Search

We are reporting on the latest growth and scintillating properties of a recently discovered intrinsic scintillating compound with the K2PtCl6 simple cubic crystal structure: the Cs-based Cs2HfCl6 (CHC) and Cs2HfCl4Br2 (CHCB), both with the density of 3.9 g/cm3, as well as the Tl-based Tl2HfCl6 (THC) and Tl2ZrCl6 (TZC). This article presents a successfully developed process of growing crack-free, single crystals of 16 mm diameter to 1-in-diameter CHC by the Bridgman method. The energy resolution of 2.8% [full-width at half-maximum (FWHM)] at 662 keV has been obtained for small diameters and typical 3.5% (FWHM) at 662 keV has been obtained for 1 in $\times 1$ in CHC. The light yield of 30 000 ph/MeV and the 3.8- $\mu \text{s}$ primary decay time have been measured for CHC. CHC’s excellent linear response to gamma rays compared to those of NaI:Tl and BGO is also reported. The mixed-halides CHC-based compound CHCB performs with a shorter primary decay time of $1.8~\mu \text{s}$ . Introducing Tl, a heavier element than Cs, increases both Zeff and density of THC and TZC, and improves gamma-ray detection efficiency. Primary decay times are also reduced to about $1~\mu \text{s}$ for THC and $2~\mu \text{s}$ for TZC.

Transparent, crack-free single crystal boules of 1-inch diameter Cs2HfCl6 (CHC) and Cs2HfCl4Br2 (CHCB) have been successfully grown using the vertical Bridgman method. Samples sized dia. 23mm×30mm and dia. 23mm×26mm, respectively, are characterized for their optical and scintillation properties. Energy resolutions of 3.5% and 3.7% (FWHM) at 662 keV as well as light yields of 23,000 ph/MeV and 20,000 ph/MeV have been calculated for CHC and CHCB, respectively. Results comparable to previously reported smaller crystals have been obtained. Studies on decay times, non-proportionality, and detector characterization are also reported. (This work was supported in part by U.S. Department of Energy under Grant #DE-SC0015733, U.S. National Science Foundation under Grant #HRD-1547757, and by U.S. National Aeronautics and Space Administration under Grant #NNX16AK42G).

This paper presents the evaluation of an alternative solution for detectors of dual energy x-ray absorptiometry systems. Bone mineral density (BMD) evaluation tools are crucial to the proper diagnosis of osteoporosis. The commercially available DEXA systems utilize CZT as the detector of the small dose of ionizing radiation passing through the area of interest. Our paper presents a novel Strontium Iodide doped with Europium (SrI2:Eu2+) scintillator crystal coupled to a Silicon Photomultiplier (SiPM) array as a less expensive alternative to CZT detector. Dual energy (60 keV and 122 keV) exposure was used for BMD measurements of a bone phantoms containing CaHPO4 to mimic bone and resin to mimic soft tissue. Calibration curves was determined for both BMD and for thickness measurements, validated by “unknown” phantoms. Both calibration curves lead to decreased errors compared to commercially available systems.

This paper describes the synthesis, crystal growth, detector fabrication, radiation hardening studies, MCNP modeling, and characterization of lithium indium diselenide or LiInSe 2 . This newly-developed room-temperature thermal neutron detector has semiconducting and scintillating properties and it is suitable for neutron detection application. LiInSe 2 was synthesized starting from elemental Li, In, Se in two steps due to high reactivity of Li. A single crystal of LiInSe 2 was grown using the Vertical Bridgman method . The room temperature band gap was found to be 2.8 eV using optical absorption measurements. Bulk resistivity was measured at ∼ 5 × 1011 Ω cm. Photoconductivity measurements of LiInSe 2 wafers identified a peak in the photocurrent around 445 nm. Nuclear radiation detectors were fabricated from single crystal wafer and the responses to alpha particles at various biases were measured. The mobility-lifetime product was estimated. Gamma irradiation studies were performed with calculated absorbed doses ranging from 0.2126 to 21,262 Gy. The characterization of the two wafers for their scintillator performance was conducted after each irradiation. The gamma irradiation produced a reduction of the light yield that translated to a lower channel number for the centroid of alpha detection spectra. It also showed a considerable reduction of the decay time after the first irradiation. These are the first studies on gamma radiation hardening with this material.

