this material system. However, these detectors are known to polarize using conventional configurations, limiting their use. Continued improvement of room temperature, high-resolution gamma ray detectors based on TlBr requires further understanding of the degradation mechanisms. While high quality material is a critical starting point for excellent detector performance, we show that the room temperature stability of planar TlBr gamma spectrometers can be significantly enhanced by treatment with both hydrofluoric and hydrochloric acid. By incorporating F or Cl into the surface of TlBr, current instabilities are eliminated and the longer term current of the detectors remains unchanged. ²⁴¹Am spectra are also shown to be more stable for extended periods; detectors have been held at 2000 V/cm for 52 days with less than 10% degradation in peak centroid position. In addition, evidence for the long term degradation mechanism being related to the contact metal is presented.

CR-39 detectors were exposed to DT neutrons generated by a Thermo Fisher model A290 neutron generator. Afterwards, the etched tracks were examined both optically and by scanning electron microscopy (SEM). The purpose of the analysis was to compare the two techniques and to determine whether additional information on track geometry could be obtained by SEM analysis. The use of these techniques to examine triple tracks, diagnostic of ≥9.6 MeV neutrons, observed in CR-39 used in Pd/D co-deposition experiments will also be discussed.

High-energy, gamma-ray calorimetry typically employs large scintillation crystals coupled to photomultiplier tubes. These calorimeters are segmented to the limits associated with the costs of the crystals, photomultiplier tubes, and support electronics. A cost-effective means for construction of a calorimeter system is to use solid-state photomultipliers (SSPM) with front-end electronics, which is at least half the cost, but the SSPM must provide the necessary energy resolution defined by the physics goals. One experiment with plans to exploit this advantage is an upgrade to the PRIMEX experiment at Jefferson Laboratories. We have developed a large-area SSPM (1 cm × 1 cm) for readout of large scintillation crystals. As PbWO₄ has excellent properties (small Molière radius and radiation hard) for high-energy gamma-rays (>1 GeV) but low light yields (~150 photons/MeV at 0 °C), evaluation of the SSPM and support readout electronics with LaBr₃ provides a measure of the device performance. Using the known detection efficiency and dark current of the SSPM, an excess noise factor associated with after pulsing and cross talk is determined. The contribution to the energy resolution from the detector module is calculated as <1% for gamma rays greater than ~2.5 GeV (0.7% at 4.5 GeV).

A fiber-coupled imaging system for monitoring the proton beam profile on the target of the Spallation Neutron Source was developed using reflective, refractive and diffractive optics to focus an image onto a fiber optic imaging bundle. The imaging system monitors the light output from a chromium-doped aluminum oxide (Al₂0₃:Cr) scintillator on the nose of the target. Metal optics are used to relay the image to the lenses that focus the image onto the fiber. The material choices for the lenses and fiber were limited to high-purity fused silica, due to the anticipated radiation dose of 10⁸ R. In the first generation system (which had no diffractive elements), radiation damage to the scintillator on the nose of the target significantly broadened the normally monochromatic (694 nm) spectrum. This created the need for an achromatic design in the second generation system. This was achieved through the addition of a diffractive optic for chromatic correction. An overview of the target imaging system and its performance, with particular emphasis on the design and testing of a hybrid refractive/diffractive high-purity fused silica imaging triplet, is presented.

Exploration in nuclear physics may require extreme conditions, such as temperatures down to a few Kelvin, high magnetic fields of several Tesla, or the small physical dimensions of a few centimeters. As a standard technique for radiation detection using scintillation materials, it is desirable to develop photodetectors that can operate under these harsh conditions. Though photomultiplier tubes (PMTs) have been used for most applications for readout of scintillation materials, they are bulky, highly susceptible to magnetic fields, and present a large heat load in cryogenic environments. Avalanche photodiodes are a reasonable alternative to PMTs in that they are extremely compact and less susceptible to magnetic fields. Avalanche photodiodes have been developed in a commercial CMOS process for operation at temperatures below 100 Kelvin. Here we present the overall operation of the photodiodes at 5 Kelvin. The diodes show a quantum efficiency of at least 30% at 532 nm at 5 Kelvin. At about 30 Kelvin, the diodes exhibit an internal resistive term, which generates a second breakdown point. The prototype diode shows a proportional response to the intensity of light pulses down to 150 detected photons with a hole to electron ionization ratio, k, of 2.3x10^-13 at 5 Kelvin. The properties of the photodiodes and the readout electronics will be discussed for general photon detection below 100 K.

