Semiconductor Neutron Detectors Research Articles

Two types of single sphere neutron spectrometers based on micro-structured semiconductor neutron detectors

Two types of single sphere neutron spectrometers based on a set of micro-structured semiconductor neutron detectors (MSNDs) were designed. Each neutron spectrometer consists of 31 MSNDs symmetrically located at certain positions along three perpendicular axes within a single moderating sphere. One uses high-density polyethylene (HDPE) moderating sphere, while the other uses borated polyethylene rather than HDPE as the outermost thin shell of moderating sphere. The second design aims to eliminate the backscattering effects in order to improve the response isotropy of slow neutrons. For two neutron spectrometers, a charged particle tracking (CPT) model was built with Monte Carlo transport code MCNPX to calculate neutron responses for 53 mono-energies from 10-9 MeV to 20 MeV for three different irradiation geometries. The feasibility of two neutron spectrometers and the isotropy of neutron responses were evaluated by simulating a set of irradiation exposures to different typical neutron spectra. A generalized regression neural network (GRNN) method was used to unfold neutron spectra without the need to input preset spectral information by the users. The neutron spectrum unfolding performance was assessed quantitatively by index Qs and the results from two spectrometers were compared and analyzed.

A novel 4H–SiC thermal neutron detector based on a metal-oxide-semiconductor structure

Energy resolution and leakage current are two key parameters for semiconductor neutron detectors. Metal-Oxide-Semiconductor (MOS) detectors introduce an additional oxide layer compared to Schottky Barrier Diodes (SBDs) detectors in order to reduce leakage current. However, this also leads to a degradation in energy resolution due to the introduction of an additional dead layer. Therefore, optimizing the thickness of the oxide layer is important for MOS detectors. In this study, a 4H–SiC-based MOS thermal neutron detector was designed with comprehensive consideration of the oxide layer. Subsequently, a prototype of the detector was fabricated and tested. The prototype of the MOS detector achieved an nA-level leakage current under a reverse bias of −200 V. Additionally, it demonstrated good energy resolution performance in a moderated D-T neutron source test. The MOS detector was able to clearly distinguish between the two reaction channels of 10B converter events, which is consistent with the theoretical branching ratio. This work highlights the potential application of 4H–SiC-based MOS detectors for thermal neutron detection.

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.

Semiconductor Detector Study for Detecting Fusion Neutrons using Geant4 Simulations

Accurate neutron flux measurements in fusion reactors are essential, in order to determine the feasibility and progress of the reaction as well as for safety issues. Semiconductor neutron detectors exhibit promising characteristics for operation in the extreme environmental conditions of fusion reactors. Silicon, Diamond and Silicon Carbide are the most studied and anticipated materials for constructing detectors with high efficiency and irradiation resistance. The ITER fusion reactor is expected to run D-D plasma measurements in the near future, so the detection of 2.45 MeV neutrons with appropriate detectors is of great and immediate importance. In the present work the study of 2.45 MeV neutrons interactions with a silicon, diamond and silicon carbide detector was made, using GEANT4 [1] simulations, in order to compare their response. An experimental study will follow at the neutron production facility of the TANDEM accelerator of the I.N.P.P. of the NCSR “Demokritos”, with detectors provided by CIVIDEC Instrumentation GmbH, so the geometry of the simulations was built accordingly. A quasi-monoenergetic neutron beam of 2.45 MeV was produced through 3H(p,n) reactions in a TiT target. Due to the low cross section of the reaction, biasing techniques were implemented in the simulation to increase the counting rate and thus producing realistic results. These biasing techniques were studied, with various tests and the parameters affecting the choice of the biasing factor are shown and discussed.

Latest developments in room-temperature semiconductor neutron detectors: Prospects and challenges

Latest developments in room-temperature semiconductor neutron detectors: Prospects and challenges

Toward achieving cost-effective hexagonal BN semi-bulk crystals and BN neutron detectors via halide vapor phase epitaxy

Presently, thermal neutron detectors fabricated from boron-10 enriched hexagonal boron nitride (h-10BN) ultrawide bandgap semiconductor grown by metal organic chemical vapor deposition (MOCVD) hold the record high detection efficiency among all solid-state detectors at 59%. To overcome the short comings of MOCVD growth, including inherently low growth rate and unavoidable impurities such as carbon in metal organic source, we demonstrate here the growth of natural hexagonal boron nitride (h-BN) semi-bulk wafers using halide vapor phase epitaxy (HVPE), which is an established technique for producing GaN semi-bulk crystals at a high growth rate. Electrical transport characterization results revealed that these HVPE grown materials possess an electrical resistivity of 1 × 1013 Ω cm, and a charge carrier mobility and lifetime product of 2 × 10−4 cm2/V s. Detectors fabricated from a 100 μm thick h-BN wafer have demonstrated a thermal neutron detection efficiency of 20%, corresponding to a charge collection efficiency of ∼60% at an operating voltage of 500 V. This initial demonstration opens the door for mass producing high efficiency h-BN semiconductor neutron detectors at a reduced cost, which could create unprecedented applications in nuclear energy, national security, nuclear waste monitoring and management, the health care industry, and material sciences.

