Microstructured Semiconductor Neutron Detectors Research Articles

Survivability of microstructure semiconductor neutron detectors (MSND) in a thermal neutron radiation environment

The performance of five representative microstructured semiconductor neutron detectors (MSND) were investigated after exposure to varying magnitudes of thermal neutrons fluences ranging from 107 cm−2 to 1012 n cm−2. The devices have 95% enriched microparticulate 6LiF granules backfilled in microscopic trenches within a Si pvn diode. Neutron interactions in the 6Li causes the emission of reaction products which ionize the trenched Si diode depletion regions. The average intrinsic thermal-neutron detection efficiency (ϵtn) of the five devices before irradiation measured as 24.09% ± 0.35% with an operating leakage current below 11 nA at a −3 V reverse bias condition. The observed threshold of performance deterioration was approximately 1010 n cm−2, resulting in an spectral shift in the reaction product spectrum to lower energy channels and a general increase in leakage current. Although damage was observed beyond a thermal-neutron fluence of 1010 n cm−2, the devices remained operable. Beyond a thermal-neutron fluence of 1012 n cm−2, the devices experienced severe damage and demonstrated poor detector performance. At the highest thermal neutron fluences tested, near 1012 n cm−2, the average ϵtn of the five devices decreased to 4.42% ± 0.10% with an average operating leakage current of 300 nA at a −3 V reverse bias condition. Performance degradation was attributed to reaction-product damage in the MSND pvn diode structure.

Charge carrier motion and effect of fixed oxide charge in a microstructured silicon radiation detector

Signal formation in a microstructured semiconductor neutron detector is more complex than in planar diode geometry. Three-dimensional microstructures are laterally smaller than the ionization cloud length, and the electric fields may be weak enough to exhibit plasma time effects. This work is the first detailed treatment of charge carrier motion in these complex semiconductor devices to replicate the time profile and signal magnitude. Simulations were performed using COMSOL Multiphysics to investigate various parameters that affect the propagation of the charge cloud. It was observed that the size of the simulated three-dimensional structure had an impact on the induced current pulse, indicating the importance of simulation geometry optimization to accurately simulate charge cloud expansion. COMSOL Multiphysics was used to replicate accurate charge creation profiles using energy deposition information imported from radiation transport codes. A detailed simulation methodology is presented to benchmark preamplifier event pulses along with complexities in modeling the charge carrier motion along the etched microstructured trenches with Si–SiO2 boundary conditions, including fixed oxide charge and interface trapping.

Simulation of charge drift in surface doped, pixelated micro-structured semiconductor neutron detectors

A semiconductor neutron imaging device is proposed (X-MSND) based on high efficiency, Micro-structured Semiconductor Neutron Detector (MSNDs) bump bonded onto a Timepix pixelated readout chip. The device serves as a combined neutron and photon imager with a 256x256 pixel array, using per-pixel time-over-threshold (ToT) energy deposition. A method of simulation was developed by combining finite-element tools (COMSOL), particle transport code (Geant4), and charge transport code (Allpix2), to model the time-dependent charge induction on pixel electrodes to be read-out by the Timepix. Pixel cluster events have been simulated as a result of incident neutron irradiation for the X-MSND and compared to a planar coated diode device. The X-MSND design produced thermal neutron detection efficiency of 14%, significantly greater than the theoretical maximum for planar devices. Simulated pixel clusters showed similar qualitative characteristics as planar neutron sensitive Timepix hybrid detectors.

A semiconductor-based neutron detection system for planetary exploration

We explore the use of microstructured semiconductor neutron detectors (MSNDs) to map the ratio between thermal neutrons and higher energy neutrons. The system consists of alternating layers of modular neutron detectors (MNDs), each comprising arrays of twenty-four MSNDs, and high-density polyethylene moderators (HDPE) with gadolinium shielding to filter between thermal neutrons and higher energy neutrons. We experimentally measured the performance of three different configurations and demonstrated that the sensor system prototypes detect and differentiate thermal and epithermal neutrons. We discuss future planetary exploration applications of this compact, semiconductor-based low-energy neutron detection system.

Design and performance considerations for dual-sided microstructured semiconductor neutron detectors

Thin-film-coated solid-state thermal neutron detectors were replaced in recent decades with the Microstructured Semiconductor Neutron Detector (MSND) technology. The basic device structure of the MSND involves micro-sized trenches that are etched into a vertically-oriented pvn-junction diode that are backfilled with a neutron converting material. Neutrons absorbed within the converting material induce fission of the parent nucleus, producing a pair of energetic charged-particle reaction products that can be counted by the diode. The deep-etched microstructures of the MSND yield good neutron-absorption efficiency and reaction-product counting efficiency, resulting in a 6-10x improvement in intrinsic thermal-neutron detection efficiency over thin-film-coated devices. Performance of present-day MSNDs are reaching an efficiency plateau; streaming paths between the conversion-material backfilled trenches, allow a considerable fraction of neutrons to pass through the device undetected. Dual-Sided Microstructured Semiconductor Neutron Detectors (DS-MSNDs) have been developed that utilize a complementary second set of trenches on the back-side of the device to capture streaming neutrons. This work investigates several of the fundamental design structures that can be etched into the semiconductor material, including repeated front-side and back-side patterns and inverse microfeature patterns. Results from MCNP6 simulations of DS-MSNDs show that intrinsic thermal-neutron detection efficiencies are often double that of their MSND counterparts and greater than 80% intrinsic thermal-neutron detection efficiency is theoretically possible with a 1.5-mm thick device.