This paper reports on successful growth and performance evaluation of two large diameter Cs2HfCl6 (CHC) and Cs2HfCl4Br2 (CHCB), both recently developed scintillator crystals. The discovery of Cs2HfCl6 (CHC) as a scintillator has lately generated much interest in this material and its family, which belongs to the K2PtCl6 cubic crystal structure. CHC is an intrinsic scintillator that is non-hygroscopic (slightly deliquescent, i.e., slightly moisture sensitive but does not deteriorate much when left in air), has no self-radioactivity, provides excellent energy resolution, and has excellent non-proportionality. CHC has a moderate density of 3.9 g/cm3 and an effective atomic number of 58. Reported in this paper are transparent crack-free single crystal CHC and CHCB boules of one inch in diameter, both grown using the vertical Bridgman method. Samples retrieved from the boules, sized ∅23 mm × 30 mm and ∅23 mm × 26 mm, respectively, are characterized for their optical and scintillation properties. Energy resolutions of 3.5% and 3.7% (FWHM) at 662 keV, respectively, are reported. Light yields of 23,000 ph/MeV and 20,000 ph/MeV have been calculated for the large diameter CHC and CHCB crystals, respectively. Results comparable to previously reported results for smaller crystals have been obtained. Studies on decay time, non-proportionality, as well as detector characterization are also reported.

Strontium iodide doped with europium (SrI 2:Eu2+ ) is a new scintillator being developed for use in high-energy astrophysical detectors with excellent energy resolution. Nonproportionality is the primary limiting factor to improving its energy resolution, although the physics of nonproportionality is not yet fully understood. In the past few years, co-dopants have been used to alter nonproportionality. By irradiating a SrI 2:Eu2+ sample with a 2255 Ci 137Cs source, we explore both the crystal's potential for space-based applications in a radiation environment and this new method of altering nonproportionality. At ~6200 Gy irradiation, a drop of 7.8% at 700 nm and a drop of 14.1% at 450 nm were seen in the transmission spectrum. Nonproportionality was also reduced after irradiation, shifting from 87% to 101% of the theoretical light yield at 32.1 keV, while the 4.7 keV peak decreased 40% closer to its theoretical value. We propose a novel method of altering the nonproportionality of scintillators, using radiation-induced F-centers in place of co-dopants.

Osteoporosis is a bone disease characterized by low Bone Mineral Density (BMD) leading to a reduction in bone quality.

Europium-doped TlSr2I5 (TSI), a new high light yield scintillator for gamma-ray detection, has been grown by the vertical Bridgman method. A 12mm and 16mm diameter boules of TSI is grown in a two-zone vertical furnaces. Samples extracted from the grown boule have been characterized for their scintillation properties. Energy resolution of < 3% (FWHM) at 662 keV and a gamma-ray light yield of approximately 54,000 Ph/MeV have been obtained. Decay times of 395 ns (89%) and 2.0 microseconds (11%) have been measured.

The performance of Cd1−x Zn x Te (CZT) materials for room-temperature gamma/x-ray radiation detection continues to improve in terms of material quality and detector design. In our prior publications, we investigated the use of multiple wavelengths of light (in the visible and infrared) to target charge carriers at various trap energies and physical positions throughout crystals. Light exposure significantly alters the charge mobility and improves carrier collection at the anode contact. This study presents an investigation of material performance as a radiation detector during such illumination. The decrease in charge trapping and increase in charge collection due to a higher probability of free electron release from traps contributed to an increase in the resolution-based performance of the detector through controlled illumination. We investigated the performance improvement of CZT crystals with previously known levels of intrinsic defects and secondary phases, at various voltages, light-emitting diode (LED) light wavelengths, and shaping times. Although our setup was clearly not optimized for radiation detector performance, it demonstrated substantial resolution improvements (based on full-width at half-maximum using 662-keV gamma rays from 137Cs upon illumination with 950-nm light) of 16% to 38% in comparison with unilluminated CZT under similar conditions. This manuscript includes discussion of the electrooptic behavior and its effect on performance. Additional testing and fabrication of a detector that incorporates such LED light optimization could lead to improved performance with existing detector-grade materials.

We present a preliminary design for a novel neutron detection system that is compact, lightweight, and low power consuming, utilizing the CubeSat platform making it suitable for space-based applications. This is made possible using the scintillating crystal lithium indium diselenide ( 6 LiInSe 2 ), the first crystal to include 6Li in the crystalline structure, and a silicon avalanche photodiode (Si-APD). The schematics of this instrument are presented as well as the response of the instrument to initial testing under alpha, gamma and neutron radiation. A principal aim of this work is to demonstrate the feasibility of such a neutron detection system within a CubeSat platform. The entire end-to-end system presented here is 10 cm x 10 cm x 15 cm, weighs 670 grams and requires 5 V direct current at 3 Watts.

