Organic materials are receiving a large attention because of their potential application as low-cost, large-area electronic devices. In particular, the long-range molecular order of organic semiconducting single crystals (OSSCs) limits charge carrier trapping due to grain boundaries and structural imperfections that dominate electronic transport in polymers and organic thin films. Very little attention has been paid so far to the application of organic materials as direct charge based radiation detection. Most of the emphasis has currently centered on the development of organic scintillators or photodiodes that act as indirect radiation detectors [1,2]. In the present report we show how millimiter-sized solution-grown OSSCs based on a dipolar molecule 4HCB (4-hydroxy-cyanobenzene) can be reliably employed as direct, room temperature X-ray detectors with a linear response and a significant long-term stability, reproducibility and radiation hardness [3]. The ease and low cost of OSSCs fabrication makes these devices quite appealing for future large-scale mass applications. Flexible large area detecting systems consisting of 2D matrices where each pixel is a single crystal detector have been fabricated with inkjet printed metal or conducting polymer electrodes, reaching operating voltages down to few volts.
References
[1] G. Hull et al. IEEE Trans. Nucl.Sci. 56, 899 (2009)
[2] S. Tedde et al. Nano Lett. 9 980-983 (2009)
[3] B. Fraboni et al. Adv.Mater. 24, 2289 (2012)

High efficiency devices are required for hard radiation detection (such as X-rays and gamma-rays) for the purposes of homeland security, biomedical and dental diagnostics, as well as other scientific applications. Semiconductors with wide band gaps and high densities are promising all-solid state detector materials for room temperature applications promising unsurpassed resolution and detection efficiencies. These materials must have structures composed of heavy elements (Z > 40), have wide band gap (1.6 - 2.5 eV), and high carrier mobility-carrier lifetime product (mu;tau;). We report that heavy metal compound semiconductors with chalcogens (Q = S, Se, Te) and halogens (X =Cl, Br, I) can be a promising family of materials for radiation detection investigations. We will present one chalcohalide compound, SbSeI (5.8 g/cm3 and 1.7 eV), in terms of optimized crystal growth by vertical Bridgman method and its physical characterization along with electrical resistivity as well as mu;tau; product.

ZnO is a low cost, solution processable, radiation hard, and environmentally friendly material, making it an attractive candidate for a variety of semiconductor applications. In this work we evaluate polycrystalline films of ZnO as a material for large-area charged particle radiation detectors. We are particularly concerned with controlling the charge carrier concentration, while also measuring other important parameters for detector materials such as charge carrier lifetime and mobility. We study films deposited by multiple techniques, including a sol-gel solution process, RF magnetron sputtering, and pulsed laser deposition. We also assess the effects of doping the films with Mg and/or Li, because these elements are reported to increase the resistivity of the films. We analyze films with thicknesses in the range of 700-3000 nm; this thickness range is based on simulations results suggesting that this would be the minimum thickness required to absorb enough energy from an impinging heavy charged particle to generate a detectable pulse. In order to measure the carrier concentration we fabricate metal-oxide-semiconductor capacitors as well as Hall measurement test structures. We fabricate thin film transistors and diodes to extract the carrier mobility and lifetime, respectively. We also investigate the structure and chemistry of the films using characterization techniques including x-ray diffraction, atomic force microscopy, Rutherford backscattering spectrometry, and electron microscopy.

X-ray and γ-ray detection have been applied widely in biomedical imaging, industrial process monitoring, national security and scientific research. Wide band gap semiconductors with high specific density and heavy elements have obvious advantages as X-ray and γ-ray detector, because they in general offer far better energy resolution than scintillation detectors. Currently, there are only a few compound semiconductors such as HgI2, TlBr and CdZnTe which can be used for room temperature operation. However, all these compoundss have limitations, HgI2 undergoes destructive phase change and is mechanically weak. The polarization effect of TlBr detector degrades spectroscopic performance over time. Poor crystallinity remains a major problem affecting the availability and cost of CdZnTe detectors. Therefore, new compound semiconductors that match the specific property requirements for detection can have a significant impact. Solid state metal chalcogenides present potential source of materials for the detection of hard radiation, because they have a desirable semiconductor properties (e.g. , a wide range of energy band gaps ) and can be prepared in complex stoichiometries and crystal structures. During this research, we have discovered Cs2Hg6S7, CsHgInS3, CsCdInSe3 and CsCdInTe3 compound semiconductors. We have grown large single crystals of all these compounds and measured their mobility lifetime products. Electronic structure calculations at the density function theory were performed based on the refined crystal structures and suggest a relatively small carrier effective mass for carriers, which indicate promising intrinsic properties for X-ray and γ-ray detection. The band gap energy of all the compounds are in a suitable range (~ 1.6 eV to ~2.5 eV). The electrical resistivity of the “as grown” crystals is large enough to limit leakage current. The mobility-lifetime product (mu;tau;) of electrons and holes estimated on our samples are comparable to the commercial materials.

Nonlinear quenching of luminescence at high excitation densities created in electron tracks is considered a root cause of nonproportional response in scintillators. By measuring decay time vs excitation density using ultrashort ultraviolet laser pulses, rate constants for 2nd order quenching in several oxide scintillators, mainly tungstates [Kirm et al], and in CsI excited in the exciton bands [Grim et al] have been determined. These findings seemed to confirm a common assumption that nonlinear quenching in scintillators would be dominantly 2nd order. Measuring luminescence yield versus excitation density at constant pulse energy by a z-scan technique described in this work allows 3rd and 2nd order kinetics to be distinguished for the first time in scintillators, as well as extraction of rate constants for quenching in a method applicable to a wider array of materials than decay time studies. We show that the iodide scintillators SrI2, SrI2:Eu, NaI, NaI:Tl, CsI, and CsI:Tl all display nearly pure 3rd order quenching kinetics when excited above the band gap, whereas the oxides BGO and CdWO4 display nearly pure 2nd order kinetics. This implies that the excitations interacting during the time of nonlinear quenching are free carriers in the iodides and excitons in the oxides. It raises the interesting question of what determines whether initially free carriers will remain independent or pair as excitons in the ~ 10 ps time of main nonlinear quenching, and why it should differ between iodide and oxide classes of insulators that both have large exciton binding energies. We suggest that hot carrier thermalization time mediated by LO phonon frequency [see also Wang et al, Kozorezov et al, Kirkin et al], as well as hot electron group velocity and hole self-trapping that affect charge separation, are important determiners of 3rd vs 2nd order quenching.[Li at al, 2012] Calculation of electron energy response using experimentally measured material parameters with Geant4 simulations of electron tracks are compared with SLYNCI and K-dip data to show how the kinetic order of quenching directly affects proportionality and total light yield and also interacts with carrier diffusion to determine performance trends. We report z-scan luminescence yield measurements on semiconductors including CdTe, CZT, ZnSe:Te, and ZnO to examine the limit of high carrier mobility. A characteristic feature of semiconductor z-scans is that the luminescence yield forms a peak vs excitation density if carrier mobility is high, or rises initially and then plunges near the beam waist when mobility is moderate, approaching the pure plunge due to nonlinear quenching as seen in insulators. The rising wings imply competition between radiative decay and quenching of a lower order, including zero-order quenching on saturable surface sites. Interestingly, undoped CsI shares some characteristics with the paradigm of semiconductor z scans. NNSA NA-22 DE-NA0001012 and DE-AC02-05CH11231.

Hydrothermal synthesis of ThO₂, U_xTh_1-xO₂, and UO₂ at temperatures between 650°C and 700°C has been demonstrated. ThO₂ possesses the highest known melting point of any refractory oxide at 3,390°C. Thus, single crystal synthesis of the refractory oxides ThO₂, U_xTh_1-xO₂, and UO_x at these temperatures represents a significant improvement over traditional melt-based crystal growth techniques, which rely on temperatures around 1800°C. Hydrothermal synthesis offers a comparatively low temperature and low thermal strain method for crystal growth. Hydrothermal synthesis also represents a cost effective, environmentally friendly way of growing bulk actinide materials of optical quality. These refractory oxide single crystals offer potential applications in thorium nuclear fuel technology, wide-band-gap uranium-based direct-conversion solid state neutron detectors as a replacement for traditional neutron detectors like 3He. ThO₂ single crystals of dimensions 6.49mm x 4.89mm x 3.89 mm and weighing 0.633g have been synthesized. Single crystal U_xTh_1-xO₂ crystals with mole fractions up to x=0.25 have also been grown. The largest alloyed crystal with mole fraction x asymp;0.15 has dimensions of 7.68mm x 6.07mm x 1.00 mm Additionally, single crystal UO₃ has been synthesized. Single crystal UO₂ is expected to be synthesized by using a template substrate for phase stabilization. X-ray diffraction of single crystal ThO₂ determined the unit cell to be of the calcium fluorite structure with a lattice parameter within variance of database values. Energy-dispersive spectroscopy (EDS) identified the chemical species as ThO₂, U_xTh_1-xO₂, and UO_x without additional impurities. Structural studies on the single crystals have enabled us to quantify the strain associated with incorporating uranium into the lattice along with measuring the dislocation density as the mole fraction of uranium is increased. Cathodoluminescence measurements have quantified the shift in bandgap energy with increased mole fraction of U along with providing band structure information for the single crystals. Photoluminescence has produced an intraband excitation peak at 233 nm and an emission at 350 nm along with an additional peak at 291 nm and an emission at 371 nm for single crystal ThO₂. Preliminary studies suggest a possible europium impurity associated with the 233 nm peak. Electron paramagnetic resonance (EPR) preformed at 4K shows a defect-free spectrum for single crystal ThO₂, and an Er³⁺ impurity for U_xTh_1-xO₂ single crystals. This research represents the first steps on the road to commercially available hydrothermally synthesized bulk actinide crystals for the detection of weapons of mass destruction, the study of the affect of aging on actinides, and thorium fuel cycle reactors.