Megavoltage imaging has applications in nondestructive imaging for homeland security, radiotherapy, and industrial manufacturing. Current commercial systems are limited by low image quality as measured by detective quantum efficiency (DQE). These systems yield measured DQE=0.01-0.02, limiting efficacy for detection based on automated signal processing. Past efforts to improve DQE have included novel scintillators and manufacturing of large crystal structures. An alternative novel design for a 2D x-ray imager, based on a modification of existing amorphous silicon (a:Si) or flat-panel imagers, is presented. The panel utilizes a fiber-optic scintillation glass array (FOSGA) consisting of scintillation fibers bundled within a pixilated thick sintered tungsten housing. The tungsten housing is constructed using a lithographic manufacturing technique for high fabrication accuracy. The Tb-doped fibers emit light in the 555-565nm range (matched to the sensitive region of current a:Si photodiodes), with a decay time of 2ms (100-to-40%). Monte Carlo simulations, linear cascaded systems analyses, and film studies have been carried out to validate and optimize image quality for radiation beams in the 1-6MV range. An 8cmx8cm prototype array was fabricated using Tb-doped fibers (9mm length, 0.9mm diameter) loaded into a tungsten matrix (1.1mm pixel pitch, 0.1mm septa), yielding measured DQE=0.05 (vs theoretical DQE=0.07) for 6MV imaging , an order of magnitude improvement in image quality over current commercial imagers. Design parameters of a large field-of-view FOSGA imager for cargo container security imaging are presented: 5cm thick FOSGA array, 0.4-1mm pixel pitch, 50-70% fill factor, DQE>0.2 for 1-6MV range.

Covariance spectroscopy is a method of processing second order moments of data to obtain information that is usually absent from average spectra. In nuclear radiation detection it represents a generalization of nuclear coincidence techniques. Correlations and fluctuations in data encode valuable information about radiation sources, transport media, and detection systems. Gaining access to the extra information can help to untangle complicated spectra, uncover overlapping peaks, accelerate source identification, and even sense directionality. Correlations existing at the source level are particularly valuable since many radioactive isotopes emit correlated gammas and neutrons. Correlations also arise from interactions within detector systems, and from scattering in the environment. In particular, correlations from Compton scattering and pair production within a detector array can be usefully exploited in scenarios where direct measurement of source correlations would be unfeasible. We present a covariance analysis of a few experimental data sets to illustrate the utility of the concept.

Nuclear radiation detection is continuously becoming more important for today's society. Conventional scintillator based gamma-ray detectors use single crystal materials such as NaI:Tl, LaBr₃:Ce, which provide excellent radiation detection properties, but suffer from their environment-related fluctuation, high cost and size limitation. The incorporation of nanophosphors or quantum dots (QD) into a transparent host matrix has been studied recently as a cost-saving alternative that may solve the scalability and stability problems while still providing considerable optical performance. In this work, a new glass based detecting material with promising gamma-ray detection performance is reported. Transparent alumino-silicate glasses containing cerium-doped gadolinium halide nanocrystals were prepared by a melt-quench method followed by annealing to form nanocrystal precipitates. Samples were cast and polished for optical and radiation characterization. The preliminary results indicated a similar gamma-ray detection efficiency compared to a conventional NaI:Tl detector and a gamma-ray peak resolution of ~27% at 662 KeV from some of these samples. By controlling elemental composition and ratio of the in-situ precipitated nanoparticles, radiation detection performance is expected to be improved.

Mobile detection of kilogram quantities of special nuclear materials (SNM) during maritime transportation is a challenging problem for the U.S. Department of Homeland Security. Counting neutrons emitted by the SNM and partitioning them from background neutrons of multiple origins is the most effective passive means of detecting the SNM. Unfortunately, neutron detection, counting, and partitioning in a maritime environment is complex due to the presence of spallation neutrons (commonly known as "ship effect") and to the complicated nature of the neutron scattering in that environment. This work studied the possibilities of building a prototype neutron detector using boron- 10 (¹⁰B) as the converter in a novel form factor called "straws" that would address the above problem by examining multiplicity distributions of neutrons originating from a fissioning source. Currently, commercially manufactured fission meters (FM) are available that separate cosmic neutrons from non-cosmic neutrons and quantitatively determine the strength of a fissioning source; however, these FMs use ³He, which is becoming increasingly difficult to procure; also the size and weight of a commercial FM is not conducive to manual neutron detection operations in a maritime environment. The current project may provide a near-term solution to the crisis that has arisen from the global scarcity of ³He by offering a viable alternative to the FM. The prototype detector provides a large-area, efficient, lightweight, more granular neutron responsive detection surface (to facilitate imaging) to ease the application of the new FMs.

