J.K. Shultis

Perforated Diode Fabrication for Neutron Detection

Excessive leakage current in perforated pin diodes was identified and addressed through simple changes in processing techniques. The first pulse height spectra from a perforated diode operated as a radiation detector is reported. Also, methods to load 6LiF neutron absorbing material into deep perforations are reported.

Characteristics of the stacked microstructured solid state neutron detector

Silicon diodes with large aspect ratio perforated microstructures backfilled with 6LiF show a dramatic increase in neutron detection efficiency beyond that of conventional thin-film coated planar devices. Described in this work are advancements in the technology with increased microstructure depths and detector stacking methods that work to increase thermal-neutron detection efficiency. Models for ion energy deposition and intrinsic thermal-neutron detection efficiency for the straight trench design are described and results presented. A dual stacked device was fabricated by coupling two detectors back-to-back, along with counting electronics, into a single detector. Experimentally verified results and modeled predictions are compared. The stacked device delivered 37% intrinsic thermal-neutron detection efficiency, lower than the predicted value of 47%. It was determined that this lower observed efficiency is due to detector misalignment in the stacked structure and ballistic deficit from slow charge collection from the deep trench structures. The intrinsic thermal-neutron detection efficiency depends strongly upon the geometry, size, and depth of the perforated microstructures. This work is part of on-going research to develop solid-state semiconductor neutron detectors with high detection efficiencies.

Perforated Semiconductor Neutron Detector Modules for Detection of Spontaneous Fission Neutrons

Compact neutron detectors are being designed and tested for use as low-power real-time personnel dosimeters and for remote neutron sensing. The neutron detectors are pin diodes that are mass produced from high-purity Si wafers. Each detector has thousands of circular perforations etched vertically into the device. The perforations are backfilled with 6LiF to make the pin diodes sensitive to thermal neutrons. The prototype devices delivered over 3.8% thermal neutron detection efficiency while operating on only 15 volts. The highest efficiency devices thus far have delivered over 12% thermal neutron detection efficiency. Devices moderated with high density polyethylene (HDPE) can be used for fast neutron detection. Compact packages with or without remote readout electronics are under construction and characterization.

Wireless neutron and gamma ray detector modules for dosimetry and remote monitoring

Compact neutron detectors are being designed and tested for use as wireless low-power real-time personnel dosimeters and for remote neutron sensing. The neutron detectors are pin diodes that are mass produced from high-purity Si wafers. Each detector has thousands of perforations etched vertically into the device. The perforations are backfilled with 6LiF to make the pin diodes sensitive to thermal neutrons. The first prototype devices delivered over 3.8% thermal-neutron detection efficiency while operating at 15 volts. Recent high-efficiency perforated devices have delivered over 20% thermal-neutron detection efficiency. Compact packages with wireless communication capability have been constructed and are under test. The compact packages record counts every second and transmit ten data points every ten seconds. The platforms have been designed to incorporate an additional fast neutron detector and a Frisch- collar CdZnTe gamma-ray spectrometer. The overall goal is to manufacture a compact package capable of remote and wireless neutron dosimetry and gamma ray spectroscopy/dosimetry.

Angular design considerations for perforated semiconductor detectors

The efficiencies of channel-type, chevron-type, and sinusoid-type perforated semiconductor neutron detectors have been calculated. Azimuthal streaming problems exhibited by rod- type perforations are eliminated using sinusoidal perforations but cannot be completely removed with channel-type and chevron- type perforations.

Micro-Pocket Fission Detector (MPFD) performance characteristics

Micro-Pocket Fission Detectors (MPFDs) show improved performance over typical fission chambers due to their miniature size. With chambers of only 0.5 mm thick, an applied bias of 200 volts will allow pulse mode counting rates in excess of 56,000 counts per second with a dead time of only 1%. The ability to discriminate background induced events from neutron induced events also improves with the miniature chamber width. In-core tests have demonstrated linear pulse mode operation through seven orders of neutron flux magnitude. Specialized neutron reactive coatings have been formulated to maintain less than 1% signal deviation in a neutron flux of 1013 n cm-2 s-1 for over 58 years.

3D real-time in-core neutron flux mapping with micro-pocket fission detectors (MPFD)

A micro-pocket fission detector (MPFD) array is being deployed in the Kansas State University TRIGA Mark-II nuclear reactor in order to produce a real-time 3-dimensional neutron flux map. A total of 75 MPFD3-Ts make up the array where each MPFD3-T consists of an uncoated detector for background subtraction, a 93% enriched 235U coated detector for thermal neutron sensitivity, a natural uranium coated detector for mixed energy neutron sensitivity, and a thermocouple for thermal monitoring. The array is located in fixed in-core positions between fuel elements and is capable of being read out in pulse, MSV, and current modes allowing for operation from shutdown to full power. In addition, using off-the-shelf electronic components, the system is able to track reactor pulses with power changes spanning 7 orders of magnitude in less than 100 milliseconds.