As the demand for energy continues to rise, nuclear power plants are a great asset for creating inexpensive energy, but with the risk of radiation exposure, mainly related to the stored high radioactive nuclear wastes. Hence, there exists a need for novel materials solutions for creating a cheap and effective way to measure such exposure dose. Thin film chalcogenide glasses fill this need because germanium containing chalcogenide glasses have a high thermal stability, high radiation sensitivity and are easily integratable into semiconductor manufacturing. The radiation sensitivity is due to the formation of defects and structural reorganization, which are known as Radiation Induced Effects (RIE). The addition of silver (Ag) into the structure of the chalcogenide glass changes the optical and electrical properties of the hosting glass. Silver can be ionized by radiation and these ions can be used to capture RIE and help in amplifying the occurring electrically measurable effects.
Films of Ge_xSe_1-x (x=20, 30, 40), ranging from chalcogen rich to germanium rich, were created as bare films for studying of the RIE. Along with these films, devices build up by a thin film of chalcogenide glass, topped with two non-diffusive metal electrodes and a source of silver in a close proximity to them were also fabricated to measure the electrical performance. These devices along with the thin films were radiated with γ rays, using a ⁶⁰Co source with a dose rate of approximately 20rad/s. We present data on structural studies, silver diffusion studies and discuss the diffusion process based on the free volume in the compositions of glasses using Raman Spectroscopy, X-ray Diffraction (XRD), X-ray Photoelectron Spectroscopy (XPS) and Energy Dispersion X-ray Spectroscopy (EDS). Electrical performances of the devices are also presented using impedance and current vs. voltage (I-V) measurements.
Data obtained from the film characterization methods was used to create mathematical and device models, which have been generated using Silvaco software. These models have been used to study the silver transport and the electrical changes observed experimentally. Since silver diffusion is the primary mechanism for device performance, silver diffusion simulations have also been studied using ion transport simulator. In addition, electrical simulations were performed with Atlas simulator to study the effect of the electric fields and the behavior of the silver ions under different conditions. The parameters for diffusion and electrical simulations were obtained from the structural study and the results of these simulations validate the I-V and impedance measurements.

In many applications materials are exposed to particle radiation: The metal walls of nuclear reactors in future fusion or current fission systems are subject to ion bombardment. Solar cells and semiconductor components in satellites are exposed to ions impacting from cosmic rays. Protons interact with DNA and biological cells during cancer therapy and interesting interactions occur in radiation detection devices based on scintillator materials. In order to design materials with a high radiation tolerance, it is essential to be able to investigate the interaction of a fast projectile with the ionic and the electronic system of the target material at a fundamental level.
Based on the real-time propagation of Kohn-Sham states we recently developed a highly parallel plane-wave implementation of non-adiabatic Ehrenfest molecular dynamics to overcome the adiabatic Born-Oppenheimer approximation. Thanks to the excellent scalability of our approach on modern supercomputers, it allows for the rigorous computation of the electronic stopping with several hundreds of atoms in the calculations.
In this talk, we briefly summarize our explicit integration scheme of the time-dependent Kohn-Sham equations with some attention to important computational details. The influence due to different charge states of the projectile will be outlined and we present results for H, He, and Li ions and atoms penetrating an Al target, finding very good agreement with experiment up to the maximum of the electronic stopping. The deviations of the stopping power observed for the high velocity range are discussed in the light of the limits of this theoretical framework. Prepared by LLNL under Contract DE-AC52-07NA27344.

The cerium-doped lanthanum halide crystals have gained special interest due to their high density and atomic number, which results in excellent scintillation properties and higher detection efficiencies in comparison to NaI(Tl). A new analytical method of efficiency calibration is proposed for cylindrical lanthanum bromide (LaBr3:Ce) scintillation detectors. This method depends on the accurate analytical calculation of two important factors; the path length d, the photon traverses within the active volume of a gamma detector, and the geometrical solid angle , subtended by the source to the detector at the point of entrance. In addition, the attenuation of photons by the detector housing materials is also treated by calculating the photon path length through these materials. The comparisons with the experimental and Monte Carlo method data reported in the literature indicate that the present method is useful in the efficiency calibration of the cylindrical lanthanum bromide (LaBr3:Ce) scintillation detectors.

Detector grade Cd_0.9Zn_0.1Te (CZT) single crystals have been grown from zone refined Cd, Zn, and Te (~ 7N) precursor materials using a tellurium solvent method. Current-voltage (I-V) measurements have shown leakage currents of the order of 5 nA at a bias voltage of 1000 V. Resistivity of the order of ~ 8.5x10¹¹ Omega;-cm has been calculated from the I - V measurements. The average size of the Te inclusions was estimated to be ~ 8 mu;m. A drift mobility of ~ 840 cm²/V.s for electrons has been determined using a time-of-flight technique. Mobility-lifetime product for electrons (mu;tau;_e) of 2.7x10^-3 cm²/V was determined using alpha-ray spectroscopy. Pockel&’s measurement has shown a very good depth-wise uniformity of the electric field distribution in the crystals biased at -1000 V. Coplanar grid structures were fabricated on the tellurium face of the CZT crystals. Pulse-height spectra were acquired using these detectors and a ¹³⁷Cs radiation source. The 662 keV peak from the ¹³⁷Cs source was seen to be well resolved and a corresponding energy resolution le; 9.8 % was found.

Stabilized alloys of amorphous selenium (a-Se) have been synthesized for room-temperature real-time digital x-rays and high-energy nuclear radiation detector applications. The synthesis of well-defined a-Se alloy materials have been carried out using a specially designed alloying reactor at the University of South Carolina (USC) and installed in an argon controlled glove box. The alloy composition has been optimized to ensure good charge transport properties and device performance. The synthesis of a-Se (As, Cl) alloys has been carried out by thoroughly mixing zone-refined (ZR) Se (~7N) with previously synthesized Se-As and Se-Cl master alloys. The synthesized alloys have been characterized by x-ray diffraction (XRD), glow discharge mass spectroscopy (GDMS), differential scanning calorimetry (DSC), Fourier transform infra-red spectroscopy (FTIR), x-ray photoelectron spectroscopy (XPS), temperature varying current-voltage (I-V) characteristics, capacitance-voltage (C-V) measurements, and thermally stimulated current (TSC) measurements . The a-Se alloy detectors have been fabricated and tested and the preliminary results show high promise for radiation detectors with its high dark resistivity (10¹² - 10¹³ Omega;-cm), good charge transport properties, and cost-effective large area scalability. Details of various steps involved for detector fabrication and characterization will be presented.

As a p-type semiconductor with a high cross-section for neutron capture, boron carbide is of interest for solid-state direct-conversion neutron detection. There has been particular interest in thin-film amorphous hydrogenated boron carbide (a-B_xC:H_y) grown by plasma-enhanced chemical vapor deposition (PECVD) from orthocarborane because it demonstrates atypically high resistivities (10¹⁰ Omega; cm and higher), important for suppressing detector leakage current. Although some research investigating this material for detection applications has been done, rigorous charge transport studies are still needed if efficient devices are to be realized, including an investigation of charge carrier mobility, concentration, lifetime, and recombination mechanism(s). The experimental determination and physical interpretation of these quantities is complicated by the high-resistivity and complexity of the solid, which is based on a disordered network of molecular icosahedral subunits. This poster will describe preliminary efforts to establish phenomenological descriptions and experimental measurements of several charge transport properties in thin-film a-B_xC:H_y.