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.

Comparison of Neutron Detection Performance of Four Thin-Film Semiconductor Neutron Detectors Based on Geant4.

Third-generation semiconductor materials have a wide band gap, high thermal conductivity, high chemical stability and strong radiation resistance. These materials have broad application prospects in optoelectronics, high-temperature and high-power equipment and radiation detectors. In this work, thin-film solid state neutron detectors made of four third-generation semiconductor materials are studied. Geant4 10.7 was used to analyze and optimize detectors. The optimal thicknesses required to achieve the highest detection efficiency for the four materials are studied. The optimized materials include diamond, silicon carbide (SiC), gallium oxide (GaO) and gallium nitride (GaN), and the converter layer materials are boron carbide (BC) and lithium fluoride (LiF) with a natural enrichment of boron and lithium. With optimal thickness, the primary knock-on atom (PKA) energy spectrum and displacements per atom (DPA) are studied to provide an indication of the radiation hardness of the four materials. The gamma rejection capabilities and electron collection efficiency (ECE) of these materials have also been studied. This work will contribute to manufacturing radiation-resistant, high-temperature-resistant and fast response neutron detectors. It will facilitate reactor monitoring, high-energy physics experiments and nuclear fusion research.

Enabling Ga2O3’s neutron detection capability with boron doping and conversion layer

There is a growing necessity to develop revolutionary neutron detectors for nuclear energy, nuclear physics, medical physics, astrophysics, biological imaging, nonproliferation, and national security. The often-used Helium-3 (He-3) neutron detector is becoming increasingly difficult to obtain due to He-3 shortages. As an emerging oxide semiconductor material, Ga2O3 exhibits excellent physical properties. These physical merits enable Ga2O3’s potential as a high-performance semiconductor neutron detector for extreme condition applications. Here, two approaches are explored, i.e., applying an exterior conversion layer of boron-10 (B-10) on Ga2O3 and directly doping B-10 into Ga2O3 to demonstrate Ga2O3’s capability for neutron detection. Using Monte Carlo simulation, we show the distinct difference in neutron detection efficiency of Ga2O3 when applying direct doping of B-10 into Ga2O3 vs applying a uniform B-10 conversion layer on top of Ga2O3. Our results exhibit that the theoretically predicted maximum doping level of B-10 in Ga2O3 does not lead to the same detection efficiency as that of a simple B-10 conversion layer when detecting 480 keV gammas. Except for the most thermalized neutrons at 0.01 eV, direct doping simulations are not able to achieve comparable results to that of the conversion layer method.

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.

LiInSe2 for Semiconductor Neutron Detectors

Lithium Indium Selenide (LiInSe2) is being developed for use as a room temperature semiconductor detector for thermal neutrons The material has been studied for a number of applications including nonlinear optics such as parametric oscillators, as anode material for lithium ion batteries, piezoelectrics, as a scintillation detector material and as a semiconductor detector material. The recent advances of the crystal growth, material processing and detector fabrication have led to semiconductor neutron detectors with up to 100 mm2 active area. The thermal neutron detection sensitivity and gamma rejection ratio (GRR) are comparable to 10atm, He-3 tubes of similar size. Synthesis, crystal growth, detector fabrication, and characterization are discussed.

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.

Simulation of GaN micro-structured neutron detectors for improving electrical properties**Project supported by the National Natural Science Foundation of China (Grant Nos. 11675198, 11875097, 11975257, 61774072, 61574026, and 61971090), the National Key Research and Development Program of China (Grant Nos. 2016YFB0400600 and2016YFB0400601), the Fundamental Research Funds for the Central Universities, China (Grant No. DUT19LK45), the China Postdoctoral Science Foundation (Grant No.

Nowadays, the superior detection performance of semiconductor neutron detectors is a challenging task. In this paper, we deal with a novel GaN micro-structured neutron detector (GaN-MSND) and compare three different methods such as the method of modulating the trench depth, the method of introducing dielectric layer and p-type inversion region to improve the width of depletion region (W). It is observed that the intensity of electric field can be modulated by scaling the trench depth. On the other hand, the electron blocking region is formed in the detector enveloped with a dielectric layer. Furthermore, the introducing of p-type inversion region produces new p/n junction, which not only promotes the further expansion of the depletion region but also reduces the intensity of electric field produced by main junction. It can be realized that all these methods can considerably enhance the working voltage as well as W. Of them, the improvement on W of GaN-MSND with the p-type inversion region is the most significant and the value of W could reach 12.8 μm when the carrier concentration of p-type inversion region is 1017 cm−3. Consequently, the value of W is observed to improve 200% for the designed GaN-MSND as compared with that without additional design. This work ensures to the researchers and scientific community the fabrication of GaN-MSND having superior detection limit in the field of intense radiation.

