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Abstract: A 4.7 ×4.7×9.5mm3 CdZnTe Frisch collar device was characterized through probing the device with a highly collimated 662keV gamma ray source. In a systematic series of experiments, the detector was probed along the length and width with a gamma ray source using a 43.0mm long Pb-collimator with a 0.6mm circular hole. The detector was probed along the central line under different operating voltages of 1200, 1000, 800, 600 and 400V. The experimental results correlated well to charge collection calculations for a modeled device with the same size and operating conditions. It was proved that, unlike the planar configuration, the charge collection efficiency profile along the length of Frisch collar device is considerably improved. The CdZnTe raw materials for this study were acquired from Redlen Technologies, and the Frisch collar device was fabricated and characterized at S.M.A.R.T. Laboratory at Kansas State University. [ABSTRACT FROM AUTHOR]

Abstract: A collimated 198Au source was used to determine the charge collection efficiency (CCE) at several locations along the length of a 3.4×3.4×5.5mm3 CdZnTe bar detector, both in planar configuration and with Frisch collars of varying length. For each configuration, a 0.50-mm-long region spanning the width of the device was irradiated with 411-keV gamma rays produced by a neutron-activated gold foil. Irradiation began at the cathode and stepped in 0.50-mm steps toward the anode, with a spectrum being collected at each location. By observing the channel location of the full-energy peak in each collected spectrum, an average CCE was determined for each irradiated region. The CCE was found to vary nearly linearly along the length of the device in the planar configuration, starting at a peak value of 89% and dropping to a minimum measured value of 26% near the anode. The addition of a Frisch collar covering the entire length of the crystal greatly altered the CCE profile, which remained near 87% for approximately two-thirds of the length, then sharply dropped near the anode. Results were confirmed by theoretical models. Further CCE mapping was also completed for devices with Frisch collars of various lengths. Those results are reported as well. [Copyright &y& Elsevier]

This paper presents an enabling technology for high-efficiency fast-neutron detection for the Transient Reactor Test (TREAT) Facility hodoscope. The hodoscope is currently outfitted with 25.4-mm long Hornyak buttons for fast-neutron detection. However, they require improvements, as they suffer from low detection efficiency, Ĉerenkov radiation contaminating the signal, and non-linearity of detector response at large transients. To address these issues, a layered ZnS:Ag device has been constructed. The layered design provides various advantages: it incorporates greater scintillation volume, Ĉerenkov background is significantly reduced by eliminating the need for additional light guides, and the detection process is simplified by achieving gamma-ray rejection using only pulse-height discrimination. The result is a greater number of fast neutron counts with reduced background noise contaminating the signal. On being irradiated with a 252 Cf source (whose neutron energy spectrum closely mimics a reactor fission spectrum), a 25.4-mm long layered device was found to exhibit an intrinsic fast-neutron detection efficiency of 8.1% for neutrons with an average energy of 2 MeV, while a 43-mm long device allows for a greater number of fast neutron to be detected, increasing the efficiency to 9.1%. Further lengthening of the device up to 20 cm has been numerically predicted to exhibit even higher efficiencies. The detectors can be mass-produced easily and inexpensively. [ABSTRACT FROM AUTHOR]

The performance of traditional Hornyak buttons and two proposed variants for fast-neutron hodoscope applications was evaluated using Geant4. The Hornyak button is a ZnS(Ag)-based device previously deployed at the Idaho National Laboratory’s TRansient REActor Test Facility (better known as TREAT) for monitoring fast neutrons emitted during pulsing of fissile fuel samples. Past use of these devices relied on pulse-shape discrimination to reduce the significant levels of background Cherenkov radiation. Proposed are two simple designs that reduce the overall light guide mass (here, polymethyl methacrylate or PMMA), employ silicon photomultipliers (SiPMs), and can be operated using pulse-height discrimination alone to eliminate background noise to acceptable levels. Geant4 was first used to model a traditional Hornyak button, and for assumed, hodoscope-like conditions, an intrinsic efficiency of 0.35% for mono-directional fission neutrons was predicted. The predicted efficiency is in reasonably good agreement with experimental data from the literature and, hence, served to validate the physics models and approximations employed. Geant4 models were then developed to optimize the materials and geometries of two alternatives to the Hornyak button, one based on a homogeneous mixture of ZnS(Ag) and PMMA, and one based on alternating layers of ZnS(Ag) and PMMA oriented perpendicular to the incident neutron beam. For the same radiation environment, optimized, 5-cm long (along the beam path) devices of the homogeneous and layered designs were predicted to have efficiencies of approximately 1.3% and 3.3%, respectively. For longer devices, i.e., lengths larger than 25 cm, these efficiencies were shown to peak at approximately 2.2% and 5.9%, respectively. Moreover, both designs were shown to discriminate Cherenkov noise intrinsically by using an appropriate pulse-height discriminator level, i.e., pulse-shape discrimination is not needed for these devices. [ABSTRACT FROM AUTHOR]