The re-discovery of Cs2HfCl6 (CHC) as a scintillator has lately generated much interest in this material and its family, which belongs to the K2PtCl6 cubic crystal structure [1] . CHC is an intrinsic scintillator that is non-hygroscopic, has no self-radioactivity, provides good energy resolution, and has good non-proportionality. In this paper we are reporting growth and scintillation properties of two new and Tl-, Hf- and Zr-based compounds of Tl2HfCl6 and Tl2ZrCl6, with physical densities of 5.1 g/cm3 and 4.5 g/cm3, respectively and effective atomic numbers (Zeff) of 71 and 69, respectively. Samples from successfully grown 16 mm diameter boules were cut, processed, and characterized for their scintillation and radiometric properties. Energy resolutions of 3.7% (FWHM) for THC and 3.4% for TZC (FWHM) at 662 keV are measured. Primary luminescence decay times of 1.1 μs and 2.3 μs, for Tl2HfCl6 and Tl2ZrCl6, respectively, as well as good proportionality for both materials have been observed.

Lithium containing semiconductor-grade chalcogenide single crystals were grown using the vertical Bridgman method. The source material was synthesized from elementary precursors in two steps, (i) preparing the metal alloy LiIn or LiGa, and (ii) reaction with chalcogen – Se or Te. In a number of experiments, enriched 6 Li isotope was used for synthesis and growth. The composition and structure of the synthesized materials was verified using powder X-Ray diffraction. The energy band gaps of the crystals were determined using optical absorption measurements. The resistivity of LiInSe 2 and LiGaSe 2 , obtained using current–voltage measurements is on the order of 10 8 –10 11 Ω cm. Photoconductivity measurement of a yellow LiInSe 2 sample showed a peak in the photocurrent around 445 nm. Nuclear radiation detectors were fabricated from single crystal wafers and the responses to alpha particles, neutrons and gammas were measured and presented. It suggests that this material is a promising candidate for neutron detection applications.

Cd0.9Zn0.1Te (CZT) based pixelated radiation detectors have been fabricated and characterized for gamma ray detection. Large area CZT single crystals has been grown using a tellurium solvent method. A 10 ×10 guarded pixelated detector has been fabricated on a ~ 19.5 ×19.5 ×5 mm3 crystal cut out from the grown ingot. The pixel dimensions were 1.3 ×1.3 mm2 and were pitched at 1.8 mm. A guard grid was used to reduce interpixel/inter-electrode leakage. The crystal was characterized in planar configuration using electrical, optical and optoelectronic methods prior to the fabrication of pixelated geometry. Current-voltage (I-V) measurements revealed a leakage current of 27 nA at an operating bias voltage of 1000 V and a resistivity of ~ 3.1 ×1010 Ω-cm. Infrared transmission imaging revealed an average tellurium inclusion/precipitate size less than 8 μm. Pockels measurement has revealed a near-uniform depth-wise distribution of the internal electric field. The mobility-lifetime product in this crystal was calculated to be 6.2 ×10 - 3 cm2/V using alpha ray spectroscopic method. Gamma spectroscopy using a 137Cs source on the pixelated structure showed fully resolved 662 keV gamma peaks for all the pixels, with percentage resolution (FWHM) as high as 1.8%.

We report measurements of luminescence and its ultraviolet excitation spectra in SrI2 and SrI2:Eu2+ at temperatures of 10 and 300 K. Attention is focused on determining the exciton energy and its temperature shift from features of the excitation spectra and limits placed by absorption spectroscopy on a 120 μm thin crystal, on observation of a broadened Eu emission band attributed to trace Eu associated with oxygen in nominally undoped crystals, and on adding observations concerning the 3.4 eV band at low temperature attributed by Pustovarov et al. to the self-trapped exciton.

The variation in bulk resistivity during infrared (IR) illumination above 950 nm of state-of-the-art CdZnTe (CZT) crystals grown using the traveling heating method or the modified Bridgman method is documented. The change in steady-state current with and without illumination is also evaluated. The influence of secondary phases (SP) on current–voltage (I–V) characteristics is discussed using IR transmission microscopy to determine the defect concentration within the crystal bulk. SP present within the CZT are connected to the existence of deep, IR-excitable traps within the bandgap.