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.

Boric acid as converter material for semiconductor neutron detector- A GEANT4 simulation study

Monte Carlo based GEANT4 simulations have been carried out to estimate the efficiency (η) of thermal neutron detection with boric acid (H3BO3) as a converter material. Different geometries of detector configurations — planar stack, sphere, cylinder and cuboid were considered and their efficiency were computed for relevant geometric parameters along with different 10B enrichment (BE) levels content in boric acid, with Low Level Discriminator (LLD) value fixed at 300 keV. The planar stack configuration detector was designed by replicating two dimensional detector units (DUs) – each consisting of boric acid as a converter material and Silicon (Si) as a detector material – in the third dimension. The simulation carried out for varied layer thicknesses (0.25μm to 10μm) of boric acid showed that efficiency (η) increased with increasing thickness, reached a maximum at a critical thickness (tsc) and thereafter found to decrease. Typically, at a critical thickness of ∼3.5μm, the η for 100% BE content in boric acid was found to be 15.96% for 30 DUs. For the case of sphere, cylinder and cuboid of boric acid embedded in Si, the simulations were performed for diameter and width both varying from 0.5μm to 9.5μm in steps of 0.5μm. In particular for cylinder and cuboid geometries, the depth was also varied from 25μm to 275μm with a step size of 25μm. In all these configurations η was found to increase initially and was maximum at a critical diameter and width of ∼8–9μm, and thereafter it was found to reduce. Apart from the simulated η estimation, histogram plots depicting the energy deposited in Si detector region by the generated charged particles (7Li = 0.839 MeV and α=1.47MeV) upon thermal neutron interaction in boric acid have also been studied for various geometrical parameters such as thickness, diameter and width of detector.

Boron film laser deposition by ultrashort pulses for use as neutron converter material

This study investigated the production of boron films by femtosecond pulsed laser deposition (PLD) to be used as converters on bulk semiconductor neutron detectors. The ablation threshold of metallic boron was determined and the film growth was studied as a function of deposition time (5–90 min) and laser pulse energy (35–530 μJ). The films were characterized by scanning electron microscopy (SEM), revealing a flaky morphology, optical profilometry, which determined the films thicknesses (from 80 nm up to 4 μm), Ion Beam Analysis (IBA) that assessed their elemental composition and X-ray diffraction (XRD), which revealed an amorphous structure. In addition, a thermal load study was performed to evaluate the heat flux onto the substrate during deposition process. Stable boron films obtained show that the femtosecond PLD process is reliable and reproducible for the fabrication of thick boron coatings.

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.

Efficiency of depleted UO2 based semiconductor neutron detectors in direct and indirect configuration—A GEANT4 simulation study

In the present work, Monte Carlo simulations using GEANT4 are carried out to estimate the efficiency of semiconductor neutron detectors with depleted UO2 (DUO2) as converter material, in both planar (direct and indirect) and 3D geometry (cylindrical perforation and trenches structure) configurations. The simulations were conducted for neutrons of variable energy viz., thermal (25 meV) and fast (1 to 10 MeV) that were incident on varying thicknesses (0.25 μm to 1000 μm), diameters (1 μm to 9 μm) and widths (1 μm to 9 μm) along with depths (50 μm to 275 μm) of DUO2 for planar, cylindrical perforated and trench structures, respectively. In the case of direct planar detectors, efficiency was found to increase with the thickness of DUO2 and the rate at which efficiency increased was found to follow the macroscopic fission cross section at the corresponding neutron energy. In the case of indirect planar detector, efficiency was lower as compared to direct configuration and was found to saturate beyond a thickness of ∼3 μm. This saturation is explained on the basis of mean free path of neutrons in the DUO2 material. For the 3D perforated silicon detectors of cylindrical (trench) geometry, backfilled with DUO2, the efficiency for detection of thermal neutrons ∼25 meV and fast neutrons ∼ typical energy of 10 MeV was found to be ∼0.0159% (∼0.0177%) and ∼0.0088% (0.0098%), respectively. These efficiency values were two (one) order values higher than planar indirect detector for thermal (fast) neutrons. Histogram plots were also obtained from the GEANT4 simulations to monitor the energy distribution of fission products in planar (direct and indirect) and 3D geometry (cylindrical and trench) configurations. These plots revealed that, for all the detector configurations, the energy deposited by the fission products are higher as compared to the typical gamma ray background. Thus, for detectors with DUO2 as converter material, higher values of low level discriminator (LLD) can be set, so as to achieve good background discrimination.

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.