Luke C. Henson

Characterization of 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.

Microstructured semiconductor neutron detector (MSND)-based Helium-3 Replacement technology

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.

Development of the dual-sided microstructured semiconductor neutron detector

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.

Advancements on dual-sided microstructured semiconductor neutron detectors (DSMSNDs)

Microstructured semiconductor neutron detectors (MSNDs) represent a low-cost, high-efficiency means of solid-state thermal neutron detection. Trenches are etched into a pn-junction diode and backfilled with nano-sized 6LiF neutron converting material. Neutrons absorbed within the conversion material produce charged particle reaction products that interact within the semiconductor substrate and generate electron-hole pairs. The electron-hole pairs are collected by the applied bias, generating an electronic pulse and indicating a neutron event. Presently, single-sided MSNDs performance is approaching the theoretical maximum detection efficiency, with devices nearing 35% intrinsic thermal neutron detection efficiency. Single-sided MSNDs are limited in their detection efficiency due to neutron streaming between the trenches; incident neutrons will pass through the semiconductor substrate without detection. Dual-side microstructured semiconductor neutron detectors (DSMSNDs) alleviate this issue with an additional set of trenches etched into the backside of the diode. Neutrons streaming through the first set of trenches can be absorbed in the second set of trenches. Careful design of DSMSNDs can allow for detection efficiencies exceeding 70% for a single 1-mm thick detector. DSMSNDs etched 20-μm wide, 350-μm deep trenches, and 10-μm wide fins have achieved an intrinsic thermal neutron detection efficiency of 53.54±0.61% for neutrons normally incident on the front face of the detector using p-type contacts on both the front- and back-side fins.

Present Status of the Microstructured Semiconductor Neutron Detector-Based Direct Helium-3 Replacement

A small form-factor, third-generation microstructured semiconductor neutron detector (MSND)-based ³He replacement (HeRep Mk III) detector has been developed to serve as a direct replacement to expensive small-diameter, high-pressure ³He gas-filled proportional neutron counters. Previously, the larger, 2-in diameter by 6-in-long HeRep Mk II utilized thirty 4-cm² active area single-sided MSNDs to replace a similarly sized 4-atm ³He counter. The net count rate of the HeRep Mk II was 95.15% ± 9.04% of the net count rate of a similarly sized 4-atm ³He detector for bare ²⁵²Cf at a distance of 1 m. The HeRep Mk II net count rate measured 102.71% ±2.65% of the ³He detector net count rate when each were placed in 6-in diameter by 9-in-long cylindrical high-density polyethylene (HDPE) moderator casks. The HeRep Mk III was developed as a test bed for new-generation dual-sided MSNDs (DSMSNDs), as well as for reduced-power electronics. The 0.75-in diameter by 4.50-in-long HeRep Mk III was populated with twelve 1-cm² active area DSMSNDs. The pvp-type diodes have thus far realized 53.54% ± 0.61% intrinsic thermal-neutron detection efficiency. The HeRep Mk III was tested against a 0.75-in diameter by 3.0-in-long 10-atm ³He detector with a 30-ng ²⁵²Cf source at a distance of 0.25 m. The HeRep Mk III reported 45.4% ± 0.3% and 56.7% ± 0.2% of the count rate of the 10-atm ³He detector for detectors with no exterior moderator and in a 3-in by 5.75-in cylindrical HDPE moderator cask, respectively. The HeRep Mk III gamma-ray sensitivity was measured at a dose rate of 50 mR/hr from ¹³⁷Cs resulting in a net count rate of 0.01 cps, which corresponds to a gamma-ray rejection ratio of approximately 5:10⁷.

Fourth-generation microstructured semiconductor neutron detector (MSND)-based 3 He replacement (HeRep) for high pressure 3 He detectors

