Douglas S. McGregor

Design considerations for thin film coated semiconductor thermal neutron detectors-I: basics regarding alpha particle emitting neutron reactive films

Semiconductor-based thermal neutron detectors provide a compact technology for neutron detection and imaging. Such devices can be produced by externally coatingsemiconductor-charg ed-particle detectors with neutron reactive films that convert free neutrons into charged-particle reaction products. Commonly used films for such devices utilize the 10 B(n,a) 7 Li reaction or the 6 Li(n,a) 3 H reaction, which are attractive due to the relatively high energies imparted to the reaction products. Unfortunately, thin film or ‘‘foil’’ type thermal neutron detectors suffer from self-absorption effects that ultimately limit neutron detection efficiency. Design considerations that maximize the efficiency and performance of such devices are discussed. Theoretical and experimental results from front coated, back coated, and ‘‘sandwich’’ designs are presented. r 2002 Elsevier Science B.V. All rights reserved. PACS: 29.40.W

Room-temperature compound semiconductor radiation detectors

A summary is presented of the present status of several compound semiconductor radiation detectors, including detectors fabricated from GaAs, HgI2, CdTe, Cd1−xZnxTe and PbI2.

Single-charge-carrier-type sensing with an insulated Frisch ring CdZnTe semiconductor radiation detector

Performance optimization of an insulated Frisch ring design was investigated for a 3×3×6 mm CdZnTe planar semiconductor detector. The Frisch ring was composed of copper and was insulated from the detector surface with Teflon. Optimization variables included the Frisch ring length and the bias voltage. Optimized overall device performance was found using a 5 mm long Frisch ring extending from the cathode toward the anode, leaving a 1 mm separation between the Frisch ring and the anode. The best energy resolution observed was 1.7% full width at half maximum at 662 keV with the ring extending 4 mm from the cathode toward the anode.

Evidence for field enhanced electron capture by EL2 centers in semi-insulating GaAs and the effect on GaAs radiation detectors

The performance of Schottky contact semiconductor radiation detectors fabricated from semi‐insulating GaAs is highly sensitive to charged impurities and defects in the material. The observed behavior of semi‐insulating GaAs Schottky barrier alpha particle detectors does not match well with models that treat the semi‐insulating material as either perfectly intrinsic or as material with deep donors (EL2) of constant capture cross section compensated with shallow acceptors. We propose an explanation for the discrepancy based on enhanced capture of electrons by EL2 centers at high electric fields and the resulting formation of a quasineutral region in the GaAs. Presented is a simple model including field enhanced electron capture which shows good agreement with experimental alpha particle pulse height measurements.

Thin-film-coated bulk GaAs detectors for thermal and fast neutron measurements

GaAs-based structures are presently under investigation as the substrate for 10 B-coated and polyethylene-coated neutron detectors. The semi-insulating (SI) GaAs-based devices operate at low bias voltages by employing the truncated electric field effect, which allows for acceptable signals to be produced with bias voltages ranging between 10 and 50 V. At this time, the 10 B-coated devices have demonstrated over 3.5% intrinsic thermal neutron detection efficiency with reactive films 1.8mm thick. Relatively high neutron/g-ray rejection ratios can be achieved with an appropriate choice of lower level discriminator setting. Polyethylene-coated GaAs devices are being studied as fast neutron detectors and have shown evidence of (n,p) reactions for 14 MeV neutrons. Theoretical neutron responses and experimental neutron detection data are presented and compared. # 2001 Elsevier Science B.V. All rights reserved.