Knowledge of optical properties of scintillators is important both for the optimization of scintillator systems and the development of new materials particularly ceramic scintillators. Recent theoretical developments, especially new density functionals that enable accurate prediction of band gaps, have made it possible to perform quantitative calculations of the optical properties of scintillator materials. We used these techniques to obtain optical properties of a large number of high light output halide scintillators. These calculations showed that many halide scintillators have remarkably little optical anisotropy and may be good candidates for development as ceramic scintillators. These include materials such as CaI₂:Eu²⁺ that have very high light output and other favorable properties but are difficult to develop due to crystal growth issues. We review some of our recent results and present new results for CeCl₃.

Recently discovered scintillators for gamma ray spectroscopy - single-crystal SrI₂(Eu), GYGAG(Ce) transparent ceramic and Bismuth-loaded plastics - offer resolution and fabrication advantages compared to commercial scintillators, such as NaI(Tl) and standard PVT plastic. Energy resolution at 662 keV of 2.7% is obtained with SrI₂(Eu), while 4.5% is obtained with GYGAG(Ce). A new transparent ceramic scintillator for radiographic imaging systems, GLO(Eu), offers high light yield of 70,000 Photons/MeV, high stopping, and low radiation damage. Implementation of single-crystal SrI₂(Eu), Gd-based transparent ceramics, and Bi-loaded plastic scintillators can advance the state-of-the art in ionizing radiation detection systems.

It has been known for more than 50 years that the light emitted by a scintillator under high-energy excitation (gamma, alpha, proton) is not always proportional to the amount of absorbed energy. The deviation from the linearity of luminosity versus absorbed energy is known as nonproportionality. In addition to its academic interest, this phenomenon has been considered central for scintillator development due to its implication in the limitation of achievable detector performance. Although non-proportional response was studied intensively during the second part of the 20th century, the understanding of its origin and implications on scintillator performance are mainly qualitative. Research in the 1960s uncovered a correlation between proportional response and ionization density, while in the 1980s nonproportionality was proposed as the main reason of energy resolution deviation from the counting statistics limit. It is only recently that the bridge between qualitative and quantitative understanding has been crossed, mainly driven by the large effort undertaken to discover new high-resolution scintillators. Developing such detector materials prompted efforts to gain a deeper understanding of the microscopic processes involved in scintillation mechanisms and so in nonproportionality. In this manuscript, the phenomenology of past and present understanding of non-proportional response will be reviewed. Based on recent experimental, computational and theoretical works, the relation between nonlinear response and energy resolution degradation will be addressed. Finally, the relation between material parameters and proportionality will be evaluated. These recent works are leading towards a deeper understanding of nonlinearity in scintillator detector materials and should result in the development of new high performance scintillator materials.

The dipole-dipole and free-carrier Auger quenching processes that are generally regarded to be at the root of nonproportionality depend respectively on the 4th or 6th power of the electron track radius if modeled as cylindrical. In an initial time interval τ when nonlinear quenching and diffusion compete to reduce the density of excited states, the track radius expands as (D_effτ)^1/2 where D_eff is the effective diffusion coefficient for the mixture of excitons and charge carriers. The range of D_eff across semiconductor and scintillator radiation detectors is large, illustrated by 8 decades between mobilities of self-trapped holes in CsI:Tl and holes in high purity Ge. We present the functional form of nonlinear quenching predicted by diffusive track dilution and show that the simple model provides a surprisingly good fit of empirical nonproportionality across a wide range of semiconductor and oxide radiation detectors. We also show how diffusion drives nonlinear branching between excitons and free carriers in the track when electron and hole mobilities are unequal, and that this nonlinear branching coupled with linear trapping on defects can produce the "halide hump" seen in electron yield data for activated halide scintillators. Picosecond time-resolved spectroscopy in alkali halides, as well as quantitative comparison of recently measured 2nd order quenching rate constants K₂ and results of K-dip spectroscopy, provide experimental benchmarks for consideration of carrier thermalization and the initial track or cluster radius r₀ from which (nearly thermalized) diffusion is assumed to commence. The ratio of initial rate of 2nd order quenching to that of dilution by diffusion in a cylindrical track is proportional to K₂/D_eff and does not depend on r₀ in lowest order; however, the absolute rates of both processes decrease with increasing r₀.

