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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.

We report on the preparation, structure and scintillation properties of single crystal cesium hafnium chloride bromide (Cs2HfCl4Br2 or CHCB). The crystals were grown using a vertical Bridgman technique process. Single crystal X-ray diffraction showed that the as grown CHCB crystal structure was isostructural to Cs2HfCl6 crystals exhibiting the cubic K2PtCl6 –type crystal structure. The Cs2HfCl6−x - Cs2HfBrx system is a solid solution with the lattice parameter of Cs2HfCl6−xBrx increasing linearly with x in the range 0

Professor Jurchescu, Dr. Wolszczak, Professor Burger, Dr. Bourret-Courchesne, and Professor Dorenbos introduce the Journal of Materials Chemistry C–Materials Advances themed collection in memoriam of Prof. Richard T. Williams.

A scintillating lithium indium diselenide (LISe) single crystal has recently been found to exhibit an eight-fold increase in its apparent light yield within 80 µm across the surface when excited by cold neutrons. The cause of this observation is currently unknown. In this paper, we report on our investigations to explain the previous observations of nonuniformity in scintillating LISe. Using a two-photon interband excitation instrument, we found that the lower and higher light yield regions “scintillate” at peak emission energies of 1.98 eV and 2.4 eV, respectively, and no region investigated exhibited both. Volumetric evaluation found a relatively uniform scintillation response in each region, verifying that the relatively flat spatial resolution of LISe substrates of varying thickness applied to neutron imaging is not from only near-surface scintillation mechanisms. Raman investigations indicate that the vibrational modes of the scintillating LISe substrate is somewhat different than that reported in literature, but no apparent difference between the two regions was observed. Using ToF-SIMS in each region, we found that the 1.98 eV emission region contains additional lithium, which is consistent with neutron absorption results. We suspect that the sharp transition in peak scintillation emission is due to a change in the charge state of Li-In antisites, but that the observed scintillation light yield and emission energy change is not directly correlated to crystal color. Qualitative comparison between the observed noise in the two-photon “scintillation” spectra indicates that the light yield between each region is not due to the quantum efficiency of the CCD used during cold neutron interrogation experiments, but an actual change in the light yield.

Gamma-ray detection and spectroscopy is the quantitative determination of their energy spectra, and is of critical value and critically important in diverse technological and scientific fields. Here we report an improved melt growth method for cesium lead bromide and a special detector design with asymmetrical metal electrode configuration that leads to a high performance at room temperature. As-grown centimeter-sized crystals possess extremely low impurity levels (below 10 p.p.m. for total 69 elements) and detectors achieve 3.9% energy resolution for 122 keV 57Co gamma-ray and 3.8% for 662 keV 137Cs gamma-ray. Cesium lead bromide is unique among all gamma-ray detection materials in that its hole transport properties are responsible for the high performance. The superior mobility-lifetime product for holes (1.34 × 10−3 cm2 V−1) derives mainly from the record long hole carrier lifetime (over 25 μs). The easily scalable crystal growth and high-energy resolution, highlight cesium lead bromide as an exceptional next generation material for room temperature radiation detection., Detection and spectroscopic measurements of gamma-ray used to rely on expensive materials such as CdZnTe crystals. Here He et al. develop a melt method to grow large size CsPbBr3 perovskite crystals and the devices achieve low cost, high energy resolving capabilities and stability.

Spectroscopic results on the infrared emission properties of trivalent holmium (Ho3+) doped potassium lanthanum chloride (K2LaCl5) are presented. Using ~900 nm excitation, Ho3+ doped K2LaCl5, which has a maximum phonon energy of 235 cm−1, exhibited the infrared emissions at ~1660, ~1995, and ~3900 nm at room temperature. The mid-infrared emission at 3900 nm originated from the 5I5 → 5I6 transition of Ho3+ ions with an exponential decay time of ~7.8 ms at room temperature. The fluorescence lifetimes were almost independent of temperature, indicating a negligibly small non-radiative decay rate for the 5I5 excited state, as predicted by the energy gap law for low phonon energy hosts. The Stark level energies of the lower 5IJ manifolds in Ho:K2LaCl5 were resolved from spectroscopic results performed at cryogenic temperatures. The transition line-strengths, radiative lifetimes, and fluorescence branching ratios were investigated by using the Judd-Ofelt method. In addition, the peak emission cross-section of the 5I5 → 5I6 transition was calculated to be 1.8 × 10–20 cm2 at ~3890 nm.