Detector grade Cd0.9Zn0.1Te single crystals were grown using a tellurium solvent method. Single crystal blocks of volume ~1 cm3 were prepared for detector fabrication and characterization. The grown crystals were characterized using infra-red transmission imaging and Pockel's effect measurements. Two detectors in single-polarity charge sensing configurations viz., small pixel, and virtual Frisch grid were fabricated on two crystals obtained from the same section of the ingot. Current-voltage measurements performed in planar configuration exhibited a very low leakage current of ~5 nA at 1000 V and resistivities of the order of 8.5×1010 Ω·cm. Electron drift mobilities of the order of 840 cm2/V.s and electron mobility-lifetime products of the order of 2.7×10-3 cm2/V were calculated from alpha spectroscopy using detectors in planar configuration. The small pixel and the virtual Frisch grid detector showed similar energy resolution of 3.7% for 662 keV gamma rays however, the virtual Frisch grid configuration revealed a better overall performance with a peak-to-Compton ratio of 2.8. A digital spectrometer and related software has been developed using a digitizer card and used to employ offline correction schemes to compensate for the charge loss effects, resulting in significant improvement of the 662 keV peak resolution (1.8% as compared to 3.7% without correction) obtained in the case of small pixel detector.

In this study we review the state-of-the-art for double salt iodide scintillators, in particular cesium barium iodide (CBI), cesium calcium iodide (CCI) and barium bromine iodide (BBI), as well as report on their scintillation and optical properties. Double salt iodides inherently have high density and atomic number which translates to good stopping power for energetic particles, in particular gamma rays. Light yields of 54,000 ph/MeV for CBI, 51,000 ph/MeV for CCI, and 46,000 ph/MeV for BBI were measured. A FWHM energy resolution for the 662 keV full absorption peak was observed at 5.7% for CBI, 16.3% for CCI and 3.56% for BBI. The principal scintillation decay timing for CBI was 840 ns, 462 ns for BBI, and two distinct time components of 9 ns and 1900 ns were observed for CCI.

In this article we report the evaluation of thick (up to 10 mm) as-grown Cd0.9Zn0.1Te(CZT):In detectors fabricated from ingots grown by traveling heater method (THM). The crystals were investigated by mapping the electron mobility (μe), (μτ)e and the internal electric field of as-grown CZT samples. The detector performance was also evaluated in co-planar grid geometry. The excellent (μτ)e value and the detector performance of 10 mm thick as-grown detectors is the indication that the THM technique can be employed successfully to fabricate the detectors directly from the as-grown boule at substantially lower cost.

Europium-doped barium and strontium iodide crystals are high light yield scintillator materials with excellent energy resolution. In this communication, BaI2:Eu and SrI2:Eu single crystals with space groups of Pnma ( D 2 h 16 ) and Pbca ( D 2 h 15 ) respectively were grown from the vertical Bridgman method. The crystals were investigated with polarized Raman spectroscopy at temperatures varied from 77 to 300 K. The observed Raman-active modes for each crystal were assigned with the help of group theory analysis. It is found that the absence of the site symmetry leads to splitting of a spectral line in Pnma structure into two lines in the Pbca structure. Structural defects including dark spots formed during crystal growth and new species produced from hydration were characterized.

We report the scintillation properties of both KPb 2 Cl 5 and K 2 CeCl 5 grown by vertical Bridgman method from synthesized and zone refined material. Both crystals exhibit minimal hygroscopic properties, which is desirable in applications where hermitization is not feasible. The KPb 2 Cl 5 crystal, doped with 2.5 mol% of EuCl 2 , appeared yellow in color and did not scintillate, while the self activated transparent K 2 CeCl 5 did. The energy resolution and decay time at 662 keV were measured to be 6.1% and 78 ns respectively, with a light yield of 31,000 photons/MeV.