The use of high doses of gamma and electron radiation in industrial processes has encountered several applications in different fields, as for example in the sterilization of medical products, to modify polymer properties and in food irradiation. In this work, we report the radio-degradation of MEH-PPV polymer film as a tool for measuring high doses of ionizing radiation. The poly[2-methoxy-5-(2&’-ethylhexyloxy)-p-phenylene vinylene] (MEH-PPV) was blended with poly(butylene adipate-co-terephthalate) (PBAT), a flexible and biodegradable polymer. In order to evaluate the possibility of using them as high-dose dosimeters, we have investigated the changes in the optical absorbance of ultraviolet-visible (UV-Vis) and Fourier transform infrared (FTIR) spectra and in the photoluminescence (PL) emission spectra, when exposed to gamma doses ranging from 1 to 3000 kGy. We have found out a strong linear correlation between the gamma dose and the absorption peak intensities in the UV-Vis region of the spectrum (335 nm) for doses ranging from 1 to 500 kGy. The analysis of PL emission, when excited at 405 nm, revealed a decreasing of luminescent intensity between 590 and 630 nm, which has been attributed to the MEH-PPV emission. Changes in the color intensities for increasing radiation doses have been also observed. Among other advantages, as for instances sample ease preparation and dose evaluation, the films are easily handled, inexpensive (U$ < 1,00) and very flexible. This work was sponsored by CAPES, FAPEMIG, CNPq and CNEN.

In order to understand the fundamental charge transport in a-B:H and a-BX:H (X = C, N, P) compound heterostructure devices, we have utilized x-ray photoelectron spectroscopy to determine the valence band offset and Schottky barrier present at interfaces formed by Plasma Enhanced Chemical Vapor Deposition of these materials on Si (100) and polished poly-crystalline Cu substrates. For a-B:H/Si and BX:H (X=C, P)/Si interfaces, we observed relatively small valence band offsets of +/- 0.25 eV. However for the a-BN:H interface, we observed a much larger valence band offset of 1.9+/-0.15 eV. For a-B:H and a-BN:H interfaces with Cu, we determined the Schottky barrier to be 0.8+/-0.15 and 3.25+/-0.25 eV respectively.

Boron phosphide (BP) is potentially useful in solid state neutron detectors because of the large thermal neutron capture cross-section of the boron-10 isotope. Unlike many other boron compound semiconductors which exhibit only a single conductivity type, both n- and p-type conductivity have been reported for BP. Cubic BP crystals were grown at a relatively low temperature (1050-1150°C) by crystallizing dissolved boron and phosphorus from nickel solvent in a sealed quartz tube using the flux growth method. The impact of the growth conditions and reactant compositions on the structural and electrical properties of the crystals were examined. Transparent red crystals with mostly hexagonal shape were obtained. The crystal size increased with amount of time the solution was held at the highest temperature, and a decrease in the solution cooling rate. The largest crystals were about 3 mm x 4 mm x 1mm, produced at a cooling rate of 3°C per hour. The structural characterization with various techniques confirmed that the crystals were zinc blende structure, highly crystalline in nature with boron to phosphorus atomic ratio of 1:1. Dislocation sensitive etching (DSE) in a molten NaOH-KOH eutectic mixture produced etch pit densities on the order of 2 x 107 cm-2. The crystals had n-type electrical conductivity with an electron mobility of 40 cm2/V*s. Thus, the high crystal quality, good charge carrier mobility and the relative ease by which crystals are grown demonstrate that BP is suitable for applications in neutron detection.

Silicon Carbide (SiC) semiconductor radiation detectors offer many advantages for measurement applications in high-temperature and high-radiation environments. In addition to possessing many of the advantages of conventional Si radiation detectors, the relatively wide band gap for SiC (3.25 eV for 4H SiC) leads to detector leakage currents that are almost three orders of magnitude lower than Si at room temperature. In addition, the leakage currents remain low at elevated temperatures allowing the detectors to operate reliably at temperatures up to 300 degrees C and higher. Furthermore, SiC detectors have been shown to perform well after very high gamma-ray, charged-particle and neutron cumulative doses.
SiC radiation detectors were first demonstrated more than a half century ago in 1959. The availability of suitable SiC materials hindered further development until the mid 1990&’s, when high-quality SiC substrates and epitaxial layers were developed enabling a resurgence of SiC detector technology. SiC detectors have been demonstrated for alpha and other charged particles, gamma rays, X-rays, beta particles and thermal and fast neutrons. SiC detector technology has now been demonstrated and applied by nearly one hundred laboratories in more than a dozen countries worldwide.
Although SiC materials technology has advanced rapidly and significantly, many limitations remain for continued development of SiC detectors. A fundamental limitation of detectors produced using epitaxial layers grown on crystal substrates is the presence of defects in the substrate material. Although many types of defects can inhibit detector performance, the most important are threading-type defects, which can propagate into the epitaxial layer and result in micropipe defects causing unacceptably high detector leakage currents. The thickness and quality of the epitaxial layer presents further limitations. Although epitaxial detectors from 50 to 200 micrometers thick are routinely available for alpha and charged-particle detection, much higher thicknesses are required for X-ray and fast-neutron detection. These higher thickness layers must also be achieved with low dopant concentrations (preferably 10E12 Nitrogen atoms cm-3 or less) to enable full depletion at modest reverse voltages. In the case of alpha-particle detectors, very thin Schottky or p+ contacts are required (50 Å or less), because the contact is the entrance window for the detector and can limit energy resolution.
The present paper will review the history and progress in SiC detector development, discuss materials and device fabrication limitations as it effects ongoing development, and discuss the prospects and directions for SiC detector applications.

Silicon carbide (SiC) based radiation detectors are well-known for their radiation hardness properties. In recent years, SiC detectors have found application in a wide variety of field such as alpha-detection, x-ray imaging, and neutron counting spectrometer. In the present work, high resolution Schottky barrier detectors have been fabricated from ~ 20 mu;m thick 4H-SiC (n-type) epilayer on 360 mu;m 4^o off-cut [11-20] (n+) SiC substrates for alpha particle detection and x-ray imaging applications. Schottky contacts were obtained by depositing ~ 10 nm thick nickel contacts on the epilayer surface with an active area of ~ 11 mm². The detectors exhibited very low leakage currents e.g., ~ 0.2 nA at 250 V at room-temperature, due to the extremely low level of micropipe defect density (< 1 cm^-2) in the epitaxial layer. A barrier-height of ~ 1.5 eV and a diode ideality factor of 1.1 was obtained from the current-voltage (I-V) measurements. Capacitance-voltage (C-V) measurements have revealed an effective doping concentration of ~ 3 × 10¹⁴ cm^-3. The low doping concentration has helped to achieve full depletion (~ 20 mu;m) which is needed for effective absorption of high-energy x-rays. The full depletion was achieved at a comparatively lower bias voltage of ~ 110 V which is necessary for low noise performance. Room-temperature alpha particle spectroscopy has revealed high resolution pulse-height spectra at bias voltages less than required for full depletion. An energy resolution of ~ 0.4 % was obtained for 5.48 MeV alpha particles at an optimum bias voltage of 60 V.

Advancement of nuclear power technology has led to the critical questions of detecting emissions of harmful radioisotopes and monitoring the exact amount of fissile material present. Thus, finding devices that allow precise detection and monitoring in even the harshest nuclear environment has become one of the key challenges in nuclear energy technology. The detector materials and device structure need to allow fast and accurate measurements at high temperatures as well as survive significant radiation and corrosive environments. While semiconductor based devices fulfill the measurement requirements, detectors made of common materials (predominantly silicon) are prone to radiation damage and cease functioning at approximately 150 °C. Among alternative detector materials, silicon carbide (SiC) has been suggested to potentially overcome these deficiencies. In this project, we designed a SiC based detector, demonstrated its function, and developed computational modeling that can predict the performance of the detectors in harsh nuclear environments. For this work, we targeted the extreme conditions found in pyroprocessing (nuclear fuel dissolved in molten salt at 500 °C or higher), a method to reprocess spent nuclear fuel. While especially the high temperatures limit many design choices for the device structure, we show that a Schottky diode detector made with 4H-SiC and nickel-based Schottky and Ohmic contacts indeed functions at temperatures up to at least 500 °C. We found that most common methods of contacting the device fail (soldering, brazing, “gluing” with metal pastes), yet we were able to develop mechanical contacts that work throughout the entire temperature regime. In order to computationally simulate temperature and radiation effects, we have developed a novel multiscale modeling methodology consisting of continuum-level simulation of irradiation damage and its evolution as a function of temperature and quantum-mechanical modeling of the effect of damage on the electrical properties of 4H-SiC. In addition, device modeling has been developed to predict the detector operation as a function of environmental conditions, which has been used for calibrating the charge measured in the detector versus impacting α-particle energy and to deliver a proof-of-concept for operation in the pyroprocessing environment.