The crystal growth and behavior of a high-performance, non-hygroscopic scintillator, Cs 2 HfCl 6 (CHC) and its variant, Cs 2 HfCl 4 Br 2 (CHCB) are presented in this work. CHC and CHCB were prepared by melt compounding of sublimed HfCl 4 with CsCl and CsBr to produce material for Bridgman growth. The resulting 1 cm dia. and 1 in. dia. CHC crystals showed in certain parts evidence of CsCl as a secondary phase—a result of a non-stoichiometric (CsCl-rich) melt composition caused by the high vapor pressure of HfCl 4 during compounding. This was verified using micro X-ray fluorescence spectrometry. There is also evidence of a secondary phase in CHCB, which has been presented for the first time. A clear CHC sample had a light yield and energy resolution of 30,000 ph/MeV and 3.3%, respectively, and decay components of 0.39 and 3.9 µs. A sample of CHCB with a secondary phase present in the core had a light yield and energy resolution of 18,600 ph/MeV and 4.4%, and with decay components of 0.38 and 2.0 µs for CHCB. Both crystals showed minimal moisture sensitivity.

An array of lithium indium diselenide (LISe) scintillators were investigated for application in neutron imaging. The sensors, varying in thickness and surface roughness, were tested using both reflective and anti-reflective mounting to an aluminum window. The spatial resolution of each LISe scintillator was calculated using the knife-edge test and a modulation transfer function analysis. It was found that the anti-reflective backing case yielded higher spatial resolutions by up to a factor of two over the reflective backing case despite a reduction in measured light yield by an average of 1.97. In most cases, the use of an anti-reflective backing resulted in a higher spatial resolution than the 50 μ m -thick ZnS(Cu): 6 LiF comparison scintillation screen. The effect of surface roughness was not directly correlated to measured light yield or observed spatial resolution, but weighting the reflective backing case by the random surface roughness revealed that a linear relationship exists between the fractional change (RB/ARB) of the two. Finally, the LISe scintillator array was used in neutron computed tomography to investigate the features of halyomorpha halys with the reflective and anti-reflective backing.

The internal contamination (radio-purity) of a cesium hafnium chloride (Cs 2 HfCl 6 ) crystal scintillator and its starting material (CsCl, HfCl 4 initial and after 3-fold static sublimation purification) was studied by HR-ICP-MS and with an ultra-low-background high purity germanium (ULB-HPGe) detector deep underground. This study was conducted for the purpose of assessing the feasibility of using CHC as a potential detector to the search for rare α decay occurring in Hafnium isotopes. Despite the low radio-purity of starting material, the resulting analysis concluded that the grown CHC crystal is free of nuclides of the U/Th natural decay chain. Only a limit was set on their activities to a few mBq/kg. However, the crystal was found to contain artificial and cosmogenic radionuclides: 137 Cs at 0.8 Bq/kg, 132 Cs at 25 mBq/kg, 134 Cs at 52 mBq/kg and 181 Hf at 14 mBq/kg.

The results of investigation into cesium hafnium chloride (Cs 2 HfCl 6 ) scintillating crystals as a promising detector to search for rare nuclear processes occurring in Hf isotopes is reported. The light output, quenching factor, and pulse-shape characteristics have been investigated at room temperature. The scintillation response of the crystal induced by α -particles and γ -quanta were studied to determine possibility of particle discrimination. Using the optimal filter method we obtained clear separation between signals with a factor of merit (FOM) = 9.3. This indicates that we are able to fully separate signals originating from α -particles and γ -quanta. Similar fruitful discrimination power was obtained by applying the mean time method (FOM = 7) and charge integration method (FOM = 7.5). The quenching factor for collimated 4 MeV α -particles is found to be 0.36, showing that α -particles generate more than a third of the light compared to γ -quanta at the same energy.