This paper reports on the charge carrier properties of several lithium indium diselenide (LISe) semiconductors. It was found that the charge collection efficiency of LISe was improved after high flux thermal neutron irradiation including the presence of a typically unobservable alpha peak from hole-only collection. Charge carrier trap energies of the irradiated sample were measured using photo-induced current transient spectroscopy. Compared to previous studies of this material, no significant differences in trap energies were observed. Through trap-filled limited voltage measurements, neutron irradiation was found to increase the density of trap states within the bulk of the semiconductor, which created a polarization effect under alpha exposure but not neutron exposure. Further, the charge collection efficiency of the irradiated sample was higher (14–15 fC) than that of alpha particles (3–5 fC), indicating that an increase in hole signal contribution resulted from the neutron irradiation. Finally, it was observed that significant charge loss takes place near the point of generation, producing a significant scintillation response and artificially inflating the W-value of all semiconducting LISe crystals.This paper reports on the charge carrier properties of several lithium indium diselenide (LISe) semiconductors. It was found that the charge collection efficiency of LISe was improved after high flux thermal neutron irradiation including the presence of a typically unobservable alpha peak from hole-only collection. Charge carrier trap energies of the irradiated sample were measured using photo-induced current transient spectroscopy. Compared to previous studies of this material, no significant differences in trap energies were observed. Through trap-filled limited voltage measurements, neutron irradiation was found to increase the density of trap states within the bulk of the semiconductor, which created a polarization effect under alpha exposure but not neutron exposure. Further, the charge collection efficiency of the irradiated sample was higher (14–15 fC) than that of alpha particles (3–5 fC), indicating that an increase in hole signal contribution resulted from the neutron irradiation. Finally, it wa...

The production of high-quality ternary single-crystal materials for radiation detectors has progressed over the past 15 years. One of the more common materials being studied is CdZnTe (CZT), which can be grown using several methods to produce detector-grade materials. The work presented herein examines the effects of environmental conditions including temperature and humidity on detector performance [full-width at half-maximum (FWHM)] using the single pixel with guard detector configuration. The effects of electrical probe placement, reproducibility, and aging are also presented.

This work focuses on the 3. Resultsanddiscussioncharacterization of 10×10×10 mm 3 THM-grown CdZnTe detector-grade crystals that have been post-growth annealed to remove the secondary phases (SPs). All three detectors showed an average energy resolution of ∼1.63% for a small guarded pixel with 3.5 mm diameter, measured using 137 Cs—662 keV with an average peak-to-Compton ratio of 2.7. The characterization showed vestiges of SPs and micro-twins present in some of the crystals indicating that the SPs prior to annealing were large and had size in the range of 100–500 μm. The various detectable structural features, such as micron twins, strains and sub-micron level of Te inclusions seemed to have little or no influence in the radiation spectrometer performance of the detectors; this is possibly because they are either having low density or electrically inactive.

Synthetic Cd1–xZnxTe or “CZT” crystals are highly suitable for γ-spectrometers operating at room temperature. Secondary phases (SP) within CZT, presumed to be Te metal, have detrimental impacts on the charge collection efficiency of fabricated device. Using analytical techniques rather than arbitrary theoretical definitions, we identify two SP morphologies: (i) many void, 20-μm “negative” crystals with 65-nm nanoparticle residues of Si, Cd, Zn, and Te and (ii) 20-μm hexagonal-shaped bodies, which are composites of metallic Te layers with cores of amorphous and polycrystalline CZT material that surround the voids.

Raman laser studies of detector-grade CdZnTe crystals show an increase in intensity of the Te peaks of the Raman spectra even at very low laser powers. In this study, atomic force microscopy (AFM) was used to characterize the extent of damage to the CdZnTe crystal surface following exposure to the Raman laser. AFM images revealed localized surface damage in the areas exposed to the Raman laser beam. Additional studies using conductive-probe AFM techniques provided localized electrical information for the laser-induced Te-rich areas.

CdZnTe (CZT) crystals can be grown under controlled conditions to produce high-quality crystals to be used as room-temperature radiation detectors. Even the best crystal growth methods result in defects, such as tellurium secondary phases, that affect the crystal's performance. In this study, CZT crystals were analyzed by micro-Raman spectroscopy. The growth of Te rich areas on the surface was induced by low-power lasers. The growth was observed versus time with low-power Raman scattering and was observed immediately under higher-power conditions. The detector response was also measured after induced Te enrichment.

Synthetic CdZnTe (CZT) semiconducting crystals are highly suitable for the room temperature-based detection of gamma radiation. The surface preparation of Au contacts on surfaces of CZT detectors is typically conducted after (1) polishing to remove artifacts from crystal sectioning and (2) chemical etching, which removes residual mechanical surface damage however etching results in a Te rich surface layer that is prone to oxidize. Our studies show that CZT surfaces that are only polished (as opposed to polished and etched) can be contacted with Au and will yield lower surface currents. Due to their decreased dark currents, these as-polished surfaces can be used in the fabrication of gamma detectors exhibiting a higher performance than polished and etched surfaces with relatively less peak tailing and greater energy resolution.

