D.S. McGregor

Thermal neutron detection with cadmium1−x zincx telluride semiconductor detectors

Abstract Cadmium zinc telluride (CdZnTe) detectors have been used to detect thermal neutrons. The CdZnTe detectors were placed in a double diffracted thermal neutron beam and the prompt gamma ray emissions from thermal neutron interactions in the Cd have been detected. The devices clearly show the 558.6 and 651.3 keV prompt gamma ray emission peaks from 113 Cd(n, γ) 114 Cd reactions. The intrinsic detection sensitivity is dependent on the gamma ray absorption efficiency, which is measured to be approximately (3.7±1.9)% at 558.6 keV for a 3 mm × 10 mm × 10 mm CdZnTe device.

Semi-insulating bulk GaAs thermal neutron imaging arrays

Prototype thermal neutron imaging arrays have been fabricated from semi-insulating (SI) bulk GaAs. The arrays are 1 mm square Schottky diodes arranged in a 5/spl times/5 matrix. GaAs Schottky barrier radiation detectors are relatively radiation hard and can withstand higher neutron and gamma ray exposure fields than MOS-based Si diode imaging arrays. The devices use /sup 10/B to convert incident thermal neutrons to energetic Li ions and alpha particles. The truncated field effect observed with SI bulk GaAs detectors produces high and low field regions in the device. Electron-hole pairs produced in the active (or high field) region of the device contribute to the observed induced charge, whereas electron-hole pairs produced in the low field region contribute very little to the induced charge. The effect is manipulated to reduce the background gamma ray interaction rate in the devices. Preliminary results show no indication of device degradation after exposure to a total thermal neutron fluence of 1.73/spl times/10/sup 13/ n/cm/sup 2/. Images have been formed of 1, 1.5, and 2 mm holes and crosses from 2 mm thick Cd templates.

Material properties and room-temperature nuclear detector response of wide bandgap semiconductors

Several semiconductor materials for room-temperature X-ray and gamma-ray detectors, including HgI2, Cd1−xZnxTe (CZT), GaAs, and Pbl2 have been studied at Sandia National Laboratories, California. A comparison of the spectral response of these detectors will be given and related to material properties, such as charge carrier drift length, crystal purity, structural perfection, and material stoichiometry, as well as to the crystal growth techniques and device fabrication processes published elsewhere. Room-temperature detector spectral responses for each of these materials are presented, for photon energies in the range of 5.9 to 662 keV. CZT and HgI2 detectors demonstrate excellent energy resolution over the entire energy range, while PbI2 detectors exhibit reasonable response only up to about 30 keV. Some of the semi-insulating GaAs detectors fabricated from vertical gradient freeze materials show good spectral resolution for lower energies up to ∼60 keV, whereas other SI-GaAs detectors studied at Sandia function only as counters. Finally, some predictions on the future materials development of these wide bandgap semiconductors for room-temperature radiation detector applications will be discussed.

Semi-insulating bulk GaAs as a semiconductor thermal-neutron imaging device

Abstract Thermal-neutron Schottky barrier detector arrays fabricated from semi-insulating bulk GaAs are presently being tested. The devices use a film of 10B to convert the incident thermal-neutron field into α particles and lithium ions, either of which interact in the detector. Bulk GaAs Schottky barrier detectors are relatively radiation hard to thermal neutrons and γ rays and have shown reasonably good energy resolution for α particles. Additionally, reverse biased radiation detectors fabricated from semi-insulating bulk GaAs have been shown to have truncated electric field distributions, resulting in the formation of a high field active region and a considerably lower field dead region. The device is sensitive only to electron-hole pairs excited in the high field region, thus the truncated field effect is advantageous as an inherent γ-ray discrimination feature for neutron detectors. Preliminary results show no indication of device degradation after over 2400 hr in a thermal-neutron beam from a reactor. Images have been formed of 1, 1.5, and 2 mm holes and crosses from 2 mm thick Cd templates.

LARGE VOLUME ROOM TEMPERATURE GAMMA-RAY SPECTROMETERS FROM CDXZN1-XTE

Abstract Recent developments in Cd1−xZnxTe (CZT) crystal growth technology have enabled the production of large samples of detector grade material. In this paper we discuss the technical issues associated with building large volume CZT detectors (> 10 cm3) for use in gamma-ray spectroscopy. In particular, an analysis of the electronic noise in CZT devices and how it scales with detector size is presented, along with a discussion of methods to reduce noise in large CZT detector systems. The degradation of energy resolution caused by charge collection problems is also discussed and the use of electronic signal processing and improved device design to minimize charge collection effects are described.