We report the generation of La2Zr2O7:5%Eu3+@Y2O3 (LZO5E@YO) core@shell crystalline inorganic-inorganic heterogeneous nanoparticles (NPs). The Y2O3 (YO) shell coating process based on a chemical sol-gel method led to the growth of a thin YO shell on the ordered pyrochlore La2Zr2O7:5%Eu3+ (LZO5E) core NPs. Photoluminescence (PL) analyses demonstrated a blue shift of 15 nm on charge transfer (CT) excitation band of the core@shell NPs from that of the core NPs. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) along x-ray diffraction (XRD), Fourier-transform Infrared spectroscopy (FTIR) and x-ray photoelectron spectroscopy (XPS) studies confirmed the formation of the thin YO layer over the LZO5E core NPs. The PL intensity of the LZO5E@YO core@shell NPs was enhanced by three fold compared to that of the LZO5E core NPs, and higher quantum yield (QY) was observed for the former compared to the original NPs by more than 70%. Higher radioluminescence (RL) emission was also observed for the core@shell NPs compared to the core NPs. Our ability of obtaining near-perfect core@shell heterostructure with enhanced luminescence performance opens the door for the development of efficient La2Zr2O7:5%Eu3+@Y2O3 NPs for both optical and x-ray scintillation applications.

Lithium containing chalcogenide single crystals have become very promising materials for photonics and radiation detection. Detection applications include nuclear nonproliferation, neutron science, and stellar investigations for the search of life. Synthesis and single crystal growth methods for lithium containing chalcogenide, specifically LiIn 1− x Ga x Se 2 , single crystals are discussed. This study elucidates the possibility of improving neutron detection by reducing the indium capture contribution; with the incorporation of the lithium-6 isotope, gallium substitution may overcome the neutron detection efficiency limitation of 6 LiInSe 2 due to appreciable neutron capture by the indium-115 isotope. As a figure of merit, the ternary parent compounds 6 LiInSe 2 and 6 LiGaSe 2 were included in this study. Quality crystals can be obtained utilizing the vertical Bridgman method to produce quaternary compounds with tunable optical properties. Quaternary crystals of varying quality depending on the gallium concentration, approximately 5×5×2 mm 3 or larger in volume, were harvested, analyzed and revealed tunable absorption characteristics between 2.8–3.4 eV.

Strontium iodide (SrI 2 ), an important new scintillator crystal having a high light yield and excellent energy resolution, was grown for the first time by the edge-defined film-fed (EFG) growth method. Using high purity starting materials and floating dies made of graphite, fused quartz or AlN, large cylindrical, planar or square cross-section single crystals (12–15 mm across and >7 cm long) were produced at growth rates up to 15 mm/h, significantly faster than the current Bridgman growth technology. Details on the equipment used to grow this deliquescent material and on its growth behavior are given along with some discussion of crystalline quality.

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.

Semiconducting lithium indium diselenide, 6 LiInSe 2 or LISe, has promising characteristics for neutron detection applications. The 95% isotopic enrichment of 6 Li results in a highly efficient thermal neutron-sensitive material. In this study, we report on a proof-of-principle investigation of a semiconducting LISe pixel detector to demonstrate its potential as an efficient neutron imager. The LISe pixel detector had a 4×4 of pixels with a 550 µm pitch on a 5×5×0.56 mm 3 LISe substrate. An experimentally verified spatial resolution of 300 µm was observed utilizing a super-sampling technique.

Lithium indium diselenide, 6 LiInSe 2 or LISe, is a newly developed neutron detection material that shows both semiconducting and scintillating properties. This paper reports on the performance of scintillating LISe crystals for its potential use as a converter screen for cold neutron imaging. The spatial resolution of LISe, determined using a 10% threshold of the Modulation Transfer Function (MTF), was found to not scale linearly with thickness. Crystals having a thickness of 450 µm or larger resulted in an average spatial resolution of 67 µm, and the thinner crystals exhibited an increase in spatial resolution down to the Nyquist frequency of the CCD. The highest measured spatial resolution of 198 µm thick LISe (27 µm) outperforms a commercial 50 µm thick ZnS(Cu): 6 LiF scintillation screen by more than a factor of three. For the LISe dimensions considered in this study, it was found that the light yield of LISe did not scale with its thickness. However, absorption measurements indicate that the 6 Li concentration is uniform and the neutron absorption efficiency of LISe as a function of thickness follows general nuclear theory. This suggests that the differences in apparent brightness observed for the LISe samples investigated may be due to a combination of secondary charged particle escape, scintillation light transport in the bulk and across the LISe-air interface, and variations in the activation of the scintillation mechanism. Finally, it was found that the presence of 115 In and its long-lived 116 In activation product did not result in ghosting (memory of past neutron exposure), demonstrating potential of LISe for imaging transient systems.

