Intrinsic Thermal Neutron Detection Efficiency 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.

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.

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.

Neutron sensitivity of 10B4C-coated aluminum honeycomb using a single-anode wire, P-10 continuous-gas-flow proportional counter

The intrinsic thermal-neutron detection efficiency of a 10B4C-coated aluminum honeycomb sample was measured using a single-anode wire, P-10 continuous-gas-flow proportional counter. The aluminum honeycomb sample was 1.27 cm thick with a diameter of 4.1275 cm (1 5/8 in.) and was composed of hexagonal-shaped cells with a cell size of 1.5875 mm (1/16 in.). The 10B4C coating was applied by Exothermics, Inc. via magnetron sputtering to a coating thickness of 4.68±1.25μm as measured using a scanning electron microscope. Using a lower level discriminator setting of 50 keV, intrinsic thermal-neutron detection efficiency was measured to be 21.45 ± 0.26% in reference to a 4.0 atm, 5.08-cm (2-in.) diameter, 15.24-cm (6-in.) long Reuter Stokes 3He tube with a known efficiency of 80.7 ± 0.5%. MCNP6 simulations yielded a theoretical intrinsic thermal-neutron detection efficiency of approximately 24% for a 10B4C-coating thickness of 4.68μm with a lower level discriminator setting of 50 keV.

Electrophoretic Deposition of 10B Nano/Micro Particles in Deep Silicon Trenches for the Fabrication of Solid State Thermal Neutron Detectors

We present a cost effective and scalable approach to fabricate solid state thermal neutron detectors. Electrophoretic deposition technique is used to fill deep silicon trenches with 10B nanoparticles instead of conventional chemical vapor deposition process. Deep silicon trenches with width of 5-6 μm and depth of 60-65 μm were fabricated in a p-type Si (110) wafer using wet chemical etching method instead of DRIE method. These silicon trenches were converted into continuous p-n junction by the standard phosphorus diffusion process. 10B micro/nano particle suspension in ethyl alcohol was used for electrophoretic deposition of particles in deep trenches and iodine was used to change the zeta potential of the particles. The measured effective boron nanoparticles density inside the trenches was estimated to be 0.7 gm cm-3. Under the self-biased condition, the fabricated device showed the intrinsic thermal neutron detection efficiency of 20.9% for a 2.5 × 2.5 mm2 device area.

Neutron sensitivity of 6Li-based suspended foil microstrip neutron detectors using Schott Borofloat® 33 microstrip electrodes

6Li foils, each 75-µm thick, were positioned between a Schott Borofloat® 33 microstrip electrode and a planar drift electrode to construct suspended foil microstrip neutron detectors. MCNP6 simulations of two detector configurations, one containing a single 6Li foil and the other containing five 6Li foils, indicated expected maximum intrinsic thermal-neutron detection efficiencies of 18.36% and 54.08%, respectively. For comparison, the intrinsic thermal-neutron detection efficiency as a function of thermal-neutron beam position along the foil span was experimentally measured for both detector configurations. A non-uniform intrinsic thermal-neutron detection efficiency distribution was observed along the span of the 6Li foil(s) between the microstrip and drift electrodes. Maximum intrinsic thermal-neutron detection efficiencies of 12.58 ± 0.15% and 29.75 ± 0.26% for the single and five 6Li foils were measured, respectively. Gamma-ray rejection ratios of 6.46 × 10−5 ± 4.32 × 10−7 and 7.96 × 10−5 ± 4.65 × 10−7 were also measured, respectively, for a 137Cs exposure rate of 50 mR h−1. All measurements were conducted with the 6Li foil(s) contained within a sealed aluminum enclosure pressurized with 10 psig of P-10 gas.

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.

Boron-10 nanoparticles filled silicon trenches for thermal neutron detection application

This paper reports on the use of 10B nano/microparticles in order to fill microstructures of deep trenches fabricated in n-type Si (110) bulk wafers for the development of solid-state thermal neutron detectors. The high aspect-ratio trenches were fabricated in the wafer by wet etching, with a trench width of 3.5 to 6 μm and a maximum depth of 120 μm. Boron was diffused at a temperature of ∼1000 °C in order to convert the entirety of the delicate Si microstructures into a p+-n junction diode. The deep trenches of the diode were completely filled with 10B nanoparticles using a simple room-temperature process involving the pumping and venting of a vacuum chamber containing the etched wafer with 10B nanoparticles atop. The simple filling process was reproduced consistently, and the best 2.5 × 2.5 mm2 device demonstrated an intrinsic thermal neutron (En < 0.5 eV) detection efficiency of 32.2 ± 1.5% under a self-biased condition. This result is promising as it demonstrates a complete, low-cost fabrication process for the development of efficient thermal neutron detectors.