We present a preliminary design for a novel neutron detection system that is compact, lightweight, and low power consuming, utilizing the CubeSat platform making it suitable for space-based applications. This is made possible using the scintillating crystal lithium indium diselenide ($^6$LiInSe$_2$), the first crystal to include $^6$Li in the crystalline structure, and a silicon avalanche photodiode (Si-APD). The schematics of this instrument are presented as well as the response of the instrument to initial testing under alpha radiation. A principal aim of this work is to demonstrate the feasibility of such a neutron detection system within a CubeSat platform. The entire end-to-end system presented here is 10 cm x 10 cm x 15 cm, weighs 670 grams and requires 5 Volts direct current at 3 Watts., Comment: 14 pages, 6 figures

We have employed photoluminescence mapping at room temperature of detector grade CZT grown by modified Traveling Heater Method (THM) technique and estimated the spatial variation of Zn composition. The composition was found to be fairly uniform over the major portion of the ingot length except near the conical part of the ingot and the upper part of the ingot. Unlike Bridgman growth technique, no detectable striations were observed. The composition was within ±0.3% over the length little more than 4 cm of the ingot. The growth interface revealed from the composition mapping was found to be reasonably flat.

The change in bulk resistivity of CdZnTe (CZT) crystals was measured during infrared (IR) light between 950 and 1000 nm. The crystals are grown using one of the state-of-the-art methods either the traveling heating method or the modified Bridgman method. The change resistivity was evaluated using the steady-state current with and without light. Additionally, the change in current with both IR sources were correlated to the influence of secondary phases (SP) in each crystal using IR transmission microscopy to determine whether the number and size of the impurities has a drastic effect based on the current-voltage (IV) characteristics. SP at various depths within CZT are connected to the existence of variable depth, IR-excitable traps that lie within the bandgap. The release of these traps will significantly affect the overall current in the system. However, the current increase may not match the overall energy of the light utilized are more dependent on the size and quantity for each energy range.

Cd 0.9 Zn 0.1 Te (CZT) gamma radiation detectors are presently among the best semiconductor materials for room temperature operation. CZT crystals in pixilated geometry can be efficiently used in the field of Homeland security and medical diagnosis for high resolution imaging purpose and position sensitive measurements using x-rays and high energy gamma rays. Large area CZT single crystals has been grown using a tellurium solvent method. A 10×10 guarded pixilated detector has been fabricated on a ∼19.5×19.5×5 mm3 crystal cut out from the grown ingot. A full area planar Au contact was used as the cathode. Each pixel of the pixilated detector was 1.3×1.3 mm2 in area and pitched at 1.8 mm. A 100 μm wide guard grid was used to reduce inter-pixel leakage. The crystal was characterized in planar configuration using electrical and optoelectronic methods prior to the fabrication of pixilated geometry. Current-voltage (I–V) measurements revealed a leakage current of 29 nA at an operating bias voltage of 1100 V. A resistivity of ∼3.1×1010 ∖.cm was calculated from the I–V measurements. Infrared transmission imaging revealed an average tellurium inclusion/precipitate size less than 8μm. Pockels measurement has revealed a near-uniform depth-wise distribution of the internal electric field. The mobility-lifetime product in this crystal was calculated to be 6.2 × 10−3 cm2/V using alpha ray spectroscopic method. Gamma spectroscopy using a 137Cs source on the pixelated structure showed fully resolved 662 keV gamma peaks for all the pixels, with percentage resolution (FWHM) as low as 1.8 %.