Scintillator nonproportionality is a key mechanism by which the energy resolution is degraded in gamma spectroscopy. Herein, we survey the results obtained for a number of inorganic scintillating materials, including alkali and multivalent halides, fluorides, simple oxides, silicates, and a tungstate. The results are interpreted in the context of a model that accounts for carrier attraction by the Onsager mechanism and exciton-exciton annihilation by the Birks model. We then utilize the theory of Landau fluctuations in combination with the fitted experimental nonproportionality curves to deduce the predicted value of the resolution degradation.

The effects of a traveling magnetic field (TMF) on vertical gradient freeze (VGF) growth of cadmium zinc telluride (CZT) are studied using a coupled model of magnetic induction, fluid dynamics, and heat transfer. Simulations are performed to determine the influences of current and frequency on melt flow and growth interface shape. A downward traveling electromagnetic wave drives flow downward at the wall, which tends to flatten the interface, whereas an upward traveling wave has the opposite effect. TMF makes a significant impact on interface shape in the absence of thermal buoyancy, but is ineffectual under realistic conditions in a 4 inch diameter ampoule, for which buoyancy dominates Lorentz force throughout the melt.

CdZnTe, commonly known as CZT is the material of paramount importance for hard X-ray and gamma ray spectroscopy and imaging applications at room temperature. Over the years, the quality of CZT crystals and its charge transport properties has improved significantly making it an attractive detector material especially for homeland security applications. The applications for homeland security demand large and thick detectors to provide a sufficient stopping power for fast detection of high energy gamma photons. In this present report we have grown two inch diameter CZT by Traveling heater method (THM) technique. The as-grown crystals were characterized through photoluminescence (PL) mapping for composition uniformity, growth interface study, Te precipitations/inclusions studies. In order to evaluate our as-grown samples, charge transport characteristics have been studied for thick samples up to ~16mm thick. It has been demonstrated that by controlling the growth interface, visualization of large and thick (~16mm thick) detectors with fairly good response is possible from as-grown CZT detectors grown by THM technique.

Computational models are applied to analyze the segregation of zinc and the shape of the solid-liquid interface during the growth of Cadmium Zinc Telluride (CZT) via an electrodynamic gradient freeze (EDG) process. Validation of the computational model is presented via a comparison between predicted and experimentally measured zinc distributions. Analysis of a large-scale CZT EDG growth system, previously analyzed by Gasperino et al. (JCG 311, 2327-2335, 2009), shows anomalous zinc segregation that features anomalous, non-monotonic axial concentrations and several local minima and maxima radially across the boule. A mechanistic explanation is put forth for this effect based on the evolution of multi-cellular melt flow structures, a particularly common feature of CZT growth. Optimized EDG furnace profiles are presented that promote the growth of CZT crystals with a uniformly convex interface shape. Such interface shapes are expected to improve the single-crystalline yield of this material.

Although cadmium zinc telluride (CZT) is one of leading materials for fabricating room-temperature nuclear-radiation- detectors, different defects in the crystals can degrade the performance of CZT detectors. Post-growth thermal annealing potentially offers a satisfactory way to eliminate the deleterious influence of these defects. Here, we report that the annealing of CZT in Cd vapor effectively lowers the density of Te inclusions. It takes a much longer annealing time to eliminate separate large Te inclusions than small ones; however, the annealing time is greatly reduced when the large Te inclusions are distributed along grain boundaries. We found that sub-grain boundaries still exist after the annealing at 500 °C, indicating that a higher annealing temperature might be needed.

Surface dislocations were revealed in single crystal CdZnTe and CdTe samples using chemical etching. Dislocation etch pits on the Te dominant (111)B face were studied under optical, infrared and scanning electron microscopes. The samples came from crystal ingots which were grown using different chemical compositions and applied growth parameters. From these ingots, etch pit density, shape and distribution were examined and compared with the varying growth techniques. Resistivity and mobility-lifetime product, μτe, properties of the detectors were measured. A combination of detector grade and non-detector grade ingots were tested. Near infra-red microscopy was used to compare the amount of secondary phases present in the bulk of the crystals with dislocation etch pits on their surface.