An optimum thickness passivation layer of silicon nitride on AlGaN\GaN heterojunction devices can not only improve device performance by reducing electron traps at the surface but also serve to radiation harden the devices. In this study, the effects of passivation layer thickness was investigated. Various thicknesses (0, 20, 50 and 120 nanometers) of Si₃N₄ on bare epilayers of AlGaN \GaN were electron irradiated with 1.0 Mev electrons at fluences of 5 x 10¹⁵ cm^-2 to 10¹⁶ cm^-2 , thus increasing the electron trapping at the interface and allowing an enhanced measurement of the interface quality. It has been shown previously that such irradiation produces point defects and creates acceptors. In this study, Hall system measurements were used pre- and post- irradiation to observe changes in carrier concentration and mobility as a function of electron beam fluence and energy. These measurements indicate that the surface states are donors and that mobility and conductivity are preserved as a function of Si₃N₄ thickness.

New demands in X-ray and γ-ray radiation detection ranging from biomedical to homeland security applications require high efficiency devices. Detector materials with near-perfect charge transport properties similar to cryogenically-cooled Ge but capable of running at room temperature are desirable. We demonstrate that mercury compounds with chalcogens (Q = S, Se, Te) and halogens (X = I, Cl, Br) can be a promising family for radiation detection materials. By using a hybrid chalcohalide approach between HgQ and HgX2 tuning of the electronic structure of Hg3Q2X2 is achieved. Chalcogen orbitals in HgQ located near the Fermi level are responsible for relative high mobilities but band gap decreases (from S to Te) due to their extended interactions. Halogens in HgX2 on the other hand have their bands well below the Fermi level and salts between mercury and halogen are usually insulators. Incorporation of halogen atoms in a mercury chalcogenide framework (Hg3Q2X2) can give rise to intermediate properties between the two end members (HgQ and HgX2), i.e. structures composed of heavy elements (Z > 40), wide band gap (1.6 - 2.5 eV), and high carrier mobilities. We will present two members in the new chalcohalide family, Hg3Q2X2. Crystal growth of the Hg3Te2Br2 phase (7.8 g/cm3 and 2.5 eV) by a vapor transport method and of the β-Hg3S2Cl2 phase (6.8 g/cm3 and 2.5 eV) by a Vertical Bridgman method gave mm-sized crystals. Physical characterization and transport properties relevant to detector materials will be reported.

This presentation will describe several new scintillators for gamma spectroscopy and imaging radiography, being developed by LLNL and our partners. Scintillator discovery efforts over the past six years have identified new scintillators that can provide higher resolution gamma spectroscopy than Thallium-doped Sodium Iodide. We started with high-risk discovery efforts, down-selected among candidates, and are now progressing into scale-up, reproducible production and integration into devices. The latest performance of prototype devices based on Europium-doped Strontium Iodide, transparent ceramic GYGAG(Ce) and Bismuth-loaded plastic will be presented. We are also developing Lutetium-based transparent ceramics that improve X-radiography throughput and spatial resolution, due to their high light yields, stopping power and radiation hardness, compared to scintillator glass or single crystals such as Cadmium Tungstate.

Acknowledgements
This work was performed under the auspices of the U.S. DOE by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344, and has been supported by the US Department of Homeland Security, Domestic Nuclear Detection Office, under competitively awarded IAA HSHQDC-09-x-00208/P00002, the U.S. DOE, NNSA, Office of Nonproliferation Research and Development (NA-22) and DOE ESC program. This support does not constitute an express or implied endorsement on the part of the Government.

Among energy-resolving gamma-ray detector technologies, scintillators offer a cost— performance balance that makes them an intriguing option for portal-monitoring applications. However, there is a recognized need for novel, processible, high-performance alternatives to current single-crystal scintillating materials that are difficult to scale to portal-relevant detector sizes. Semiconductor nanocrystal quantum dots (NQDs) are of interest because of their potentially high gamma-stopping power, excellent stability, high emission quantum yields, and facile, solution-based processibility into a range of composites. However, development of NQD-based scintillators dramatically slowed after initial studies revealed only mediocre performance [1-2]. We suggest that this is due not to a definitive, insurmountable problem with NQDs themselves, but rather to the general lack of insight into the gamma-to-photons transduction process in these complex materials, which reduces the development and refinement of candidates to simple trial-and-error. In contrast, the recent explosion in performance of NQD-based light-emitting diodes, detectors and solar cells was facilitated by ultrafast spectroscopic studies of carrier processes that informed rational optimization of the materials themselves. Thus, we hold that a major stumbling block to NQD scintillator development is the lack of equivalent methods for observing carrier relaxation upon high-energy excitation. Here, we describe the application of ultrafast transient cathodoluminescence to probe the population of excited states formed within a granular material during scintillation, for the purpose of determining the major sources of efficiency loss. In such an experiment, a short pulse (< 10 picoseconds) of fast electrons acts as a synchronizable surrogate for gamma-rays, allowing us to examine the spectral and temporal characteristics of emitted light using, e.g., a streak camera. Our analysis shows that in the case of CdSe/ZnS core/shell NQDs, high-energy excitation produces an excited-state population dominated by charged-imbalanced and/or multiexciton states. This being the case, the losses due to previously blamed factors of low-stopping power and high reabsorptive losses are likely dwarfed by those attributable to the efficient, non-radiative Auger recombination exhibited by such states. We examine the ramifications of this conclusion, particularly how it informs the design of the next generation of scintillating NQDs and composites.
[1] I.H. Campbell and B.K. Crone, Adv. Mater. 18, 77 (2006).
[2] S.E. Letant and T.F. Wang, Nano Lett. 6, 2877 (2006).

At Lawrence Livermore National Laboratory, highly transparent polycrystalline ceramic oxide scintillators are being developed for gamma spectroscopy. The family of gadolinium-based garnet oxides doped with cerium (Gd3-XYX(Ga,Al)5O12:Ce) were down-selected from a broad list of cubic oxide candidates as the material of choice, as they offer high light yield, high stopping power, and fast decay time. These garnet materials can be grown in the single crystal form; however chemical inhomogeneity in the crystals often plagues their performance by creating non-uniform light yield degrading the resolution in large volume scintillators. Transparent polycrystalline ceramics, on the other hand, are formed by solid state sintering far below the melting point and offer an alternative means of fabrication which can lead to higher chemical homogeneity, and also creates different defect trap distributions which can lead to improved performance.
The ceramic scintillators&’ light yield and energy resolution depend on the details of various processing steps, including powder calcination, green body formation, and sintering atmosphere. We have found that gallium sublimation during vacuum sintering creates compositional gradients in the ceramic and alters the trap state distributions which affect the energy resolution. On the other hand, sintering in oxygen produces ceramics with uniform composition and little afterglow, but light yields are reduced compared to vacuum sintering. By controlling the local atmosphere during the sintering process steps, we were able to minimize the gallium sublimation, resulting in a more homogeneous composition and improved gamma spectroscopy performance. In addition, post-annealing the ceramic enables us to manipulate the trap state distributions for optimal performance.
We are in the process of optimizing the composition for the best detector. Addition of yttrium helps stabilize the garnet crystal structure for transparency, but slightly lowers the material&’s density and effective Z, reducing the stopping power. The concentrations of yttrium, gallium, and aluminum will also alter the luminescence properties. With our best performing samples, we have achieved <4% energy resolution (FWHM) at 662 keV (with silicon photodetector readout), and <5% with 1 in3 size samples with PMT readout).
Acknowledgements: Thanks to Keith Lewis, Patrick Beck, Owen Drury, Ben Sturm, Joshua Kuntz, Sean O&’Neal, Zurong Dai, Rick Ryerson, Jeff Roberts, Scott Fisher, Lindsey Haselhorst, Long Nguyen, and Todd Stefanik of Nanocerox Inc. This work was performed under the auspices of the U.S. DOE by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344, and has been supported by the US Department of Homeland Security, Domestic Nuclear Detection Office, under competitively awarded IAA HSHQDC-09-x-00208/P00002. This support does not constitute an express or implied endorsement on the part of the Government. LLNL-ABS-595896

Nanocomposites composed of high loading content of nanoparticles dispersed in polymer matrix are being extensively investigated for various electronic, photonic, and mechanical applications. They have also been proposed to detect high energy X- and γ-rays upon scintillation, but experimental successes have been limited, because light yield from the nanocomposites is often diminished due to fluorescence quenching and opacity in bulk size scintillators (over 1cm3). We have synthesized a transparent nanocomposite comprising gadolinium oxide nanocrystals uniformly dispersed in bulk-size samples at a high loading content. New techniques have been introduced to avoid fluorescence quenching and opacity in the nanocomposite, leading to higher radioluminescence light yield than standard commercial plastic scintillators. Nanocomposites containing 31 wt% of Gd2O3 nanocrystals (14 mm diameter by 3 mm thickness) generated an appreciable photoelectric peak.