Cerium doped Barium Fluoride (BaF 2 Ce) transparent ceramic was fabricated and its luminescence and scintillation properties were studied. The photoluminescence shows the emission peaks at 310 nm and 323 nm and is related to the 5d-4f transitions in Ce 3+ ion. Photo peak at 511 keV and 1274 keV were obtained with BaF 2 Ce transparent ceramic for Na-22 radioisotopes. Energy resolution of 13.5% at 662 keV is calculated for the BaF 2 Ce transparent ceramic. Light yield of 5100 photons/MeV was recorded for BaF 2 Ce(0.2%) ceramic and is comparable to its single crystal counterpart. Scintillation decay time measurements shows fast component of 58 ns and a relatively slow component of 434 ns under 662 keV gamma excitation. The slower component in BaF 2 Ce(0.2%) ceramic is about 200 ns faster than the STE emission in BaF 2 host and is associated with the dipole-dipole energy transfer from the host matrix to Ce 3+ luminescence center.

La 0.2 Y 1.8 O 3 is a new and promising scintillator which is based on the host material without doping. Here the time gated X-ray excited optical luminescence is measured by using the excitation energy below, above and at the La L 3 -edge. A relatively slow and broad emission with peak at 415 nm has been observed as the dominant emission. Also, a weak emission at 360 nm is observed at the fast window, associated with the recombination of trapped excitons in Y 2 O 3 host. The observations show that the broad emission at 415 nm is most likely due to the recombination of trapped excitons associated with the La 3+ doping into Y 2 O 3 sites.

Lithium based chalcogenides have many uses in the nuclear science field. 6LiInSe2 (or LISe) has been researched extensively because of its ability to operate in both scintillator and semiconductor modes. This makes it useful in applications including nuclear material detection for national security, space applications, and medical imaging. In some applications growth and fabrication costs are of concern, as it takes upwards of 40 days to synthesize a charge, grow a crystal and fabricate a detector. Ceramics are a solution for this long route of preparation. This research shows that using a 6LiInSe2 ceramic instead of a single crystal significantly decreases the time required to make a detector. In the present work, ceramic wafers can be formed using a mechanical press equipped with a heating attachment. The resulting ceramic wafer performs very similar to a slow-growth single crystal when used in the scintillator mode.

6LiInSe2 has gained attention recently as a semiconducting thermal neutron detector. As presented herein, the chalcogenide compound semiconductor also detects incident neutrons via scintillation, making 6LiInSe2 the only lithium containing semiconductor to respond to neutrons via both detection mechanisms. Both yellow and red crystals, which appear in the literature, were investigated. Only the yellow crystal responded favorably to ionizing radiation, similar to the semiconducting operation utilizing electrodes. The obtained light yield for yellow crystals is 4400 photons/MeV, referenced to Bi4Ge3O12 (BGO).The estimated thermal neutron light yield was 21,000 photons/thermal neutron. The two measured decay time components were found to be 31±1 ns (49%) and 143±9 ns (51%).This crystal provides efficient, robust detection of neutrons via scintillation with respectable light yield and rapid response, enabling its use for a broad array of neutron detection applications.

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 grew CdTe x Se 1− x crystals with nominal Se concentrations of 5%, 7%, and 10% by the vertical Bridgman technique, and evaluated their compositional homogeneity and structural quality at the NSLS’ X-ray fluorescence and white beam X-ray topography beam lines. Both X-ray fluorescence and photoluminescence mapping revealed very high compositional homogeneity of the CdTe x Se 1− x crystals. We noted that those crystals with higher concentrations of Se were more prone to twinning than those with a lower content. The crystals were fairly free from strains and contained low concentrations of sub-grain boundaries and their networks.