We have fabricated and characterized cadmium zinc telluride (CZT) Schottky diodes with low reverse leakage current for high resolution radiation detector applications. The diodes were made using Cd0.9Zn0.1Te detector grade crystals grown by the low temperature tellurium solvent method. The diodes were characterized using electron beam induced current (EBIC) technique to investigate crystallographic defects. The EBIC images were correlated with transmission infrared (TIR) images of CZT crystals and the EBIC contrast was attributed to the nonuniformities in spatial distribution of Te. Further characterization by the thermally stimulated current (TSC) spectroscopy revealed shallow and deep level centers with activation energies 0.25- 0.4 eV and 0.65 - 0.8 eV respectively, which we attribute to intrinsic defects associated with excess of Te. Pulse height spectra (PHS) measurements were carried out using a 241Am (59.6 keV) radiation source on the Frisch collar radiation detectors made from the suitable portions of the CZT ingot used for Schottky diode fabrication, and an energy resolution of ~4.2% FWHM was obtained.

The film radiographic inspection remains among the most reliable methods for inspection of pipelines and metalwares. The worldwide trend of transition from film radiography to digital radiography requires creation of advanced digital radiography systems. The main direction of digital radiography systems development is improvement of spatial resolution. The pixel size of the X-ray film is several microns, while solid-state detector pixels have dimensions of several hundred or several thousand microns. For the most common detectors of "scintillator-photodiode" type, it is impossible in principle to obtain the film pixel sizes. However, the detecting ability for substances that differ by their density and atomic number achieved using "scintillator-photodiode" detectors is by several orders higher. Using a standard experiment scheme with dual-energy detector arrays, our aim was to show that it could be possible to substantially increase the accuracy of supervision of the inspected object. For obtaining shadow X-ray images, a receiving-detecting circuit with a detector array was developed for 200 mm scanning field. Using model digital radiographic systems and standard testing we have evaluated spatial resolution (not worse than 1.25 line pairs/mm) and detecting ability (better than 0.2 mm steel wire behind 6 mm steel).

The photon counting CdTe detector by using pulse rise height for high count rate detection was developed. The detector can measure not only pulse rise height for energy estimation but also pulse rise time. The energy spectrum after polarization in high bias voltage and before polarization in low bias voltage was so similar, but the pulse rise time was difference. In this paper, we have compared polarization properties of M-p-n type CdTe diode and Schottky one. We will estimate changing internal electric field by using this measurement.

There is much interest in developing new scintillator detectors for radiation detection and radiographic imaging applications. The knowledge of the electron mobility (μ) is important in the basic understanding of charge transport and in the selection and optimization of many inorganic scintillator materials such as thallium-doped cesium iodide, CsI(Tl). Performance measures are used to model various scintillator responses in an effort to predict the effect of doping concentrations. Performance models will help in the new scintillator design process. Initial tests are done with cadmium zinc telluride detectors to establish measurement techniques and baselines.

Lithium containing A^IB^IIIC^VI semiconductors are being considered as alternative materials for room temperature neutron detection. One of the primary challenges in growing a high quality crystal of such a material is the reactivity of lithium metal. The presence of nitrides, oxides, and a variety of alkali and alkaline earth metal impurities prevent pure synthesis and truncate crystal growth by introducing multiple nucleation centers during growth. Multiple lithium metal purification methods have been investigated which ultimately raised the metal purity to 99.996%. Multi-cycle vacuum distillation removed all but 40 ppm of metal impurities in lithium metal. LiGa(Se/Te)₂ was then synthesized with the high purity lithium metal by a variety of conditions. Lithium metal reacts violently with many standard crucible materials, and thermodynamic studies were undertaken to insure that an appropriate crucible choice was made, with high purity iron and boron nitride crucibles being the least reactive practical materials. Once conditions were optimized for synthesis of the chalcopyrite, vertical Bridgman crystal growth resulted in red crystals. The optical, electronic, and thermodynamic properties were collected.