X-ray diffuse scattering for evaluation of wide bandgap semiconductor nuclear radiation detectors

The crystalline perfection of solid state radiation detectors was examined using triple axis x-ray diffraction. Triple axis techniques provide a means to analyze the origin of diffraction peak broadening: the effects of strain (due to deviations in alloy composition or stoichiometry) and lattice tilts (mosaic structure) can be separated. Cd1 − xZnxTe (x ≈ 0.1), HgI2, and GaAs detector materials were studied. In the cases of Cd1 − xZnxTe and HgI2 the crystalline properties of detectors with different spectral responses to γ-radiation were determined. Increased mosaicity was universally found to be related to deteriorated detector properties. For Cd1 − xZnxTe, detectors with poor performance possessed greater levels of diffuse scatter due to lattice tilts than did high quality detectors. For GaAs, low angle grain boundaries were attributed to impaired detector performance. Additionally, in large HgI2 detectors, deviations from stoichiometry were also related to reduced performance. Interestingly, HgI2 detectors which possessed a sharp spectral response to γ-radiation but also showed polarization were of comparable crystallinity to those detectors which did not exhibit polarization effects. This initial analysis suggests that polarization is related to native point defects or chemical impurities which do not significantly alter the crystallinity of the material. Overall, within a given class of materials, improved detector performance (better spectral response) always correlated with better material quality.

Comparison of vertical gradient freeze bulk GaAs and custom grown vertical zone melt bulk GaAs as radiation spectrometers

Abstract Custom grown vertical zone melt (VZM) ingots of GaAs have been zone refined and zone leveled. The crystallinity, impurity concentrations, defect concentrations, and electrical properties of the ingots have been studied. Radiation detectors fabricated from the ingots have been compared to radiation detectors fabricated from commercially available vertical gradient freeze (VGF) material. Preliminary results suggest that electrical homogeneity may be a major factor in determining the performance of GaAs gamma ray detectors. Deep level EL2 concentrations were reduced in the VZM material, however gamma ray detectors fabricated from the material exhibited inferior resolution. Commercial semi-insulating bulk VGF GaAs material demonstrated much better resolution than the VZM detectors, giving best energy resolutions of 7.8% FWHM for 122 keV gamma rays and 5% FWHM for 356 keV gamma rays.

State of the art of wide-bandgap semiconductor nuclear radiation detectors

SummaryThe leading materials which operate as room temperature nuclear radiation detectors are HgI2, CdTe, and Cd1−xZnxTe (0.05>x>0.25). However, additional materials have also been developed, such as semi-insulating GaAs and PbI2. A comparison of the charge transport properties of all these materials will be made, followed by a discussion of each of the materials separately. Crystal growth methods of spectrometer-grade materials will be mentioned, and defects which limit their performance will be discussed. Nuclear spectra measurements with detectors fabricated from these materials, for low X-ray energies as well as for high-energy gamma-rays, will be shown. Polarization effects which occur in some detectors such as HgI2 will also be discussed. Correlation between crystalline perfection and detector performance will be shown. Results of quantitative chemical analysis of various detector materials and problems encountered in determining accurate values ofx in Cd1−xZnxTe and its homogeneity in the bulk will be presented. Finally, the present state of the art and developments for the near future will be discussed.

The investigation of custom grown vertical zone melt semi-insulating bulk gallium arsenide as a radiation spectrometer

Vertical zone melt (VZM) bulk GaAs boules have been zone refined (ZR) and zone leveled (ZL) to reduce EL2 deep donor levels and impurity concentrations with the intent of improving properties for gamma ray detectors. ZR and ZL GaAs boules had background impurity levels and deep donor EL2 concentrations near or below detectable limits. The crystal mosaic of the material at locations near the seed end was slightly superior to commercial liquid encapsulated Czochralski (LEC) material, and nearly equivalent to commercial vertical gradient freeze (VGF) material. The crystal mosaic in ZL material degraded towards the tail end. The homogeneity of the electrical properties for the ZL and ZR VZM material was inferior compared to commercially available bulk GaAs material. Post growth annealing may help to homogenize some electrical properties of the material. The charge collection efficiency of the ZR GaAs detectors was only 30% maximum, and only 25% maximum for the ZL GaAs detectors. Resulting gamma ray spectra was poor from detectors fabricated with the ZL or ZR VZM material. Detectors fabricated from material that was both ZR and ZL did not demonstrate gamma ray resolution, and operated mainly as counters. The poor spectroscopic performance is presently attributed to the inhomogeneity of the electrical properties of the ZR and ZL GaAs materials. Comparisons are made with detectors fabricated from VGF SI bulk GaAs.

Material analysis and characterization on zone refined and zone leveled vertical zone melt GaAs for radiation spectrometers

Abstract GaAs is a wide band gap (1.42 eV) semiconductor that has shown promise as a room temperature operated γ-ray detector. A practical γ-ray detector would be large in volume, hence the resistivity of the material must be high to ensure large depletion volumes and low leakage currents. Commercially available semi-insulating (SI) bulk GaAs is compensated by a balance between native defect deep donors (EL2) and residual dopant impurities. The high concentrations of electrically active deep and shallow levels are believed to contribute to electric field distortions observed in γ-ray detectors fabricated from SI bulk GaAs. Hence, the controlled reduction of native defects and contaminant impurities may yield improved bulk GaAs for γ-ray detectors. Custom grown vertical zone melt (VZM) bulk GaAs is presently under investigation as an alternative material for room temperature operated γ-ray detectors. The VZM technique allows for zone refinement (ZR) and zone leveling (ZL) of the GaAs ingots. Custom growth of the material allows for controlled changes in the bulk crystal, including deliberate reductions in EL2 and impurity concentrations. Comparisons are made to commercial vertical gradient freeze (VGF) and liquid encapsulated Czochralski (LEC) bulk GaAs.