Crystals of CuGaS2 and AgGaS2 with different isotopic compositions have been grown by chemical vapor transport (CVT) using iodine as the transport agent. Before performing the CVT growth, sulfur and copper were purified by sublimation and etching, respectively. 109Ag and the etched 71Ga isotopes were purified from oxides by vacuum annealing. Transparent yellow orange crystals of CuGaS2 and greenish yellow crystals of AgGaS2 were obtained in the shape of platelets, chunks, rods and needles in sizes of up to 8 mm (CuGaS2) and 30 mm (AgGaS2). These crystals were used to study their electronic, vibrational and thermodynamic properties. Higher excitonic states (n=2,3) were observed at low temperatures with wavelength-modulated reflectivity spectroscopy, thus proving an excellent surface and crystal quality. In addition, the experimentally determined non-monotonic temperature dependence of the excitonic energies can be well fitted by using two Bose–Einstein oscillators and their statistical factors, corresponding to characteristic acoustic and optical phonon frequencies. Isotopic shift of excitonic energies has also been successfully observed in these crystals.

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.

In the pursuit of ultrasmall electronic components, monolayer electronic devices have recently been fabricated using transition-metal dichalcogenides. Monolayers of these materials are semiconducting, but nanowires with stoichiometry MX (M = Mo or W, X = S or Se) have been predicted to be metallic. Such nanowires have been chemically synthesized. However, the controlled connection of individual nanowires to monolayers, an important step in creating a two-dimensional integrated circuit, has so far remained elusive. In this work, by steering a focused electron beam, we directly fabricate MX nanowires that are less than a nanometre in width and Y junctions that connect designated points within a transition-metal dichalcogenide monolayer. In situ electrical measurements demonstrate that these nanowires are metallic, so they may serve as interconnects in future flexible nanocircuits fabricated entirely from the same monolayer. Sequential atom-resolved Z-contrast images reveal that the nanowires rotate and flex continuously under momentum transfer from the electron beam, while maintaining their structural integrity. They therefore exhibit self-adaptive connections to the monolayer from which they are sculpted. We find that the nanowires remain conductive while undergoing severe mechanical deformations, thus showing promise for mechanically robust flexible electronics. Density functional theory calculations further confirm the metallicity of the nanowires and account for their beam-induced mechanical behaviour. These results show that direct patterning of one-dimensional conducting nanowires in two-dimensional semiconducting materials with nanometre precision is possible using electron-beam-based techniques.

Crystal growth of bulk cadmium telluride (CdTe) ingot with 64 mm diameter and 75 mm height was accomplished in a modified vertical Bridgman configuration with the aim of achieving minimal nonstoichiometry related second phases (SP) defects such as inclusions and precipitates. As-grown crystal wafers were characterized with respect to infrared microscopy, bulk resistivity, low-temperature photoluminescence (PL) and foreign impurity concentration. Except for grain boundaries and twins, the wafers were found to be completely free of SP of sizes >1 μm and the distributions at different parts of the grown boule were found to be identical. Star shaped inclusions were observed at some places near the mid-section of the ingot, which were tentatively identified as Cd inclusions. At room temperature, the crystal exhibited high bulk resistivity values of 10 7 –10 8 Ω cm, which was found to be in conformity with the low density of nonstoichiometric defects. GDMS results indicated that the concentration of unintentional impurities was low enough for the electrical behavior to be explained uniquely on the basis of native defects.

Large diameter Bridgman growth of europium activated strontium iodide SrI 2 :Eu 2+ produces crystals with light yield of up to 115,000 ph/MeV with an excellent light yield proportionality. SrI 2 :Eu 2+ exhibits an outstanding energy resolution of better than 3% FWHM at 662 keV. Its emission is centered at 435 nm. The scintillation decays with a 1 μs time constant for small samples and up to 5 μs to larger crystals. This paper presents successful progress made in the vertical Bridgman crystal growth of SrI 2 :Eu 2+ and its scintillator properties. Large diameter, crack-free and transparent SrI 2 :Eu 2+ single crystals with diameters of 1 in., 1.3 in., 1.5 in. and 2 in. were all successfully grown.