Over the last two decades, it has been determined that for most scintillators, the resolution is mainly dictated by the so-called nonproportionality of the light yield. The existence of nonproportionality means that the light yield, as defined in units of photons/keV, depends on the gamma or high-energy electron energy and is therefore not a constant. We have been utilizing a specialty instrument referred to as SLYNCI (Scintillator Light yield Nonproportionality Compton Instrument) to acquire this type of data on dozens of materials. We have also developed a model which can interpret the shape of the nonproportionality curves to deduce the predicted resolution of the material. The simplest version of the model fits the data with only two parameters: one to describe the impact of exciton-exciton annihilation, and a second which accounts for the attraction of electrons and holes to form new excitons. We then invoke the existence of Landau fluctuations in the ionization density in order to describe the fluctuations in the light yield (and hence model the predicted resolution). This model works for nearly all scintillators, including standard materials as diverse as NaI(Tl), LSO(Ce), BGO, YAP(Ce), and LaBr3(Ce). We also tested many related Ce-doped garnet scintillators, comparing those that are made by ceramic-processing and melt-growth routes, as well as garnets comprised of various cations such as Gd, Y, Lu, Ga and Al. We were surprised to find major differences among these similar materials, which we attribute to differences in the shallow trap distribution. Other examples of revealing studies of scintillators include organics, and Eu-doped and undoped SrI2. Interestingly, there are a few scintillators [notably CsI(Na)] that are ostensibly “unfittable” by our standard modeling method, demanding that the basic assumptions be reconsidered as being too approximate for certain materials. The theory was therefore extended to allow for two kinds of excitons: “free” excitons that are highly-mobile but short lived; and “self-trapped” excitons which migrate by hopping in a thermally-activated manner and are long-lived. Our mechanistic models were further challenged by studying the nonproportionality of as a function of temperature, wherein changes in the shape of the curve are clearly observed for CsI(Na) at reduced temperatures, whereas little change was recorded for most other scintillators. We are also expanding the modeling framework in other ways, such as by examining the nature of the fluctuations of delta-rays and how they affects the resolution.
This work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344

IR absorption study of CdTe and CdSe annealed in CdSO4 vapor at 850°C is presented. It is shown that such a treatment results in the appearance of absorption lines previously assigned to the oxygen substituting for Te/Se in the regular lattice perturbed by the nearby Cd vacancy [G. Chen et al. Phys. Rev. Lett. 96 035508 (2006); Phys. Rev. B 75 125204 (2007)]. Each of these features is accompanied by weaker lines. The intensities of each set of lines match the natural abundance of the sulfur isotopes. These modes are reassigned to the stretch vibrations of a sulfur-oxygen complex, SO₂*. The number of oxygen atoms involved in SO₂* was deduced from the relative intensity of an IR absorption line related to the ¹⁸O isotope. Uniaxial stress studies revealed the activation barrier for the SO₂* rotation.
Additionally, two previously reported local vibrational modes (LVMs) in CdSe single
crystals at 1992 and 2001 cm^-1 [Chen et al. Phys. Rev. Lett. 101 195502 (2008)] are generated by annealing the crystals in H₂ gas at 500°C. The absorption bands shift to 1454 and 1461 cm^-1 when the samples were annealed in D₂ ambient. Each LVM consists of at least five lines which relative intensities match the natural abundance of the predominant Se isotopes. The 1992- and 2001-cm^-1 lines are assigned to two selenium-hydrogen vibrations in the Cd vacancy. Influence of these complexes on the electrical activity of CdTe and CdSe is discussed.

In this study we clarify the role of edge effects and what steps need to be taken in the CdZnTe (CZT) detector-fabrication process to decrease, and ultimately eliminate, their deleterious effects on device performance. Edge effects denote the loss of charge collection efficiency at the perimeter of devices. They may also play a role in device stability and noise. These edge effects are responsible for introducing aberrations in imaging devices and reducing the energy resolution in X- and gamma-ray spectrometers. Experimental results on trapping of photo-generated carriers at surfaces and correlations with the surface conditions will be discussed.

The ternary compound CdTe0.9Se0.1 can be a good choice for room-temperature radiation detector applications. The main advantage of CdTe0.9Se0.1 is the segregation co-efficient of Se in CdTe is near unity. The near unity segregation co-efficient of Se can potentially result in compositional homogeneity of the grown ingots, which in turn drastically can increase the yield and thus production of detectors at a lower cost compared to CdZnTe. Another advantage is the lower effective mass of electrons compared to CdTe and thus higher electron mobility, viz. better charge transport characteristics are possible compared to CdTe and CdZnTe. In the present study, we have characterized CdTe0.9Se0.1 samples grown by Bridgman technique. The samples showed fairly high resistivity of 5.5 x109 ohm-cm, and the value of mobility-lifetime product obtained was 3.5x10-3 cm2/V. The samples were also characterized by X-ray topography in order to study the intrinsic defects present in the crystals. Preliminary study of X-ray response mapping and compositional homogeneity will also be discussed. Well resolved gamma peaks from 241Am source were registered at room temperature. Our preliminary study showed that CdTe0.9Se0.1 may be a potential candidate for room-temperature radiation detector applications, and it can potentially be produced at a much lower cost compared to CdZnTe.

CdZnTe and other CdTe-based wide-band-gap compound semiconductors have gained much popularity as potential detector materials for x-ray and gamma-ray detectors. However, their widespread deployment is limited by the dearth of spectroscopic-grade crystals. The process of growing defect-free crystals is complex, and it is still not fully reproducible. We have been investigating various extended defects in these crystals, e.g. low-angle sub-grain boundaries and dislocations that significantly degrade the detector&’s performance. Therefore, the understanding and reduction of these defects in bulk crystals are a high priority for the manufacturing of high-quality devices. In this work, we studied extended defects by revealing etch pits on crystal surfaces using different chemical etchants, such as Nakagawa, Everson, and EAg with variable concentrations of Ag. The formation of etch pits with a specific shape depended upon the surface orientation and the choice of etchant. We illustrated the pits&’ shapes, sizes, and concentrations to identify the nature of the crystal&’s defects. We evaluated the areal density and distribution of the etch pits associated with dislocations and other crystallographic defects using IR transmission microscopy and a scanning electron microscopy (SEM) combined with energy-dispersive x-ray spectroscopy (EDS). Finally, we correlated our detailed data on these defects with device response to understand their influence on the performance of detectors produced from different materials.

CdZnTe (CZT) has been recognized as one of the leading materials for fabricating room-temperature radiation detectors. However, the presence of material defects, e.g. dislocations, is still a challenging problem, which affects the availability of high-quality CZT crystals. A better understanding of dislocations in CZT helps to find a way to prevent their formation during the growth and post-growth annealing processes or minimize their incorporation by the fabrication steps. In this work, we intentionally create dislocations by making indentations on CdZnTe crystals. Then we use a low-temperature spatially resolved micro-scale photoluminescence mapping (LSMPM) system to probe the spatial variation of dislocation-related emissions around the indentation area. We observed a strengthening of the dislocation-related emission when moving the sampling point towards the indentation. Experimental results with the above characterization method will be presented and discussed in detail. These results provide useful knowledge regarding the origin and nature of dislocations in detector-grade CZT crystals.

A success in development of detectors for X-ray systems depends on the progress in development and investigation of a semiconductor material capable to provide reliable registration of single particles of ionizing radiation.
Unlike conventional GaAs detectors based on growth SI GaAs compensated EL2 centers chromium compensated gallium arsenide (GaAs: Cr) allows to form a detector structure with an extremely high resistivity up to 109 ohm*cm and close to linear IV characteristics. The GaAs:Cr detector material technology devised is shown to make possible the development of detectors with the following parameters: electron mobility and lifetime product up to 10-4cm2/V, hole mobility and lifetime product about 10-6cm2/V, gamma-radiation resistance of ge; 50MRad, active layer thickness as great as 800mu;m, and detector chip dimensions within 76 mm diameter wafer.
Different type of GaAs: Cr microstrip and pixel detectors have been developed and tested. The detectors under consideration exhibit high speed of response, high X- and charged-particle radiation sensitivity, and good homogeneity. The GaAs: Cr detectors have low values of cross-talk effect and demonstrate spatial resolution that defined by pixel pitch.