Single charge carrier type sensing with a parallel strip pseudo-Frisch-grid CdZnTe semiconductor radiation detector

Wide band gap compound semiconducting materials offer great promise for use as room temperature operated radiation detectors. The most common semiconductor radiation detector design incorporates the use of a semiconducting block of material with metal contacts applied at opposite ends of the block. A voltage is applied across the block, which produces an internal electric field capable of drifting free charge carriers to the detector contacts. Gamma ray interactions occurring in the device excite electron-hole pairs which are separated by the applied electric field across the device bulk. Electrons are drifted towards the anode, and holes are drifted towards the cathode. The induced charge produced by the moving free charge carriers can be measured by an external circuit. Shockley 1 and Ramo 2 derived the dependence of the induced current and induced charge produced by point charges moving in an electric field, which was later shown to apply to semiconductor detectors as well. 3‐5 The Shockley‐Ramo theorem states that the induced charge that appears at the terminals of a planar device from moving point charges is proportional to the distance displaced by the moving point charges, regardless of the presence of space charge. Hence, the change in induced charge Q* can be represented by

Present status of undoped semi-insulating LEC bulk GaAs as a radiation spectrometer

Abstract Bulk GaAs has undergone extensive research by several groups in order to ascertain its usefulness as a room temperature radiation spectrometer. The results of an experimental program studying the properties of detectors fabricated from bulk GaAs are summarized in this paper. Electric field models of the active region are compared with measured results. Limitations of bulk LEC GaAs as a material for radiation spectrometers are discussed.

CdZnTe semiconductor parallel strip Frisch grid radiation detectors

CdZnTe wide band gap compound semiconducting material offers promise as a room temperature operated gamma ray spectrometer. Position-dependent free charge carrier losses during transport can prevent efficient charge carrier extraction from semiconductor detectors and severely reduce energy resolution. Hole trapping losses in CdZnTe radiation detectors are far worse than electron trapping losses and resolution degradation in CdZnTe detectors results primarily from severe hole trapping during transport. Coplanar radiation detectors improve energy resolution by sensing the induced charge primarily from the motion of electrons. Demonstrated is an alternative approach to single free charge carrier sensing, in which a parallel strip Frisch grid is fabricated on either side of a parallelepiped block. The detectors are three terminal devices, but require only one preamplifier for the output signal. The prototype devices demonstrate a considerable increase in energy resolution when operated in the true Frisch grid mode rather than the planar mode, with a demonstrated room temperature energy resolution for 662 keV gamma rays of 5.91% at FWHM for a 10 mm/spl times/2 mm/spl times/10 mm device. Presently, high surface leakage currents prevent large voltages from being applied to the devices, which ultimately reduces their maximum achievable energy resolution. Further improvements are expected with the realization of reduced surface leakage currents.

Self-biased boron-10 coated high-purity epitaxial GaAs thermal neutron detectors

Semiconductor thermal neutron detection devices based on /sup 10/B-coated high-purity GaAs films were investigated. The fundamental device consisted of high-purity /spl nu/-type epitaxial GaAs films grown onto n-type GaAs substrates. Two blocking contact adaptations were applied to the high-purity /spl nu/-type GaAs regions: 2000 /spl Aring/ thick p+GaAs blocking contacts and 200 /spl Aring/ thick Schottky blocking contacts. The /spl nu/-type GaAs active layers ranged between 1 micron and 5 microns in thickness. The device sensitive areas were 3 mm/spl times/3 mm, each of which was coated with a 1.5 mm diameter film of 98% enriched high-purity /sup 10/B. The built-in potential of the blocking contact interface was sufficient to operate the devices, and no external voltage bias was necessary to operate the detectors. Preliminary calculations on intrinsic detection efficiency indicate values between 1.6% and 2.6%.

Performance characteristics of Frisch-ring CdZnTe detectors

The performance characteristics of Frisch-ring CdZnTe (CZT) detectors are described and compared with other types of CZT devices. The Frisch-ring detector is a bar-shaped CZT crystal with a geometrical aspect ratio of /spl sim/1:2. The side surfaces of the detector are coated with an insulating layer followed by a metal layer deposited directly upon the insulator. The simple design operates as a single-carrier device. Despite the simplicity of this approach, its performance depends on many factors that are still not fully understood. We describe results of testing several detectors fabricated from CZT material produced by different vendors and compare the results with numerical simulations of these devices.