Numerical evaluation of fast-sensitive microstructured semiconductor neutron detectors for TREAT hodoscope

Presented is the numerical study of fast-sensitive, actinide and hydrogenous microstructured semiconductor neutron detectors (MSNDs) for the hodoscope at the Transient REActor Test facility (TREAT) using Geant4 and MCNP6. Neutron converters considered were 237Np, 235U, natural uranium, and 232Th for actinide MSNDs and paraffin wax for hydrogenous MSNDs (H-MSNDs). Paraffin wax was found to have a larger fission-spectrum-weighted macroscopic cross section (0.32 cm −1) than the actinide materials (the best being 0.067 cm −1 for 237Np). However, actinide reactants were found to allow higher lower-level discriminator (LLD) settings due to the large energy of fission fragments. Actinide MSNDs filled with 235U, natural uranium, and 232Th were evaluated in Geant4 using the fission fragment generator. With the LLD set to 5 MeV, the intrinsic neutron-detection efficiency of the 235U-filled MSNDs was 1.2% for a 2-cm device length and saturated at 2.6% for lengths beyond 14 cm, where 20-μm trench and 10-μm wall widths were assumed. The deposited energy in 235U predicted by Geant4 differed from the MCNP6-predicted value by about −0.7%. With the LLD set to 300 keV, Geant4 and MCNP6 predicted an efficiency of about 10% for a 2-cm long H-MSND with 20-μm trench and 10-μm wall widths and an efficiency of about 26% for a detector length of 20 cm. For a LLD set to achieve a signal-to-noise (S/N) ratio of 100 when including gamma-ray noise, the best-case, Geant4-predicted efficiencies were 2.5% and 9.6% for 2-cm and 20-cm long devices with 60-μm trench and 40-μm wall widths.

Wearable detector device utilizing microstructured semiconductor neutron detector technology

A Wearable Detector Device (WDD) has been outfitted with Microstructured Semiconductor Neutron Detectors (MSNDs) to aid in the search and localization of special nuclear materials (SNMs). Many SNMs decay by spontaneous fission and emit free neutrons. The WDD detects these neutrons and stores interaction rate information to alert the operator to the presence of special nuclear material. The WDD is composed of 16 Modular Neutron Detectors (MNDs), each populated with a 4 × 6 array of 1-cm2 active area, 500-μm thick MSNDs. The individual MSNDs each have an intrinsic thermal-neutron detection efficiency of approximately 30%. Each MND connects to a communications dongle, and the MND and dongle were encased in a 3-in. wide by 5-in. tall by 0.6-in. thick high-density polyethylene moderator (HDPE) case. The 16 MNDs, connected to Controller Area Network dongles, communicate with a master control board, which also contains the battery bank and power conditioning electronics. The operational lifetime of the battery-powered WDD is greater than 12 h per single charge. The WDD, mounted on an ANSI 42.53 phantom, reported 8.12 ± 0.07 cps and 12.93 ± 0.07 cps for a bare and moderated 21.9-ng 252Cf source at a distance of 1 m, respectively. The background count rate was 0.446 ± 0.002 cps. The gamma-ray rejection ratio of the WDD for 137Cs measured at a dose rate of 10 mR/h was 1.8 × 10−8.

Hexagonal boron nitride neutron detectors with high detection efficiencies

Neutron detectors fabricated from 10B enriched hexagonal boron nitride (h-10BN or h-BN) epilayers have demonstrated the highest thermal neutron detection efficiency among solid-state neutron detectors to date at about 53%. In this work, photoconductive-like vertical detectors with a detection area of 1 × 1 mm2 were fabricated from 50 μm thick free-standing h-BN epilayers using Ni/Au and Ti/Al bilayers as ohmic contacts. Leakage currents, mobility-lifetime (μτ) products under UV photoexcitation, and neutron detection efficiencies have been measured for a total of 16 different device configurations. The results have unambiguously identified that detectors incorporating the Ni/Au bilayer on both surfaces as ohmic contacts and using the negatively biased top surface for neutron irradiation are the most desired device configurations. It was noted that high growth temperatures of h-10BN epilayers on sapphire substrates tend to yield a higher concentration of oxygen impurities near the bottom surface, leading to a better device performance by the chosen top surface for irradiation than by the bottom. Preferential scattering of oxygen donors tends to reduce the mobility of holes more than that of electrons, making the biasing scheme with the ability of rapidly extracting holes at the irradiated surface while leaving the electrons to travel a large average distance inside the detector at a preferred choice. When measured against a calibrated 6LiF filled micro-structured semiconductor neutron detector, it was shown that the optimized configuration has pushed the detection efficiency of h-BN neutron detectors to 58%. These detailed studies also provided a better understanding of growth-mediated impurities in h-BN epilayers and their effects on the charge collection and neutron detection efficiencies.