Solid-state neutron detectors based on thickness scalable hexagonal boron nitride

This paper reports on the device processing and characterization of hexagonal boron nitride (hBN) based solid-state thermal neutron detectors, where hBN thickness varied from 2.5 to 15 μm. These natural hBN epilayers (with 19.9% 10B) were grown by a low pressure chemical vapor deposition process. Complete dry processing was adopted for the fabrication of these metal-semiconductor-metal (MSM) configuration detectors. These detectors showed intrinsic thermal neutron detection efficiency values of 0.86%, 2.4%, 3.15%, and 4.71% for natural hBN thickness values of 2.5, 7.5, 10, and 15 μm, respectively. Measured efficiencies are very close (≥92%) to the theoretical maximum efficiencies for corresponding hBN thickness values for these detectors. This clearly shows the hBN thickness scalability of these detectors. A 15 μm thick hBN based MSM detector is expected to yield an efficiency of 21.4% if enriched hBN (with ∼100% 10B) is used instead of natural hBN. These results demonstrate that the fabrication of hBN thickness scalable highly efficient thermal neutron detectors is possible.

Metalorganic chemical vapor deposition of hexagonal boron nitride on (001) sapphire substrates for thermal neutron detector applications

This paper reports on the growth and characterization of hexagonal boron nitride (hBN) and its use for solid-state thermal neutron detection. The hBN epilayers were grown by metalorganic chemical vapor deposition on sapphire substrates at a temperature of 1350 °C. X-ray diffraction peak from the (002) hBN plane at a 2θ angle of 26.7° exhibited the c-lattice constant of 6.66 Å for these films. A strong peak corresponding to the high frequency Raman active mode of hBN was found for the films at 1370.5 cm-1 hBN-based solid-state neutron detectors were fabricated and tested with a metal-semiconductor-metal configuration with an electrode spacing of 1 mm and hBN thickness of 2.5 μm. Fabricated detectors showed strong response to deep UV light as well. An intrinsic thermal neutron detection efficiency of 0.86% was measured, which is close to the theoretically expected efficiency of 0.87%. These results demonstrate that epitaxial hBN films are promising for thermal neutron detection applications.

Monte Carlo simulation of random, porous (foam) structures for neutron detection

Porous media incorporating highly neutron-sensitive materials are of interest for use in the development of neutron detectors. Previous studies have shown experimentally the feasibility of 6LiF-saturated, multi-layered detectors; however, the random geometry of porous materials has limited the effectiveness of simulation efforts. The results of scatterless neutron transport and subsequent charged reaction product ion energy deposition are reported here using a novel Monte Carlo method and compared to results obtained by MCNP6. This new Dynamic Path Generation (DPG) Monte Carlo method was developed in order to overcome the complexities of modeling a random porous geometry in MCNP6. The DPG method is then applied to determine the optimal coating thickness for 10B4C-coated reticulated vitreous-carbon (RVC) foams. The optimal coating thickness for 4.1275cm-thick 10B4C-coated reticulated vitreous carbon foams with porosities of 5, 10, 20, 30, 45, and 80 pores per inch (PPI) were determined for ionizing gas pressures of 1.0 and 2.8atm. A simulated, maximum, intrinsic thermal-neutron detection efficiency of 62.8±0.25% was predicted for an 80 PPI RVC foam with a 0.2µm thick coating of 10B4C, for a lower level discriminator setting of 75keV and an argon pressure of 2.8atm.

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.

Microstructured silicon neutron detectors for security applications

In this paper we present the design and performance of a perforated thermal neutron silicon detector with a 6LiF neutron converter. This device was manufactured within the REWARD project workplace whose aim is to develop and enhance technologies for the detection of nuclear and radiological materials. The sensor perforated structure results in a higher efficiency than that obtained with an equivalent planar sensor. The detectors were tested in a thermal neutron beam at the nuclear reactor at the Instituto Superior Técnico in Lisbon and the intrinsic detection efficiency for thermal neutrons and the gamma sensitivity were obtained. The Geant4 Monte Carlo code was used to simulate the experimental conditions, i.e. thermal neutron beam and the whole detector geometry. An intrinsic thermal neutron detection efficiency of 8.6%±0.4% with a discrimination setting of 450 keV was measured.