Lithium containing A I B III C VI semiconductors are being considered as alternative materials for room temperature neutron detection. Materials such as LiGaSe 2 have been synthesized for non-linear optical applications; however, when the crystal is grown enriched in the 6 Li isotope, it is possible to imagine a radiation detector. A nuclear reaction occurs with 6 Li, which can be detected within the semiconductor crystal. As such, high quality crystals are required, which have few defects which prohibit charge collection. One of the primary challenges in growing a high quality crystal is the reactivity of lithium metal. Vacuum purified lithium metal was therefore reacted with gallium to form LiGa as an intermediate to LiGaSe 2 synthesis. Vertical and horizontal Bridgman growth was then conducted to determine the optimal growth conditions. Vertical Bridgman growth resulted in more pure crystals. Annealing in lithium metal vapor reduced crystal defects and improved optical and electrical properties of the subsequent LiGaSe 2 crystal.

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.

Our review offers an overview of the Traveling Heater Method (THM) for growing crystals of CdZnTe, the most important semiconductor material available today for fabricating nuclear detectors operable at room temperature. The review compares the advantages of the THM technique with respect to melt growth techniques, and details the development and improvements in the technique from its start to the present day. It is known that the optimization of the growth parameters is highly dependent on the height of the Te-rich CZT molten zone, which in turn governs the shape of the growth interface. Special attention is paid to understand the effect of the Te-rich CZT molten zone on the growth interface (both microscopic and macroscopic) to improve the uniformity and overall quality of the grown crystals. We conclude that this technique affords us the best method today for consistently producing large homogenous detectors in mass quantities with a thickness up to 15 mm. Such detectors are need for many national-security and medical-imaging applications.

We present a review on mercuric iodide technology for room temperature gamma radiation detection. After four decades of research and development the material is attractive for a number of applications for x-ray and gamma-ray imagers with performances that match all competing materials. The crystal growth of large single crystals is challenging; however the high resistivity of 1013 Ω cm, the undoped, as-grown crystals and the high radiation damage remaining features that are not exhibited by any other wide bandgap semiconductor materials.

Single-crystal strontium iodide (SrI2) doped with relatively high levels (e.g., 3 - 6 %) of Eu2+ exhibits characteristics that make this material superior, in a number of respects, to other scintillators that are currently used for radiation detection. Specifically, SrI2:Eu2+ has a light yield that is significantly higher than LaBr3:Ce3+ -a currently employed commercial high-performance scintillator. Additionally, SrI2:Eu2+ is characterized by an energy resolution as high as 2.6% at the 137Cs gamma-ray energy of 662 keV, and there is no radioactive component in SrI2:Eu2+ - unlike LaBr3:Ce3+ that contains 138La. The Ce3+-doped LaBr3 decay time is, however, faster (30 nsec) than the 1.2 sec decay time of SrI2:Eu2+. Due to the relatively low melting point of strontium iodide (~515 oC), crystal growth can be carried out in quartz crucibles by the vertical Bridgman technique. Materials-processing and crystal-growth techniques that are specific to the Bridgman growth of europium-doped strontium iodide scintillators are described here. These techniques include the use of a porous quartz frit to physically filter the molten salt from a quartz antechamber into the Bridgman growth crucible and the use of a bent or bulb grain selector design to suppress multiple grain growth. Single crystals of SrI2:Eu2+ scintillators withmore » good optical quality and scintillation characteristics have been grown in sizes up to 5.0 cm in diameter by applying these techniques. Other aspects of the SrI2:Eu2+ crystal-growth methods and of the still unresolved crystal-growth issues are described here.« less

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.