The search for a solid form semiconductor neutron detector continues because such a device would have greater efficiency, in a compact form, than present day gas-filled 3He and 10BF3 detectors. Boron based compounds, such as BP, BN, and BAs have seen limited success, and thus far do not appear promising due to crystal growth and materials preparation problems. Although some Li compounds have been quite successful as neutron detectors, such as LiF thermoluminescent dosimeters and LiI scintillators, neutron detectors from Li based semiconductor compounds have not been explored to a similar extent as B based semiconductors. The 6Li(n,t)4He reaction yields a total Q value of 4.78 MeV, larger than 10B, and easily identified above background radiations. Hence, devices composed of either natural Li (nominally 7.5% 6Li) or enriched 6Li (usually 98% 6Li) may provide a semiconductor material for compact high efficiency neutron detectors. A sub-branch of the III-V semiconductors, the filled tetrahedral compounds, AIBIICV, known as Nowotny-Juza compounds, are known for their desirable cubic crystal structure, and were originally studied for photonic applications. Equimolar portions of Li, Zn, and P or As were sealed under vacuum (10-6 Torr) in quartz ampoules with a graphite lining, loaded into a compounding furnace, and increased to 560 C to form the ternary compound, LiZnP or LiZnAs, and further annealed to promote crystallization. The product was harvested as a powder and large clumps. The compounded material was loaded into a tantalum vessel, and vacuum welded closed in an argon environment. The tantalum tube as loaded into a vacuum chamber, and mounted into a harness between two electrodes leading to a high current, low voltage power source. The current was increased until the material became molten, then slowly decreased to room temperature without shocking the crystal. Ingots 1.0 cm in diameter and up to 1.0 cm long were harvested. The chemical makeup was confirmed at Galbraith Analytical Laboratories by Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-AES). The results showed the compounds were reacted in equal ratios, 1-1-1, to form the cubic ternary compounds. Cubic crystal structures were confirmed, and rocking curves were collected using XRD methods. Samples were prepared for neutron sensitivity testing. On a polished surface, silver epoxy was applied for the cathode contact. The sample was transferred to an electron beam evaporator, where 600 nm of titanium and 1000 nm of gold where deposited as the anode contact. With a negative 100 volt bias applied at the anode, a neutron response pulse-height was acquired at the thermalized neutron beam in the KSU TRIGA Mark II nuclear reactor. Noise was noticed in short bursts which randomly inhibited neutron counting, and resulted in inconsistent data collection. Further modification to crystal growth processes are currently under investigation.

Boron phosphide (BP) can be used in neutron detector devices due to the large neutron absorption cross section of boron. To ensure high detection efficiency, we need BP to be single crystal with a defect density as low as possible. We studied the defect evolution of epitaxial single crystal BP thin film grown on several substrates. The morphology and defects of these materials systems were investigated using high-resolution transmission electron microscopy (HRTEM) and aberration-corrected scanning transmission electron microscopy (STEM).
The substrates studied include intrinsic silicon, 4^o and 8^o miscut 4H-SiC. The growth temperature ranged from 800 to 850 ^oC. This range of growth temperatures did not have any influence on the morphology of the film grown on the 4^o miscut 4H-SiC, but it seemed to affect the growth of BP on the 8^o miscut 4H-SiC. The reason for the differences in growth is unclear and more samples need to be studied to determine the underlying mechanism. On the Si substrate, the first 50nm of BP at the interface are amorphous and then polycrystalline BP is formed. The grain sizes increases with distance from the interface. There is no preferred growth orientation observable in the TEM. On the 4^o miscut 4H-SiC substrates, epitaxial BP films with a growth direction of [111] are obtained. The structure contains twin boundaries at the interface and then single crystalline BP with a few small angle grain boundaries. On the 8^o miscut 4H-SiC substrates, the BP film is almost single crystalline under 800 ^oC while it is basically polycrystalline under 850 ^oC. However none of the crystal orientations are aligned with SiC [0001].

The best crystalline BP was grown on the 4^o miscut 4H-SiC. The samples were investigated in [11-20] and also [10-10] zone axes of the substrate. A series of HRTEM images were taken starting from substrate to study the evolution of the microstructures with the analysis of the corresponding diffractograms. This investigation in different zone axes was possible in the TEM Zeiss Libra 200 MC due to high tilt capabilities. High-angle angular dark-field (HAADF) images were also taken to study the atomic structures of the interface and defects. We found that heavy defects locate near the SiC interface which caused BP lattice distortions. The number of defects decreased along the direction of film growth. We identified all types of defects and their distribution within the film. The BP film can be considered single crystalline with a variation of defect densities and types. The crystal quality achieved should be sufficient for the requirement of a neutron detector.

High resistivity semiconductors are essential for niche applications, such as radiation detectors. The high resistivity in detector oriented semiconductors must be achieved by low concentration of free carriers rather than by low mobility. The semi-insulating resistivity implies semi-intrinsic charge carrier concentrations and it is seldom achieved by high material purity and crystalline perfection. In most cases a compensation mechanism is responsible for the low material conductivity. Such is believed to be the case of wide bandgap II-VI compound semiconductors (CdTe, Cd1-xZnxTe, etc.). Depositing electrical contacts with reproducible macroscopic behavior remains challenging. Furthermore, detailed understanding of electrical contacts to such materials remains elusive.
In this study finite element calculations are used to investigate the contact-semiconductor interfaces for various compensation mechanisms. The computation results shed light on the conditions inside the metal-semiconductor-metal (MSM) structures with various contacts. It is shown that pure Schottky contacts could yield a variety of external current-voltage (I-V) characteristics depending on the compensating species. This is an important insight since often the “nature” of the contacts, namely the resulting energy band structure is deduced from the macroscopic I-V characteristics.

Alkaline earth halides, elpasolites and rare earth halides are very interesting because many compositions from these crystal families provide efficient Ce³⁺/ Eu²⁺ luminescence, good proportionality and good energy resolution. This paper presents successful progress made in large diameter, crack-free and transparent SrI:Eu and CLYC:Ce single crystals with diameters of 1", 1.3",1.5" and 2" were all successfully grown using the vertical Bridgman technique. Recent investigations of Ce³⁺ doped Cs₂LiYCl₆ (CLYC) scintillator material (and other elpasolites: Cs2LiLaBr6 and Cs2LiYBr6) show very promising results for gamma ray and thermal neutron detection. this paper also cover growth and properties of larger Cs₂LiYCl₆ (CLYC) crystals. which is possible with the help of pulse shape discrimination (PSD). PSD allows for recognition of an incident particle&’s nature based on the shape of the corresponding scintillation pulse. CLYC has the potential to minimize the cost and complexity of dual sensing gamma ray and neutron spectrometers.

The purpose of this research is to grow and characterize SrI2: Eu2+ single crystals for room temperature radiation detector applications and high resolution x-ray imaging applications. Single crystal SrI2- scintillators exhibit scintillation properties that make them superior to other scintillators like a light yield as high as 95 000 ph/MeV, energy resolution down to 4.1% at 662 keV (FWHM) for ~1 cm3 crystal wrapped in aluminum foil.
The use of the SrI2-scintillator till now is limited to spectroscopic applications only due to the hygroscopic nature. In this research work, we embedded thin SrI2- scintillator screens between aluminum slides (210 µm thick) and glass slides (150 - 170 µm thick) and integrated them in a high resolution x-ray imaging setup at the ANKA/Topo Tomo beamline in Karlsruhe. The detector is made up of a high resolution microscope (Optique Peter, France) coupled to a PCO4000 CCD camera. The total magnification of the setup is 5x (objective 2x/NA = 0.08 and 2.5x eye piece). According to the Rayleigh criterion, the resolution limit is 3.8 µm and the effective pixel size 1.8 x 1.8 µm2.
At 12 - 15 keV x-ray energies, copper mesh images and radiography images were recorded with thin SrI2-screens and YAG-screens that served as reference samples. The resolutions of the images recorded with thin SrI2-screens were similar to those recorded with thin YAG-screens, despite the embedding layers. With thin SrI2-screens the integration was reduced by a factor 4 due to the high light yield of the SrI2-scintillator. Our results show that thin SrI2-screens are possible candidates for high resolution x-ray imaging.

Ionizing radiation impinging on a doped scintillator material creates electron-hole pairs that diffuse through the lattice to eventually reach the activator site. During the synthesis and growth of these materials, impurities and vacancies may exist and create electron and hole traps. If these trap states are accessible energetically during the charge transfer process, they can impact the brightness and the emission delay times of the scintillator. Experiments suggest the presence of oxygen impurities as well as F-centers in known europium-doped barium halide scintillators, which are believed to influence in their luminosity and proportionality. We have performed Density Functional Theory based (GGA+U) band structure calculations for this family of scintillator compounds to assess the contribution of these defects in the transfer process of the electrons and holes to the Eu site. Preliminary results show reasonable correspondence between the theoretical energy gaps introduced by the various defects and the experimental absorption spectra of the defect-containing samples.