A suspended boron foil multi-wire proportional counter neutron detector

Three natural boron foils, approximately 1.0cm in diameter and 1.0µm thick, were obtained from The Lebow Company and suspended in a multi-wire proportional counter. Suspending the B foils allowed the alpha particle and Li ion reaction products to escape simultaneously, one on each side of the foil, and be measured concurrently in the gas volume. The thermal neutron response pulse-height spectrum was obtained and two obvious peaks appear from the 94% and 6% branches of the 10B(n,α)7Li neutron reaction. Scanning electron microscope images were collected to obtain the exact B foil thicknesses and MCNP6 simulations were completed for those same B thicknesses. Pulse-height spectra obtained from the simulations were compared to experimental data and matched well. The theoretical intrinsic thermal–neutron detection efficiency for enriched 10B foils was calculated and is presented. Additionally, the intrinsic thermal neutron detection efficiency of the three natural B foils was calculated to be 3.2±0.2%.

Scalable large-area solid-state neutron detector with continuous p–n junction and extremely low leakage current

We report on the fabrication and characterization of solid-state thermal neutron detectors with detection areas up to 16cm2 that require only a single preamplifier for data acquisition. These detectors consist of a honeycomb-like micro-structured Si diode with boron-10 filled deep holes. A continuous p–n junction formed over the entire surface of the microstructure helps to achieve a low leakage current density of ~6.1×10−9A/cm2 at −1V for a 2.5×2.5mm2 detector. This low leakage current results in low electronic noise, which enables the fabrication of large-area detectors. An intrinsic thermal neutron detection efficiency of up to 26% was measured for a 2.5×2.5mm2 detector module and up to 24% was measured for a 1cm2 detector module. These measurements were obtained under zero bias voltage using a moderated californium-252 source. The relative efficiency remains almost the same when scaling the detector area up to 8cm2 by connecting 1cm2 detector modules in series. However, it decreases to 0.89 and 0.82, respectively, for 12 and 16cm2. Nevertheless, these results demonstrate the promise of using boron filled micro-structured Si diodes as a cost effective alternative to the helium-3 based neutron detection technology and the potential of fabricating scalable large-area solid-state neutron detectors that are desirable for many applications.

Characterization of a mid-sized Li foil multi-wire proportional counter neutron detector

A 550cm2 thermal neutron detector was constructed with five parallel sheets of 75µm thick 6Li foil (95% enrichment) spaced 1.63cm apart. Anode wire banks containing a plurality of anode wires were strung on both sides of each foil, six banks in total. The chamber was backfilled with P-10 proportional gas and over-pressured to 1.1, 1.5, 2.0, and 2.8atm (111, 151, 202, and 284kPa). The design was tailored to allow the products from the 6Li(n,t)4He reaction to escape both sides of the Li foil simultaneously, thereby, allowing for concurrent measurement in the proportional gas. The measured intrinsic thermal neutron detection efficiency of the detector with normal incident thermal neutrons to the foil sheets was 53.8±0.20%. When the detector was angled (55° from normal) such that a 0.5cm diameter thermal neutron beam intersected all of the foil layers, the intrinsic thermal neutron detection efficiency increased to 58.6±0.21%. A 252Cf neutron source positioned at a distance of 2.0m yielded an absolute neutron detection efficiency of 0.73cpsng−1. The gamma-ray rejection ratio (GRR) was 7.67×10–9 as measured from a 60Co source for an exposure rate of 40mRhr−1. Theoretical pulse-height spectra obtained with MCNP6 agreed well with experimental data and allowed pulse-height spectra and discriminator settings to be energy-calibrated. These results demonstrate the potential for the Li foil multi-wire proportional counter (MWPC) as a viable 3He neutron detector replacement.

Thermal neutron response and theoretical comparison of LiF coated aluminized Mylar

Thin layers of LiF were deposited on a 2.0µm thick aluminized BoPET (i.e. Mylar) in thicknesses of 4.5, 9.9, and 14.0µm using an electron beam evaporator. These coatings were thinner than the summed triton and alpha particle range from the 6Li(n,t)4He reaction, which allows both particles to escape a suspended absorber sheet simultaneously and measured in a proportional gas region concurrently. Each thickness of the LiF coated Mylar sheets were positioned separately in a test chamber that had a single anode wire positioned on each side of the absorber sheets. The thermal neutron response pulse-height spectra were collected for each LiF thickness and are presented and discussed. The coatings became fragile at thicknesses greater than 5.0µm and would flake off of the Mylar sheets. Additionally, the ideal LiF coating thickness that maximizes the intrinsic thermal neutron detection efficiency is greater than 5.0µm, which is discussed in a greater detail in the text. Overall, the detectors are capable of achieving thermal-neutron detection efficiencies greater than 30% for a 5 layer device and 60 % for 20 layers, but these devices are complex to fabricate due to flaking of the LiF coatings. Additional research is required to eliminate flaking by possibly using additional mechanical structures or adhesive materials.

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.