The current gamma-ray/neutron instrumentation development effort at NASA Goddard Space Flight Center aims to extend the use of active pulsed neutron interrogation techniques to probe the subsurface elemental composition of planetary bodies in situ. Previous NASA planetary science missions, that used neutron and/or gamma-ray spectroscopy instruments, have relied on neutrons produced from galactic cosmic rays. One of the distinguishing features of this effort is the inclusion of a high intensity 14.1 MeV pulsed neutron generator synchronized with a custom data acquisition system to time each event relative to the pulse. With usually only one opportunity to collect data, it is difficult to set a priori time-gating windows to obtain the best possible results. Acquiring time-tagged, event-by-event data from nuclear induced reactions provides raw data sets containing channel/energy, and event time for each gamma ray or neutron detected. The resulting data set can be plotted as a function of time or energy using optimized analysis windows after the data are acquired. Time windows can now be chosen to produce energy spectra that yield the most statistically significant and accurate elemental composition results that can be derived from the complete data set. The advantages of post-processing gamma-ray time-tagged event-by-event data in experimental tests using our prototype instrument will be demonstrated.

Development of the Europium-doped Strontium Iodide scintillator, SrI2(Eu), involves advances in crystal growth, optics and readout methodology for prototype detectors. We have demonstrated energy resolution of 3% at 662 keV for a 26 cm3 SrI2(Eu) crystal, which is equivalent to the performance obtained with Cerium-doped Lanthanum Bromide of equivalent size. Compared to standard analog readout, use of a digital readout method allows improved energy resolution to be obtained with large volume SrI2(Eu) crystals. Comparative gamma spectra acquired with LaBr3(Ce) and NaI(Tl) quantitatively depict the value of the high resolution of SrI2(Eu) in discriminating closely spaced gamma lines for radioisotope identification applications.

CdZnTe-based gamma detectors require a reduction of electronic noise contributed by apparent device and surface leakage current, especially for advanced readout schemes such as the Co-planar Grid or Pixelated Grid. In this work, we describe a combination of surface treatments and amorphous semiconductor layers that result in a reduction of both apparent device and surface leakage current compared to metal contacts on Br:MeOH etched devices. Characterization by scanning electron microscopy (SEM), spectroscopic ellipsometry (SE), x-ray photoelectron spectroscopy (XPS), current-voltage (IV), and pulse height spectra is performed.

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.

Numerical modeling and comparison to experiment in the materials for which suitable parameters have been measured confirm that three of the most important material parameters for predicting proportionality and the related host-dependent light yield (LY) of scintillators are (i) the carrier diffusion coefficients (including hole self-trapping if present, and hot-electron diffusion if unthermalized), (ii) the kinetic order and associated rate constant of nonlinear quenching, and (iii) deep-trapping probability. Thermalized carrier diffusion appears sufficient to describe the main trends in oxides and semiconductors. For heavier halide hosts, it appears necessary to take account of hot-electron diffusion to explain several important host-dependent trends.

A transient, coupled model has been developed to analyze the segregation of zinc in cadmium zinc telluride (CZT) grown in an electrodynamic gradient freeze (EDG) furnace. The coupled model consists of a local model that solves for time-dependent melt flow, heat transfer, melt–crystal interface position, and zinc distribution in both melt and solid phases and a quasi-steady-state global model that features realistic furnace heat transfer. After verification and validation tests, the model is applied to predict composition patterns in a large-scale CZT EDG growth system previously analyzed by Gasperino et al. [On crucible effects during the growth of cadmium zinc telluride in an electrodynamic gradient freeze furnace, J. Crys. Growth 311 (2009) 2327–2335]. Surprisingly, anomalous zinc segregation is predicted, featuring a non-monotonic axial concentration profile and several local minima and maxima across the boule. A mechanistic explanation is put forth based on the cumulative effect of changes in multi-cellular melt flow structures, a particularly susceptible occurrence for CZT systems. Additional effects of furnace translation rate and solid state diffusion are probed.

The Fisk-Vanderbilt Masters-to-Ph.D. Bridge Program is a model for substantially increasing the number of underrepresented minority students earning doctoral degrees in the physical sciences. The program presently leads the nation in master’s degrees in physics for African-Americans, and is one of the top ten producers of physics master’s degrees among all U.S. citizens. The program is on pace to become the nation’s top producer of underrepresented minority Ph.Ds. in physics, astronomy, and materials science. We summarize the main features of the program, including two of its core strategies: Partnering a minority-serving institution and a major research university through collaborative research, and using the master’s degree as a pathway to the Ph.D. We discuss our methods for recognizing and selecting for unrealized potential in students during the admissions process, and for cultivating this potential to develop successful scientists and leaders.