Two fourth-generation ³He replacement (HeRep Mk IV) detectors based on microstructured semiconductor neutron detector (MSND) technology have been fabricated and characterized against a 0.75-in diameter by 3.0-in long, 10-atm ³He neutron proportional counter. The HeRep Mk IV detectors have a 0.75-in by 0.75-in square form factor with one HeRep Mk IV measuring 3.1-in long and the other measuring 6.1-in long. Each HeRep Mk IV was populated with 15 DS-MSNDs that have an intrinsic thermal-neutron detection efficiency of approximately 50%. The void spaces between adjacent detectors in the 6.1-in long HeRep Mk IV were filled with HDPE moderator. The 3.1-in long HeRep Mk IV recorded 92.8 ± 1.8% of the count rate of the 10-atm ³He detector for bare 26.5-ng ²⁵²Cf, bare detector and 82.2 ± 0.9% for moderated ²⁵²Cf source, moderated detector configurations. The 6.1-in long HeRep Mk IV count rate was 108.5 ± 2.0% and 113.8 ± 1.2% of the count rate of the 10-atm ³He detector for bare ²⁵²Cf, bare detector and moderated ²⁵²Cf, moderated detector, respectively. The gamma-ray rejection ratio was determined for ¹³⁷Cs at a dose rate of 50 mR/hr. The gamma-ray rejection ratio was 1:1.5×10⁸ and 1:4.6×10⁸ for the 3.1-in long and 6.1-in long HeRep Mk IV, respectively. The HeRep Mk IVs exhibit a 116% (3.1-in long) and 33% (6.1-in long) improvement in intrinsic detection efficiency over the 0.75-in diameter by 4.5-in long HeRep Mk III for moderated ²⁵²Cf at 25 cm.

Lithium foil gas-filled neutron detector using microstrip electrodes

Microstrip electrodes have been fabricated and combined with one and five suspended 6Li foils positioned within a pressurized, gas-filled chamber to create a suspended foil microstrip neutron detector. This new detector offers a mechanically and electrically robust alternative to multi-wire proportional counters. Incident neutrons are converted into charged-particle reaction products that ionize the backfill gas. Charge carriers produced from the ionization of the backfill gas drift toward their respectively-charged electrodes due to the influence of the electric field formed from the potential difference between the drift electrode and the microstrip electrode anode and cathode strips. Gas multiplication occurs as electrons approach the surface of the microstrip electrode resulting in an increase in signal amplitude. Suspended foil microstrip neutron detectors containing one and five suspended 6Li foils were simulated using MCNP6 and compared to experimental results. The measured count rates from a moderated 26-ng 252Cf source positioned 18 cm from microstrip neutron detectors equipped with one and five suspended 6Li foils were 3.25 ± 0.04 and 10.62 ± 0.14 counts per second, respectively. The intrinsic thermal neutron detection efficiency of each detector was 4.02 ± 0.04% and 14.58 ± 0.11% for one and five suspended 6Li foils, respectively.

Improved low power, modular thermal neutron counter based on microstructured semiconductor neutron detectors (MSND)

A modular, mass-producible, low-power, compact thermal-neutron counter has been developed to meet the needs for modern detector instruments and to compete with aging 3He-based counter systems. The Domino™ V5.4 neutron detector package is populated with four 1-cm2 active-area microstructured semiconductor neutron detectors (MSNDs). The detectors are calibrated to an intrinsic thermal neutron detection efficiency of 30% over its 4 cm2 active area. A gamma-ray rejection ratio of at least 1:10”8 is achieved at this calibration level. Total device dimensions measure 2.5-cm wide by 3.8-cm tall and 0.47-cm thick, weighing 9.5 g. Individual Dominoes™ can be tiled together into strings and then arrayed laterally to form large-area instruments. In this form, the small sensor can populate instruments up to 1 m2, with a cost that is comparable to a similar 3He-based array. The addition of I2C-based bias and threshold controllers reduces sensor cost and complexity over the previously-reported Domino™ versions (V3.0 and V4.0) and allows for user-end calibrations and modifications. Furthermore, the Domino™ has realized 10× reduced power consumption over previous versions, to nearly 90μW at 3.3 volts, while also reducing sensitivity to thermal- and capacitance-induced noise from the MSNDs, making the Domino™ an ideal candidate for 3He replacement and implementation in low-power, portable neutron-detector instruments.

Fabrication of present-generation microstructured semiconductor neutron detectors

Microstructured semiconductor neutron detectors with large aspect-ratio, straight trenches backfilled with neutron sensitive material exhibit superior detection efficiencies over traditional thin-film-coated diodes for solid-state thermal neutron detection. The detectors operate as partial-conformal diffused pin-junction diodes with low leakage current and capacitance. The solid-state silicon substrate detectors operate on a zero to 2.7 V bias and are coupled with signal amplifying and electronic readout components. The intrinsic thermal neutron detection efficiency for a 4-cm2 single-sided MSND reported here is 30.0±0.9% for a neutron beam with normal incidence to the detector surface. The intrinsic thermal neutron detection efficiencies for 0.0253 eV neutrons were determined by calibrating against a calibrated helium-3 gas-filled proportional detector at the Kansas State University TRIGA Mk II nuclear reactor diffraction beam port.