A new x-ray luminescence based spectrometer was developed and installed to examine the scintillation properties of materials while revealing the origins of luminescence and investigating trapping defects. Measurements were performed on a number of important scintillation materials. The measured x-ray luminescence spectra provide information about the spectral range and the scintillation efficiency and linearity. The efficiency of charge-carriers production due to x-ray, their energy transfer to the luminescence centers and the efficiency of luminescence are all reflected in the efficiency of x-ray luminescence. The spectrometer offers unique tool and tremendous advantages for the study of scintillation materials

Heavy metal iodides such as mercuric iodide, bismuth tri-iodide and mercuric bromide are among the materials with the best properties for room temperature semiconductor detection. The growth of crystals and crystalline layers of these materials has been subject of interest during the last decades and is a challenge for worldwide crystal growers. The use of nanostructures of these compounds -recently synthesized- used as precursors for nucleation of oriented layers, opens new perspectives for layer growth and, through them, to the possibility of a new generation of detectors with enhanced imaging performances. Mercuric iodide, bismuth tri-iodide and mercuric bromide were synthesized in this work by the suspension method, using 1-octadecene as suspension agent, and Hg(NO₃)₂.H₂O as mercuric source and Bi(NO₃)₃.5H₂O as bismuth source. We used I₂ and NaBr as halide sources. The best synthesis conditions were as follows: for mercuric iodide, the synthesis temperature was 70°C for 240 min, and we also added octadecylamine (ODA) as capping and reduction agent. For bismuth tri-iodide and mercuric bromide, we added the reagents at 100°C and 80°C respectively, maintained that temperature for 240 min, and finally we raised the temperature to 200°C for 10 min. Compounds identity was confirmed by X-ray diffraction (XRD) and energy dispersive spectrometry (EDS). We characterized the nanostructures by transmission (TEM), high resolution (HR-TEM) and scanning (SEM) electron microscopy. In all cases we obtained crystalline nanostructures in the range 10-80 nm in size appropriated for film nucleation, opening interesting perspectives for the future of radiation detectors.

Lead iodide has been widely studied as a candidate semi-conductor gamma ray detector material and more recently as an ultra-fast scintillator for time-of-flight applications. The ultra-fast scintillation and near band-edge emission of undoped PbI2 at low temperatures are attributed to shallow native donor-acceptor recombinations. This work systematically presents first-principles electronic structure calculations of donor and acceptor levels introduced by isolated intrinsic defects and defect complexes, including a bi-vacancy pair, Schottky defect and Frenkel pairs. We also calculated the formation energies of each type of defect in all relevant charge states to determine which would be the most stable for different experimental conditions. Our study shows that only lead vacancies yield shallow acceptor levels, while donor levels introduced by isolated iodine vacancies and interstitial lead are relatively deep unless there is a nearby lead vacancy forming a defect complex. Such complexes provide both shallow donor and acceptor levels that can explain the near band-edge emission.

Material homogeneity is a critical factor in achieving high-performance in CdZnTe (CZT) radiation detectors. The subgrain boundaries accumulating impurities and secondary phases result in significant carrier trapping and, consequently, cause broadening of the energy spectra of CZT detectors. Our group at Brookhaven National Laboratory (BNL) conducts systematic studies, focusing on a better understanding of the roles of crystal defects in CZT detectors. We routinely employ infrared transmission microscopy, white beam X-ray diffraction topography, and high-spatial-resolution X-ray response mapping to correlate crystal defects and the devices&’ performances. In this talk, we will present our recent results from characterization of CZT pixel detectors to reveal the roles of subgrain boundaries in CZT detectors.

Thallium bromide (TlBr) is a promising material for room-temperature gamma radiation detectors, having recently been demonstrated to exhibit spectroscopic response with energy resolution approaching 1% at 662 keV for pixelated detectors. However, performance degradation over time, a phenomenon often called “polarization,” remains a problem. A major source of this phenomenon appears to be related to the metal contacts used in device fabrication. Here, we present an analysis based on first-principles calculations using density functional theory of the effects of more than a dozen metals at the anode and cathode electrodes of TlBr detectors. We analyze the metal contacts as sources of impurities that can penetrate into the bulk of the TlBr material and introduce carrier recombination centers. We compare the solubilities, kinetics, and electronic properties of the different metals. Finally, we suggest optimal contact metals to maximize detector performance and compare to experimental data for validation.

Prepared by LLNL under Contract DE-AC52-07NA27344; funded by the National Nuclear Security Administration Office of Nonproliferation and Verification Research and Development (NA-22).

TlBr is a promising semiconductor material for room-temperature radiation detection. However, ionic conductivity due to vacancy diffusion and related polarization phenomenon have been major challenges to the development of TlBr detectors. It has recently been proposed that doping TlBr with donor and acceptor impurities can suppress the ionic conductivity and the polarization phenomenon due to strong binding between impurities and vacancies, which renders vacancies immobile. In this paper, we show by first-principles calculations that the impurity-vacancy binding in TlBr is very weak as a result of large dielectric screening. Furthermore, the presence of charged impurities in TlBr does not reduce the concentration of isolated vacancies regardless the binding strength between the impurity and the vacancy. This is because isolated vacancies in TlBr can equilibrate with their reservoirs (e.g., surfaces can be sources of vacancies.) to maintain their equilibrium concentrations even at room temperature. Therefore, doping TlBr with donor and acceptor impurities will not suppress ionic conductivity and polarization phenomenon in TlBr radiation detectors.

For X- and γ-ray detection, dimensional reduction can be applied to produce new, functional semiconductor materials. Per this approach, compound semiconductor materials with high density can be engineered with wider or narrower band gaps by the addition of a salt to reduce lattice dimensionality. Recently, we have modified this approach to use highly dense Tl as the cation in the salt added. The incorporation of Tl has the added benefit of higher covalency in the materials produced relative to alkali metal cations as a result of larger atomic orbitals, and will serve to increase band dispersion and mobility of the electron and hole carriers generated by nuclear radiation. This last aspect is especially relevant as the figure of merit for γ-ray detector materials is the µtau; product, where µ is the mobility of the carriers and tau; is the carrier lifetime. Highly dense Tl salts have been used to produce the promising detector materials Tl2SnS3 and Tl2GeS3. Tl2SnS3, crystallizing in the C2/c space group, is derived from the addition of Tl2S (d=8.39 g/cm^3) to SnS2 (P-3m1, d=4.57 g/cm^3). This addition results in a transition from a 2-dimensional crystal structure composed of sheets of side-sharing [SnS6] octahedra in SnS2 to a 1-dimensional structure with linear chains of corner-linked [SnS4]4- tetrahedral units, as well as a larger density, in Tl2SnS3 (d=6.54 g/cm^3). A similar result is seen in the addition of Tl2S to GeS2 (FDD2, d=3.03 g/cm^3), a 3-dimensional structure of corner-sharing [GeS4] tetrahedra, to produce Tl2GeS3 (P-1, d=5.95 g/cm^3), a “0-dimensional” structure with isolated molecular [Ge2S6]4- units. Our studies on these Tl-based compounds show that their band gaps (1.65 eV

Semiconductor materials have shown promise as ionizing radiation detection devices; however, in order to be used as a neutron detector, these materials require the addition of a nucleus with a large neutron absorption cross section (such as 10B or 6L) to capture thermal neutrons and convert them into directly detectable particles. A semiconducting material which contains the neutron absorber within its regular stoichiometry has the potential to be more efficient than a layered or heterogeneous device at transferring the kinetic energy of the charged particle reaction products into the semiconducting material. One class of materials that have shown promise is Li-containing AIBIIIXVI2 compounds such as LiGaTe2, LiGaSe2, or LiInSe2. These materials have wide band gaps (2-3.5 eV) appropriate for room-temperature detection of thermal neutrons. A vacuum distillation process provided high purity 6Li metal for chalcopyrite synthesis and resulted in single crystals with reasonable resistivity. Photo response, electrical resistivity, and carrier mobility have been determined for these crystals. An alpha particle flux was observed for LiInSe2, and represents the first radiation response observed for the class of lithium containing semiconductor compounds. Additional radiation measurements indicated that a 6mm x 7mm x 1.33mm crystal of LiInSe2 detected gamma rays and despite being composed of natural abundance lithium, responded to thermal neutrons as well.

Boron Monophosphide (BP) is a promising material for use as a room temperature semiconductor detector of thermal neutrons. The absorption of a thermal neutron by a 10B nucleus in BP can yield 2.3MeV of energy, which in solid-state BP can yield sim;0.5 million electron-hole pairs that would be detectable with minimal amplification in a device. The parameters necessary to deposit oriented zincblende boron phosphide (BP) thin films on 4° off-axis C-face 4H-SiC(0001) substrates by chemical vapor deposition are reported. Ultra high purity B2H6 and PH3 were used as reactive precursors, with hydrogen as the carrier gas. The PH3 to B2H6 flow rate ratio and substrate temperature was adjusted to obtain good bulk morphology of the BP films. The best growths, exhibiting heteroepitaxial interfaces and single crystalline morphology, were observed in samples with average growth rates up to 6mu;m/hr using PH3 to B2H6 flow rate ratios of 5 to 1. Higher growth rates were obtained by increasing the B2H6 flow rate, but the morphology became disordered. The BP films were deposited on the substrate at temperatures between 800°C-900°C. The surface characteristics were investigated by SEM and the crystalline properties of the films were studied by XRD, TEM and Raman spectroscopy.