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.

AgGaSe2, AgGa0.9In0.1Se2, and AgGa0.8In0.2Se2 single crystals grown by the horizontal Bridgman technique were studied utilizing Raman spectroscopy excited with 633- and 784-nm lasers at temperatures varied from 77 to 300 K. The resonant Raman scattering associated with a transition from the Γ 6 (B) of the valence band to the conduction band Γ 6 of AgGa0.9In0.1Se2crystal is demonstrated. Resonant enhancements of the longitudinal optical (LO) polar modes of Γ 5 (or E) including Γ 5 L ( W 4 ) , Γ 5 L ( W 3 ) , Γ 5 L ( X 5 ) , and Γ 5 L ( Γ 15 ) , and their overtones and combinations in the crystal at 77 K were observed.

Carrier trapping times were measured in detector grade thallium bromide (TlBr) and cadmium zinc telluride (CZT) from 300 to 110 K and the experimental data were analyzed using a trapping model. In CZT, because the majority carrier concentration is close to the intrinsic carrier concentration, the trapping time increases exponentially as the temperature decreases below about 160 K. In TlBr, the majority carrier concentration is many orders of magnitude greater than the intrinsic carrier concentration and the trapping time followed a (1/ T ) 1/2 temperature dependence over the range of temperatures studied. The results of the model suggest that a moderately deep compensation center could be used to significantly increase the room temperature trapping time in TlBr.

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.

We have developed a procedure to purify, synthesize and grow InI single crystals using the Bridgman technique. The commercial product of InI from Alfa Aesar (99.998% InI or 4 N) was zone refined by 50 passes of a zone-heater at 420 °C, traveling at 2 cm/h (the melting point of InI is 360 °C). InI was also synthesized using high purity In and I 2 by vapor transport technique. The grown InI single crystal showed a clear band edge excitonic emission around 2.02 eV at 6 K using photoluminescence measurement. The resistivities of InI detectors were found to be ∼2×10 9 and 1×10 8 Ω cm for zone refined and vapor synthesized starting materials, respectively. The InI detector formed by Pd–Pd contact showed clear peak of alpha particle detection at room temperature using 241 Am source.

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.

The concentration, size and spatial distribution of Te inclusions in the bulk of CdMnTe crystals mined from two batches of ingots were studied. An isolated planar layer decorated with Te inclusions was identified in CdMnTe crystals from the second ingot. The internal electric field of a CMT crystal was probed by infrared (IR) imaging employing Pockels electro-optic effect. The effect of an isolated plane of Te inclusions on the internal electric-field distribution within the CdMnTe crystal was studied. Space charge accumulation around the plane of Te inclusions was observed, which was found to be higher when the detector was reverse-biased. The effects of the plane of Te inclusions on the electric-field distribution within the CdMnTe crystal, and the quality of CdMnTe crystals for nuclear detector applications are discussed.

New scintillators for high-resolution gamma ray spectroscopy have been identified, grown and characterized. Our development efforts have focused on two classes of high-light-yield materials: europium-doped alkaline earth halides and cerium-doped garnets. Of the halide single crystals we have grown by the Bridgman method-SrI2, CaI2, SrBr2, BaI2 and BaBr2-SrI2 is the most promising. SrI2(Eu) emits into the Eu2+ band, centered at 435 nm, with a decay time of 1.2 mus and a light yield of up to 115,000 photons/MeV. It offers energy resolution better than 3% FWHM at 662 keV, and exhibits excellent light yield proportionality. Transparent ceramic fabrication allows the production of gadolinium- and terbium-based garnets which are not growable by melt techniques due to phase instabilities. The scintillation light yields of cerium-doped ceramic garnets are high, 20,000-100,000 photons/MeV. We are developing an understanding of the mechanisms underlying energy dependent scintillation light yield non-proportionality and how it affects energy resolution. We have also identified aspects of optical design that can be optimized to enhance the energy resolution.