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.

Dual-sided microstructured semiconductor neutron detectors (DSMSNDs)

Microstructured semiconductor neutron detectors (MSNDs) have in recent years received much interest as high-efficiency replacements for thin-film-coated thermal neutron detectors. The basic device structure of the MSND involves micro-sized trenches that are etched into a vertically-oriented pvn-junction diode that are backfilled with a neutron converting material. Neutrons absorbed within the converting material induce fission of the parent nucleus, producing a pair of energetic charged-particle reaction products that can be counted by the diode. The MSND deep-etched microstructures produce good neutron-absorption and reaction-product counting efficiencies, offering a 10× improvement in intrinsic thermal neutron detection efficiency over thin-film-coated devices. Performance of present-day MSNDs are nearing theoretical limits; streaming paths between the conversion-material backfilled trenches, allow a considerable fraction of neutrons to pass undetected through the device. Dual-sided microstructured semiconductor neutron detectors (DSMSNDs) have been developed that utilize a complementary second set of trenches on the back-side of the device to count streaming neutrons. DSMSND devices are theoretically capable of greater than 80% intrinsic thermal neutron detection efficiency for a 1-mm thick device. The first such prototype DSMSNDs, presented here, have achieved 29.48±0.29% nearly 2× better than MSNDs with similar microstructure dimensions.

Development of compact high efficiency microstructured semiconductor neutron detectors

Semiconductor diode detectors coated with neutron reactive materials are generally fashioned as planar diodes coated with 10B, 6LiF, or Gd. Planar detectors coated with 10B or 6LiF are limited to less than 5% intrinsic thermal neutron detection efficiency. Detectors coated with Gd can achieve higher efficiencies, but the low-energy signatures are problematic in the presence of background radiations. Microstructured semiconductor neutron detectors (MSNDs) can now achieve a tenfold increase in neutron detection efficiency over the planar diode designs. These semiconductor neutron detectors are fashioned with a matrix of microstructured patterns etched deeply into the semiconductor substrate and, subsequently, backfilled with neutron reactive materials. Intrinsic thermal-neutron detection efficiencies exceeding 35% have been achieved with devices no thicker than 1 mm while operating on less than 5 V, now allowing for instrumentation to be realized with similar performance as 3He gas-filled detectors.

High-efficiency microstructured semiconductor neutron detectors for direct 3He replacement

High-efficiency Microstructured Semiconductor Neutron Detectors (MSNDs) have been tiled and arranged in a cylindrical form factor in order to serve as a direct replacement to aging and increasingly expensive 3He gas-filled proportional neutron detectors. Two 6-in long by 2-in diameter cylinders were constructed and populated with MSNDs which were then directly compared to a 4atm Reuter Stokes 3He detector of the same dimensions. The Generation 1 MSND-based 3Helium-Replacement (HeRep Mk I) device contained sixty-four 1-cm2 active-area MSNDs, each with an intrinsic neutron detection efficiency of approximately 7%. A Generation 2 device (the HeRep Mk II) was populated with thirty 4-cm2 active-area MSNDs, with an intrinsic thermal neutron detection efficiency of approximately 30%. The MSNDs of each HeRep were integrated to count as a single device. The 3He proportional counter and the HeRep devices were tested while encased in a cylinder of high-density polyethylene measuring a total of 6-in by 9-in. The 3He counter and the HeRep Mk II were each placed 1m from a 54-ng 252Cf source and tested for efficiency. The 3He proportional counter had a net count rate of 17.13±0.10cps at 1m. The HeRep Mk II device had a net count rate of 17.60±0.10cps, amounting to 102.71±2.65% of the 3He gas counter while inside of the moderator. Outside of moderator, the 3He tube had a count rate of 3.35±0.05cps and the HeRep Mk II device reported 3.19±05, amounting to 95.15±9.04% of the 3He neutron detector.

Present status of microstructured semiconductor neutron detectors

Semiconductor diode detectors coated with neutron reactive materials have been investigated as neutron detectors for many decades, and are fashioned mostly as planar diodes coated with boron-10 (10B), lithium-6 fluoride (6LiF) or gadolinium (Gd). Although effective, these detectors are limited in efficiency (the case for boron and LiF coatings) or in the ability to distinguish background radiations from neutron-induced interactions (the case for Gd coatings). Over the past decade, a renewed effort has been made to improve diode designs to achieve up to a 10-fold increase in neutron detection efficiency over the simple planar diode designs. These new semiconductor neutron detectors are fashioned with a matrix of microstructured patterns etched deeply into the substrate and, subsequently, backfilled with neutron reactive materials. Intrinsic thermal-neutron detection efficiencies exceeding 40% have been achieved with devices no thicker than 1mm while operating on less than 5V.

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

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 0.0253eV neutrons was found 6.8% compared against a calibrated 3He proportional counter.