Recently, neutrons have been proposed as a new radiation source, due to their high penetration for heavy elements, and the fabrication of neutron semiconductor detector is expected. For this research we aimed to obtain a new neutron imaging device by semiconductor compound with boron (B) converter. We have suggested a neutron detector using (n, α) reactions generated by B atom. To distinguish neutrons from the background γ-rays, we selected GaN - a wide band gap semiconductor with low γ-rays sensitivity. We tried to fabricate a neutron semiconductor detector by using BGaN.
In this work, BGaN was grown on Al2O3(0001) substrates by metal organic vapor phase epitaxy (MOVPE). On the top of the device, a gold electrode has been deposited by vacuum evaporation to obtain the desired structures: Au/BGaN/GaN/Al2O3. The BGaN samples were evaluated by I-V measurements with α-rays and neutron irradiation.
In order to detect the neutrons using (n, α) reaction, the detection sensitivity of BGaN for α-rays is studied. The IVt characteristic of the double Schottky structure BGaN on applied voltage was measured under irradiation with the α-rays from americium (Am). The current was detected when irradiating with the α-rays, while no current flow was with no irradiation. The result of detection sensitivity of BGaN to α-rays reveals that the neutron detector using BGaN could detect α-ray resulting from B atoms by neutron irradiation. Additionally, the IVt characteristic of the BGaN is measured by irradiating with the γ-rays. By irradiating with γ-rays the current does not flow, indicating that the BGaN has the sensitivity for the α-rays, while γ-rays are not detected. BGaN is expected as a material which detects only neutrons, because γ-rays generated from the neutron source was not detected.
Finally, the detection sensitivity of BGaN for neutron is studied. In the result of neutron irradiation evaluation, we realized to get the detection signal of neutron by BGaN. These results indicate that BGaN neutron detector using (n, α) reaction can be expected as a new neutron detection material.

GaN is one of the most radiation-resistant semiconductors available today, yet it presents new opportunities for detection and quantification of ionizing irradiation, particularly in high radiation environments. Depth resolved cathodoluminescence spectroscopy (DRCLS) reveals GaN surface properties that manifest fast and thermal neutron irradiation optically and electronically. DRCLS measured specific native point defects vs. depth on a nm scale and vs. neutron fluence. Bulk undoped (n-type) and Fe doped (semi-insulating or SI) GaN wafers were grown by hydride vapor phase epitaxy (HVPE) with low dislocation density (5x10⁶ cm^-2) and irradiated in the core region at The Ohio State University Research Reactor (OSURR). Samples were irradiated at about 80°C with either full reactor spectrum (fast plus thermal) or enclosed in Cd to screen out thermal neutrons. Fluence levels were varied from 10¹⁴ to 10¹⁶ n/cm² for fast neutrons and from 4 × 10¹⁴ to 2.6 × 10¹⁶ n/cm² for thermal neutrons. Neutron irradiation induces several striking effects including changes in two common native defects in GaN, a “yellow band” (YL) Ga vacancy defect with 2.2 eV luminescence and a “blue band” (BL) defect with 2.9 eV emission still under debate. Remarkably, fast neutron fluence improves the crystal quality of the irradiated GaN samples, primarily tens of nm below the free surface. YL emission decreases 4x with 1.4 × 10¹⁶ n/cm<² fast neutrons, while BL emission decreases by 2x over the same range. However, within 5 nm of the surface, these defects behave qualitatively different. Here, fast plus thermal neutrons induce a 4.4x monotonic increase in YL DRCLS intensity up to fluences of 10¹⁵ n/cm² then a ca.20% decrease between 10¹⁵ and 10¹⁶ n/cm². Fast neutron fluence alone produces only a 2.6x YL increase at 10¹⁵ n/cm², indicating extra defects created by the thermal neutrons. Likewise, BL intensity increases nearly 4x with 10¹⁴ n/cm² fast plus thermal fluence (vs. 2x for fast neutron fluence alone), then decreases steadily by 4x between10¹⁴ and 10¹⁶ n/cm², indicating additional BL emission increase due to slow neutrons. Larger near-surface defect increases are consistent with the additional defect creation by thermal neutrons, given their higher cross section entering the surface. Current-voltage (I-V) measurements of MOCVD-grown (Si= 10¹⁶ cm^-3) on sapphire correlate with near-surface defect emissions. Sheet and contact GaN resistance decrease with 10¹⁴ n/cm² fluence then increase by 10 - 20x at 10¹⁵ n/cm². Correspondingly, near-surface YL defects decrease at 10¹⁴ n/cm² then increase by 10x at 10¹⁵ cm^-3 thermal plus fast neutrons but only 2x for fast neutrons alone. Near-surface BL defects show similar effects. These results reveal the native point defects that are created by thermal neutrons at the near-surface region of GaN, dramatically altering scattering centers and current transport and thus providing a new avenue for neutron detection in high radiation environments.

We have developed a method called optical transient positron spectroscopy [1, 2] and apply it to study the optically induced carrier trapping and charge transfer processes in diamond. The decay of the illumination effect provides information about the recombination dynamics of the system, providing an unambiguous identification of the defect in question at the same time. In addition, the relaxation of the excited state does not need to be luminescent in order to be observable: only the charge state of the defect involved needs to change (in both excitation and relaxation). Excitation power and temperature dependence of the steady-state, of the onset of the excitation effects and of the decay after excitation provide additional data that allow self-consistent evaluation of defect-related optical constants and development of microscopic models of charge transfer processes. The only requirement is that the charge state of the vacancy can be manipulated using optical excitation with the vacancy being populated/depopulated to a concentration that can be detected using positron annihilation spectroscopy (>10¹⁵ cm^minus;3).
Positron annihilation spectroscopy is a method sensitive to neutral and negatively charged open volume defects. The decreased electron density in a vacancy manifests itself as an increase of positron lifetime and the narrowing of the 511 keV photo-peak in the annihilation gamma spectrum, compared to a defect-free crystal. Positron lifetime spectroscopy provides information on the atomic structure, the charge state and often the concentration of the vacancies while the Doppler broadening of the 511 keV photo-peak allows the identification of atoms surrounding the vacancy defects. Positron annihilation spectroscopy has been efficiently used to identify and quantify technologically important vacancy-related defects in, for example, SiC and ZnO.
[1] J.-M. Mäki, F. Tuomisto, A. Varpula, D. Fisher, R. U. A. Khan, and P. M. Martineau, Phys. Rev. Lett. 107, 217403 (2011).
[2] J.-M. Mäki, T. Kuittinen, E. Korhonen, and F. Tuomisto, New J. Phys. 14, 035023 (2012).

Elpasolites are a large family of halides that have attracted considerable interest for their potential applications in room-temperature radiation detection. Cs2LiYCl6 is one of the most studied elpasolite scintillators. We show hybrid density functional calculations for electronic structure, energetics of small polarons and self-trapped excitons, and excitation of luminescence centers (Ce dopants) in Cs2LiYCl6. Approaches of using electronic structure engineering for finding new elpasolite scintillators with improved light yield and scintillation response will be discussed. We will also show results for energy transport and scintillation in LiCaAlF6 scintillator. Ce activated LiCaAlF6 suffers from light reabsorption from F centers, which can be produced under X- or gamma-ray irradiation. We suggest that lowering the Fermi level by acceptor doping will stabilize positively charged F+ center against negatively charged F- center and reduce the reabsorption of Ce emission by F centers in LiCAF:Ce3+, thereby increasing light yield and radiation resistance.

The electronic structure of ThO2 and U:ThO2 single crystals, grown through a novel hydrothermal methodology, was investigated through X-ray photoemission spectroscopy (XPS), ultraviolet photoemission spectroscopy (UPS), and inverse photoemission spectroscopy (IPES). The geometrical structure for both single crystal thorium dioxide and uranium doped (1%) thorium dioxide establishes that they are indeed excellent single crystal samples. With XPS, there is evidence of a surface to bulk core level shift, indicating either a different surface oxide, or a surface electronic structure that differs from the bulk. Efforts are made to compare the Debye-Waller temperature, determined from XPS, with that obtained from EXAFS. The electronic structure of the U:ThO2 and ThO2 single crystals were investigated using a combination of UPS and IPES to characterize the occupied states in the vicinity of the valence band maximum and the unoccupied states at the conduction band minimum to ascertain the placement of the band gap. There is little evidence that the U doping perturbs the electronic structure of ThO2 significantly. Characterization of the electronic structure of these single crystal actinide oxides is an important initial step towards the ultimate goal of creating highly sensitive actinide based neutron detectors.
This work was supported by the Defense Threat Reduction Agency, the Air Force Weapons Center, and the NSF (at UNL). The views expressed in this article are those of the authors and do not reflect the official policy or position of the Air Force, Department of Defense or the U.S. Government.