W.J. McNeil

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.

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.

Characteristics of 3D Micro-Structured Semiconductor High Efficiency Neutron Detectors

Silicon diodes with large aspect ratio perforated micro-structures backfilled with 6LiF show a dramatic increase in neutron detection efficiency beyond that of conventional thin-film coated planar devices. Described in the following are advancements in the technology with increased perforation depths. Perforated silicon diodes with three different etched micro-structure patterns were tested for neutron counting efficiency. The etched micro-structure patterns consisted of circular holes, straight trenches, and continuous sinusoidal waves, with each pattern etched 200 mum deep. Normal incident neutron counting efficiencies were determined to be 9.7%, 12.6%, and 16.2% for circular hole, straight trench, and sinusoidal devices, respectively, at a reverse bias of 3 volts. The perforated neutron detectors demonstrate limited sensitivity to high-energy photon irradiation with a 60Co gamma-ray source. This work is part of on-going research to develop solid-state semiconductor neutron detectors with high detection efficiencies and uniform angular responses.

Improved High Efficiency Stacked Microstructured Neutron Detectors Backfilled With Nanoparticle

Silicon diodes with large aspect ratio trenched microstructures, backfilled with 6LiF, show a dramatic increase in thermal neutron detection efficiency beyond that of conventional thin-film coated planar devices. Described in this work are advancements in the technology using detector stacking methods to increase thermal neutron detection efficiency, along with the current process to backfill 6LiF into the silicon microstructures. The highest detection efficiency realized thus far is over 42% intrinsic thermal neutron detection efficiency by device-stacking methods. The detectors operate as conformally diffused pn junction diodes each having 1 cm2 area. Two individual devices were mounted back-to-back with counting electronics coupling the detectors together into a single dual-detector device. The solid-state silicon device was operated at 3 V and utilized simple signal amplification and counting electronic components that have been adjusted from previous work for slow charge integration time. The intrinsic detection efficiency for normal-incident 0.0253 eV neutrons was found by calibrating against a 3He proportional counter.

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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.

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Perforated silicon diodes with two different etched patterns were tested for neutron counting efficiency and angular response. The etched patterns consisted of circular holes on a square lattice or continuous sinusoidal waves. Normal incident neutron counting efficiencies were determined to be 21% and 35% for circular hole and sinusoidal devices, respectively. A nonuniform angular response was identified for the circular hole perforated devices. For the circular hole patterns, a reduction in efficiency appeared at azimuthal angles near 0deg and 90deg, each referenced at a 90deg polar angle about the normal axis. The nonuniform angular response is due to neutron streaming paths through the square lattice. The sinusoidal device angular response was uniform and matched well to that of a standard simple planar device.

Perforated Diode Fabrication for Neutron Detection

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.

Angular response of perforated silicon diode high efficiency neutron detectors

Perforated semiconductor neutron detectors are compact, high-efficiency, diode detectors that operate at low power. Microstructured neutron detector fabrication methods have been improved over previous manufacturing methods. The neutron detectors are easily fabricated from high purity n-type Si, in which patterned trenches are etched into the Si substrate, wherein shallow p-type junctions are diffused. The trenches are then backfilled with 6LiF powder, making the devices sensitive to reaction products from the 6Li(n,t)3He reaction. Pulse height spectra show improved signal-to-noise ratio, higher neutron counting efficiency, and excellent gamma-ray discrimination over previous microstructured neutron detector designs. Thermal neutron detection measurements from a 0.0253 eV diffracted neutron beam, yielded 20.4% intrinsic detection efficiency for devices with 245 micron deep trenches and 21% intrinsic detection efficiency for two back-to-back devices each having 113 micron deep trenches.

Characteristics of the stacked microstructured solid state neutron detector

Perforated semiconductor diode detectors have been under development for several years at Kansas State University for a variety of neutron detection applications. The detectors are fabricated from high purity n-type Si. Sinusoidal trenches are etched into the substrate, into which shallow p-type junctions are diffused. The trenches are then backfilled with 6LiF powder to make the device sensitive to neutrons. Thermal neutron measurements from a 0.0253 eV diffracted neutron beam yielded 17% intrinsic detection efficiency for devices with 50 micron deep trenches and 29% intrinsic detection efficiency for devices with 100 micron deep trenches.

Variant designs and characteristics of improved microstructured solid-state neutron detectors

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.