Holly K. Gersch

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

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.

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%.

Designs for thin-film-coated semiconductor thermal neutron detectors

Thin film coated semiconductor detectors have been studied and used as neutron detectors for decades. Unfortunately, with front-irradiated devices, the basic design limits the thermal neutron detection efficiency to only 3.95% for /sup 10/B-coated devices and only 4.3% for /sup 6/LiF-coated devices. Presented in the following work are several straightforward methods to increase the thermal neutron detection efficiency for thin-film-coated semiconductor thermal neutron detectors.

Development of semiconductor detectors for fast neutron radiography.

A high-energy neutron detector has been developed using a semiconductor diode fabricated from bulk gallium arsenide wafers with a polyethylene neutron converter layer. Typical thickness of the diode layer is 250 to 300 μm with bias voltages of 30 to 150 volts. Converter thicknesses up to 2030 μm have been tested. GaAs neutron detectors offer many advantages over existing detectors including positional information, directional dependence, gamma discrimination, radiation hardness, and spectral tailoring. Polyethylene-coated detectors have been shown to detect 14 MeV neutrons directly from a D-T neutron generator without interference from gamma rays or scattered neutrons. An array of small diode detectors can be assembled to perform fast neutron radiography with direct digital readout and real-time display of the image produced. In addition, because the detectors are insensitive to gamma rays and low energy neutrons, highly radioactive samples (such as spent nuclear fuel or transuranic waste drums) could be ...

A study of the effect of incremental gamma-ray doses and incremental neutron fluences upon the performance of self-biased /sup 10/B-coated high-purity epitaxial GaAs thermal neutron detectors

High-purity epitaxial GaAs /sup 10/B-coated thermal neutron detectors advantageously operate at room temperature without externally applied voltage. Sample detectors were systematically irradiated at fixed grid locations near the core of a 2 MW research reactor to determine their operational neutron dose threshold. Reactor pool locations were assigned so that fast and thermal neutron fluxes to the devices were similar. Neutron fluences ranged between 10/sup 11/ n/cm/sup 2/ to 10/sup 16/ n/cm/sup 2/. GaAs detectors were exposed to exponential fluences of base ten. Twelve detector designs were irradiated and studied, differentiated between p-i-n diodes and Schottky barrier diodes. The irradiated /sup 10/B-coated detectors were tested for neutron detection sensitivity in a thermalized neutron beam.

Non-Destructive Spent Fuel Characterization with Semi-Conducting Gallium Arsinde Neutron Imaging Arrays

High resistivity bulk grown GaAs has been used to produce thermal neutron imaging devices for use in neutron radiography and characterizing burnup in spent fuel. The basic scheme utilizes a portable Sb/Be source for monoenergetic (24 keV) neutron radiation source coupled to an Fe filter with a radiation hard B-coated pixellated GaAs detector array as the primary neutron detector. The coated neutron detectors have been tested for efficiency and radiation hardness in order to determine their fitness for the harsh environments imposed by spent fuel. Theoretical and experimental results are presented, showing detector radiation hardness, expected detection efficiency and the spatial resolution from such a scheme. A variety of advanced neutron detector designs have been explored, with experimental results achieving 13% thermal neutron detection efficiency while projecting the possibility of over 30% thermal neutron detection efficiency.