Ryan G. Fronk

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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Silicon diodes with large aspect ratio 3D microstructures backfilled with 6LiF show a significant increase in 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 and summed-detector 6×6-element arraying methods to dramatically increase the sensitivity to thermal neutrons. The intrinsic detection efficiency of the 6×6 array for normal-incident 0.0253 eV neutrons was found 6.8% compared against a calibrated 3He proportional counter.

LiF

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.

High-efficiency microstructured semiconductor neutron detectors that are arrayed, dual-integrated, and stacked.

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 using detector stacking methods to increase thermal neutron detection efficiency. The highest efficiency devices thus far have delivered over 42% intrinsic thermal neutron detection efficiency by device-coupling stacking methods. The detectors operate as conformally diffused pn junction diodes each having 1cm2 square-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 operated at 3V and utilized simple signal amplification and counting electronic components. The intrinsic detection efficiency for normal-incident 0.0253 eV neutrons was found by calibrating against a calibrated 3He proportional counter.

Characteristics of the stacked microstructured solid state neutron detector

Microstructured semiconductor neutron detector (MSND) devices have achieved 42% intrinsic efficiency when operated as a dual-detector device. The neutron energy spectrometer has alternating layers of MSND arrays, cadmium, and high-density polyethylene (HDPE) in a linear arrangement. The detector arrays consist of 4 dual-detector devices with 1-cm2 detector elements in a stacked configuration. A 2 mm thick layer of cadmium separates each detector from the following layer of HDPE. The cadmium layer prevents thermalized neutrons from backscattering into the previous detector. The 3-cm thick HDPE layers act as a moderator for the epithermal and fast neutrons, allowing them to disperse energy within the spectrometers volume and reach thermal energies to be detected by the MSND arrays. The resultant device is a neutron energy spectrometer with a large surface area for the incident neutron beam.

High efficiency dual-integrated stacked microstructured solid-state neutron detectors

Low-power microstructured semiconductor neutron detector (MSND) devices have long been investigated as a high-efficiency replacement for thin-film diodes for thermal neutron detection. The detector devices were improved by stacking two 1cm2 devices and integrating their responses together to act as a single diode, increasing detection efficiency to over 42%. The need for larger active area devices has driven further improvement of the technology. A large active area device has been developed by arraying seventy-two 1cm2 devices together into two 6×6 configurations, dual-stacking them, and integrating their responses together in order to act as a single detector device. The intrinsic thermal neutron detection efficiency was found to be 7.03±0.04%. The leakage current of the 36cm2 device was −42nA at −5V bias and the capacitance was found to be 54pF at −5V bias.

Neutron energy spectrum with microstructured semiconductor neutron detectors

Silicon diodes with large aspect ratio 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. An individual 4-cm2 MSND was fabricated. A stacked 4-cm2 MSND was fabricated by coupling two detectors back-to-back, along with counting electronics, into a single detector. The individual MSND delivered 16% intrinsic thermal-neutron detection efficiency and the stacked MSND delivered 32% intrinsic thermal-neutron detection efficiency. The intrinsic thermal-neutron detection efficiency depends strongly upon the geometry, size, and depth of the silicon microstructures. This work is part of on-going research to develop solid-state semiconductor neutron detectors with high neutron detection efficiencies.

Arrayed high efficiency dual-integrated microstructured semiconductor neutron detectors

Silicon diodes with large aspect ratio microstructures backfilled with neutron-reactive material show a dramatic increase in neutron-detection efficiency beyond that of conventional thin-film coated planar devices. Described here are advancements in applications of the technology. The highest single-chip efficiency devices thus far have delivered over 30.1% intrinsic thermal-neutron detection efficiency at normal incident, and 37.6% at a 45° incident. The detectors operate as diffused pn-junction diodes each having 4-cm2 active area. The solid-state silicon device operates on zero to 2.7V and utilizes simple signal amplification and counting electronic components. The intrinsic detection-efficiency for normal-incident 0.0253 eV neutrons was found for the detectors by calibrating against a calibrated 3He proportional counter.

Characteristics of the large-area stacked microstructured semiconductor neutron detector

The present work employs MSND technology as a ³He gas-tube direct-replacement technology. The MSND-based Helium Replacement (HeRep) is a single instrument utilizing thirty 4-cm² active area MSNDs to directly replace ³He-based proportional neutron counters. The HeRep Mk II prototype yielded 102.71±0.84% of the neutron detection efficiency of a 4-atm Reuter Stokes ³He gas-filled detector with the same active dimensions and test setup from a ²⁵²Cf source. The HeRep Mk II matched the efficiency of the ³He detector while using MSNDs that each had 20% intrinsic-efficiency for thermal-neutrons.

Characterization of microstructured semiconductor neutron detectors

Microstructured semiconductor neutron detectors (MSNDs) have long been investigated as a replacement for inefficient thin-film-coated semiconductor neutron detectors. Thin-film-coated semiconductor thermal neutron detection efficiency is restricted to 4-5%. MSNDs improved upon these devices with etched perforations into the diode backfilled with neutron conversion material. Neutron absorption and reaction-product detection efficiency was greatly improved, leading to theoretical intrinsic thermal neutron detection efficiencies greater than 45%. Previous attempts at double-stacking MSNDs to increase the detection efficiency were successful, but were accomplished with great difficulty, where device alignment and proved to be challenging. The development of the dual-sided microstructured semiconductor neutron detector (DSMSND) provides the simplicity of a single device with the detection efficiency of a double-stacked detector. Trenches were etched into the top and bottom of a single vertical pvn-junction Si diode and backfilled with 6LiF neutron conversion material. The first such devices fabricated yielded thermal neutron detection efficiencies between 9.6-16.6%. Theoretical intrinsic thermal neutron detection efficiencies of greater than 79% are possible with a single 1-mm thick silicon diode.