Models of nonproportional response in scintillators have highlighted the importance of parameters such as branching ratios, carrier thermalization times, diffusion, kinetic order of quenching, associated rate constants, and radius of the electron track. For example, the fraction ηeh of excitations that are free carriers versus excitons was shown by Payne and coworkers to have strong correlation with the shape of electron energy response curves from Compton-coincidence studies. Rate constants for nonlinear quenching are implicit in almost all models of nonproportionality, and some assumption about track radius must invariably be made if one is to relate linear energy deposition dE/dx to volume-based excitation density n (eh/cm3) in terms of which the rates are defined. Diffusion, affecting time-dependent track radius and thus density of excitations, has been implicated as an important factor in nonlinear light yield. Several groups have recently highlighted diffusion of hot electrons in addition to thermalized carriers and excitons in scintillators. However, experimental determination of many of these parameters in the insulating crystals used as scintillators has seemed difficult. Subpicosecond laser techniques including interband z scan light yield, fluence-dependent decay time, and transient optical absorption are now yielding experimental values for some of the missing rates and ratios needed for modeling scintillator response. First principles calculations and Monte Carlo simulations can fill in additional parameters still unavailable from experiment. As a result, quantitative modeling of scintillator electron energy response from independently determined material parameters is becoming possible on an increasingly firmer data base. This paper describes recent laser experiments, calculations, and numerical modeling of scintillator response.

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, 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.

Synthetic CdZnTe or “CZT” crystals are highly suitable for γ-spectrometers operating at the room temperature. Secondary phases (SP) in CZT are known to inhibit detector performance, particularly when they are present in large numbers or dimensions. These SP may exist as voids or composites of non-cubic phase metallic Te layers with bodies of polycrystalline and amorphous CZT material and voids. Defects associated with crystal twining may also influence detector performance in CZT. Using transmission electron microscopy, we identify two types of defects that are on the nano scale. The first defect consists of 40 nm diameter metallic Pd/Te bodies on the grain boundaries of Te-rich composites. Although the nano-Pd/Te bodies around these composites may be unique to the growth source of this CZT material, noble metal impurities like these may contribute to SP formation in CZT. The second defect type consists of atom-scale grain boundary dislocations. Specifically, these involve inclined “finite-sized” planar defects or interfaces between layers of atoms that are associated with twins. Finite-sized twins may be responsible for the subtle but observable striations that can be seen with optical birefringence imaging and synchrotron X-ray topographic imaging.

Europium-doped strontium iodide (SrI 2 :Eu) crystals are reported to be high light yield scintillator materials with good energy resolution, the best energy resolution of SrI 2 :Eu at 662 keV having reached 2.6%. To improve the crystals growth quality and to protect the crystals from hydration, it is practical to distinguish the pure crystals from impurity and structural defects by Raman spectroscopy. In this report, SrI 2 :Eu crystals grown from the vertical Bridgman method have been characterized with the polarized Raman spectroscopy. Orientations of the crystals were determined with the spectroscopy assisted with the polarized optical microscopy. The crystals are highly hygroscopic, the intensity of new peak at 87 cm−1 associated with SrI 3 − anions continues to increase with time even the samples are kept in mineral oil or encapsulated in parylene. Micro-Raman mapping on the distributions of SrI 3 − anions in SrI 2 crystals stored in mineral oil are reported.

We report on the continued development and testing of unique types of Cadmium Zinc Telluride (CZT) detectors.Using large volume (10 × 20 × 20 mm 3 ) CZT crystals, we contact variou s dual anodeŽ detector designs. Weincorporate a segmented cathode with “ve regions so that the charge on all seven contacts can be used todetermine the energy and the 3-D interaction location of detected X-ray and gamma-ray photons. We describethe status of the detector development program, emphasize strengths and weaknesses of the dierent contactcon“gurations, and discuss possible applications of Dual Anode Detectors in radiation detection applications.Keywords: CZT, X-ray Detector , Contact Geometry 1. INTRODUCTION The high average atomic number ( 50) and room temperature operation of Cadmium Zinc Telluride (CZT)detectors make them desirable for Homeland Security a pplications where high eciency of detecting nuclearartifacts is required, such as a stand o detector at a border crossing. We currently are working to developlarge-volume CZT detectors with few re adout channels by exploring novel contact designs. Such designs presentcertain advantages while avoiding the complications, limitations, and requirements of using the standard contactpatterns.The most common CZT detector designs in use today employ either pixel contacts or a coplanar grid (CPG)for the anode.

CdZnTe (or CZT) crystals can be used in a variety of detector-type applications. This large band gap material shows great promise for use as a gamma radiation spectrometer. Historically, the performance of CZT has typically been adversely affected by point defects, structural and compositional heterogeneities within the crystals, such as twinning, pipes, grain boundaries (polycrystallinity) and secondary phases (SP). The synthesis of CZT material has improved greatly with the primary performance limitation being attributed to mainly SP. In this presentation, we describe the extensive characterization of detector grade material that has been treated with post growth annealing to remove the SPs. Some of the analytical methods used in this study included polarized, cross polarized and transmission IR imaging, I-V curves measurements, synchrotron X-ray topography and electron microscopy.