Abstract: An alternative method for the detection of thermal neutrons has been investigated utilizing the Pockels effect with CdZnTe and LiNbO3. A photodiode was used to indicate a change in light intensity transmitted through an assembly consisting of polarizer, Pockels cell, and analyzer. Ionization due to neutron reaction products can perturb an established electric field within the cell, thus changing the transmitted light intensity through the assembly. The CdZnTe cell demonstrated a repeatable change in transmitted light intensity, increasing with neutron flux, marking the first recorded use of this technology to detect neutrons. However, the LiNbO3 cell gave no response to neutron irradiation, and is assumed to be a result of the extremely short carrier lifetimes found in the material. [ABSTRACT FROM AUTHOR]

The Micro-Layered Fast-Neutron Detector (MLFD) is a proton-recoil scintillation detector constituting microscale alternating layers of hydrogenous medium and ZnS:Ag. The device employs a peripheral photon detection for maximizing light collection while minimizing gamma-ray background. The layered configuration offers several novel advantages inherently by design, e.g., it overcomes the light-opacity limitation of polycrystalline ZnS, recoil protons have a high probability of reacting with the scintillation grains in their forward-directional path, the design and photomultiplier orientation eliminates the need for extraneous light guides, and minimizes Ĉerenkov generation and collection. The most striking ability of the MLFD is that its efficiency can be scaled higher by simply adding layers and increasing the length, up to 20 cm. Presently, a 40-mm MLFD has an intrinsic detection efficiency of 9.2% for bare 252Cf. Described is a comparison between experimental results and a theoretical model, the respective performances of which differ in high energy regions. The differences in the two models suggest that epoxy in the MLFD contributes to an increased count rate in higher channels. [ABSTRACT FROM AUTHOR]

Abstract: Direct measurement of the amount of Pu and U in spent nuclear fuel represents a challenge for the safeguards community. Ideally, the characteristic gamma-ray emission lines from different isotopes provide an observable suitable for this task. However, these lines are generally lost in the fierce flux of radiation emitted by the fuel. The rates are so high that detector dead times limit measurements to only very small solid angles of the fuel. Only through the use of carefully designed view ports and long dwell times are such measurements possible. Recent advances in multilayer grazing-incidence gamma-ray optics provide a possible means of overcoming this difficulty. With a proper optical and coating design, such optics can serve as a notch filter, passing only narrow regions of the overall spectrum to a fully shielded detector that does not view the spent fuel directly. We report on the design of a mirror system and a number of experimental measurements. [Copyright &y& Elsevier]

Abstract: The spatial resolution of a collimated CdZnTe Frisch collar detector was experimentally investigated for two different tungsten collimator designs using a 198Au gamma-ray source at 412keV. A two-dimensional model for the detector–collimator–source geometry was developed and applied with some simplifying assumptions. The CdZnTe detector was fabricated from a single crystal using a copper shim as the Frisch collar. The detector fabrication process is briefly described. Pulse height spectra were collected from 241Am, 198Au and 137Cs using a 3.4×3.4×5.8mm3 CdZnTe detector utilizing an insulated Frisch collar mounted inside a tungsten collimator. The resulting energy spectra are presented. Room-temperature energy resolution of 1.72% full-width at half-maximum (FWHM) was obtained for 137Cs at 662keV without electronic correction. The two-dimensional model reasonably predicts the angular response of the CdZnTe detectors when inserted into two different collimators. [Copyright &y& Elsevier]

Abstract: Low-energy γ-ray spectra were collected from 241Am, 57Co, 133Ba, 198Au, 137Cs and 235U using a 3.4×3.4×5.7mm3 CdZnTe detector utilizing an insulated Frisch ring. The CdZnTe detector was fabricated from a single crystal and a copper shim was used as the Frisch collar. Room-temperature energy resolution of 1.45% full-width half-maximum (FWHM) was obtained for 137Cs at 661.7keV without electronic correction. The detector fabrication process is described and the resulting energy spectra are discussed. The detector fabrication process is described and the resulting energy spectra are discussed. The detector full-energy-peak intrinsic efficiency is reported for different γ-ray energies, specifically from 241Am, 57Co, 133Ba and 137Cs. [Copyright &y& Elsevier]