Pacific Northwest National Laboratory (PNNL) is performing a computational assessment of the impact of several important gamma-ray detector material properties (e.g. energy resolution and intrinsic detection efficiency) on the scenario-specific spectroscopic performance of these materials. The research approach combines 3D radiation transport calculations, detector response modeling, and spectroscopic analysis of simulated energy deposition spectra to map the functional dependence of detection performance on the underlying material properties. This assessment is intended to help guide formulation of performance goals for new detector materials within the context of materials discovery programs, with an emphasis on applications in the threat reduction, nonproliferation, and safeguards/ verification user communities. The research results will also provide guidance to the gamma-ray sensor design community in estimating relative spectroscopic performance merits of candidate materials for novel or notional detectors.

We detailed, by differential thermal analysis (DTA), some features of the melting- and cooling-processes in CdTe-ZnTe with a ZnTe content of up to 10 mol. % by evaluating their influence on the parameters of melting and crystallization. We focused on the heating/cooling rate, the melt's maximum temperature, and the duration of holding the melt at this maximum. Using the interrelationship in the Kissinger equation, ln(V _h/(T_L^mp)²) = A+ E_a/(R T_L^mp), between the heating rate (V _h) and the maximal temperature of the melting process's endothermic effect ( T_L^mp ), we estimated the melting process's activation energy (E_a). In addition, we studied the temperature dependencies of the volume fraction of the solid phase that remained in the CdTe-ZnTe melts, and the influence of the melt's "thermal prehistory" on the parameters of its crystallization parameters. Due to these studies we assessed the temperature intervals of the solid phases' existence and the temperature intervals of melts' crystallization.

Optical properties and morphology of the opposite polar faces of CdTe(111) single crystals subjected to irradiation with nanosecond pulses of the second harmonic of a YAG:Nd laser in a wide range of energy densities have been studied by multiple-angle-of-incidence single-wavelength ellipsometry and atomic force microscopy. The ellipsometric data have been interpreted by the semi-infinite medium model and it is assumed that there are no films on the crystal surface. Also the one-layer model is applied and the crystal relief is considered as a top layer. The refraction n and absorption k indexes of the surface layer are calculated using the fundamental ellipsometric equation and method of thickness curves. Laser irradiation results in an increase in the refractive index of the CdTe surface layer due to laser-induced transformation of the surface structure and it is associated with enriching of the CdTe surface with tellurium. The surface morphology of the crystals is modified at higher laser pulse energy densities because of the beginning of melting of prominent irregularities and thin surface layer. The effects of irradiation with nanosecond laser pulses on the polar surfaces of CdTe(111) crystals are analyzed.

Cadmium Zinc Telluride (CdZnTe/CZT) crystals were grown using a modified vertical Bridgman growth technique with 10 % Zn concentration at Washington State University (WSU). Analyses of the effects of volume (vol.) %, number density (cm^-3), mean diameter (μm) of secondary phases (SPs) and thickness (mm) of the CZT crystals on single crystal properties such as carrier mobility lifetime (μτ_e) and resistivity (ρ) were performed. Some correlations were observed between μτ_e values of different CZT crystals and vol. %, number density, mean diameter of SPs and thickness of the crystals. High μτ_e and lower SP vol. % values were obtained for the ingots grown with rapid cool down times and with no intentional amounts of excess Te/Cd. For the selected samples, the effects of the SPs on the μτ_e values were established for the SPs whose mean diameters were ≤4 μm and >4 μm. These studies indicate vol. % and mean diameter of SPs are the important parameters for CZT crystal performance as a radiation detector.

Thermal neutron detectors in planar configuration were fabricated from LiInSe₂ and B₂Se₃ crystals grown at RMD Inc. All fabricated semiconductor devices were characterized for the current-voltage (I-V) characteristic and neutron counting measurement. Pulse height spectra were collected from ²⁴¹AmBe (neutron source on all samples), as well as ¹³⁷Cs and ⁶⁰Co gamma ray sources. In this study, the resistivity of all crystals is reported and the collected pulse height spectra are presented for fabricated devices. Note that, the ²⁴¹AmBe neutron source was custom designed with polyethylene around the source as the neutron moderator, mainly to thermalize the fast neutrons before reaching the detectors. Both LiInSe₂ and B₂Se₃ devices showed response to thermal neutrons of the ²⁴¹AmBe source.

A new large area imager for X-ray crystallography is described based on active pixel sensor technology. In order to meet the demanding requirements for crystallography the detector is designed with real time correction for reset noise, nonlinearity, spatial uniformity and bad pixels. The design of the detector is described along with its operating characteristics including noise, DQE and quantum gain. We describe new techniques to rapidly calibrate and characterize the X-ray imager.