Semi-conducting CdZnTe (or CZT) crystals can be used in a variety of detector-type applications. CZT shows great promise for use as a gamma radiation spectrometer. However, its performance is adversely affected by point defects, structural and compositional heterogeneities within the crystals, such as twinning, pipes, grain boundaries (polycrystallinity), secondary phases and in some cases, damage caused by external forces. One example is damage that occurs during characterization of the surface by a laser during Raman spectroscopy. Even minimal laser power can cause Te enriched areas on the surface to appear. The Raman spectra resulting from measurements at moderate intensity laser power show large increases in peak intensity that is attributed to Te. Atomic Force Microscopy (AFM) was used to characterize the extent of damage to the CZT crystal surface following exposure to the Raman laser. AFM data reveal localized surface damage in the areas exposed to the Raman laser beam. The degree of surface damage to the crystal is dependent on the laser power, with the most observable damage occurring at high laser power. Moreover, intensity increases in the Te peaks of the Raman spectra are observed even at low laser power with little to no visible damage observed by AFM. AFM results also suggest that exposure to the same amount of laser power yields different amounts of surface damage depending on whether the exposed surface is the Te terminating face or the Cd terminating face of CZT.

6LiInSe2 is a promising thermal neutron semiconductor detector material. The performance of the detector is affected by the carrier mobility-lifetime products. Therefore, defects that function as carrier recombination centers need to be identified. In this letter, characterization of defect levels in 6LiInSe2 by photo-induced current transient spectroscopy (PICTS) and photoluminescence is reported. PICTS measurements revealed electron-related defects located at 0.22, 0.36, and 0.55 eV and hole-related defects at 0.19, 0.30, and 0.73 eV. Free exciton and donor-acceptor pairs (DAP) emissions were observed. The PICTS defect level values are consistent with those extracted from DAP transitions.

The distribution of the internal electric field in Cd1−xZnxTe (CZT) materials has significant effects on the charge collection ability. Light exposure at various wavelengths is a relatively unexplored process that alters charge collection at the anode contact. The use of multiple wavelengths can target charge carriers at various trap energies and positions throughout the crystal. The controlled illumination increases charge collection by releasing trapped electron and hole carriers in the crystal despite differences in light energy. Our study presents the results from our investigation of the effect of external illumination of CZT on the internal electric field via the Pockels effect. The space charge collection is further analyzed based on location and intensity relative to the specific wavelength of illumination.

We report variations in the currents of CdZnTe semiconductor crystals during exposure to a series of light emitting diodes of various wavelengths ranging from 470 to 950 nm. The changes in the steady-state current of one CdZnTe crystal with and without illumination along with the time dependence of the illumination effects are discussed. Analysis of the de-trapping and transient bulk currents during and after optical excitation yield insight into the behaviour of charge traps within the crystal. Similar behaviour is observed for illumination of a second CdZnTe crystal suggesting that the overall illumination effects are not crystal dependent.

Post-growth manipulation of the internal electric field in CdZnTe crystals using sub-bandgap illumination is measured as a function of temperature through infrared (IR) transmission measurements. Using near sub-bandgap IR illumination, both the optical de-trapping of charge carriers and the reduction in carrier recombination increased the mobility lifetime in the crystal. The increased carrier transport is a direct result of decreased hole and electron trapping in addition to other underlying mechanisms. Concentration of the electric field near the cathode is also observed. We measured the electric field distribution with sub-bandgap illumination as a function of temperature via the Pockels effect.

The Pockels electro-optic effect can be used to investigate the internal electric field in cadmium zinc telluride (CZT) single crystals that are used to fabricate room temperature x and gamma radiation detectors. An agreement is found between the electric field mapping obtained from Pockels effect images and the measurements of charge transients generated by alpha particles. The Pockels effect images of a CZT detector along two mutually perpendicular directions are used to optimize the detector response in a dual anode configuration, a device in which the symmetry of the internal electric field with respect to the anode strips is of critical importance. The Pockels effect is also used to map the electric field in a CZT detector with dual anodes and an attempt is made to find a correlation with the simulated electric potential in such detectors. Finally, the stress-induced birefringence effects seen in the Pockels images are presented and discussed.