Neutron Converter Material Research Articles

Improving Detection Efficiency of Silicon Carbide Neutron Detector Using Double Trench

In this article, we propose a double-trench-type silicon carbide (SiC) neutron detector and model it using the concept of a unit cell on the GEANT4 platform. When the low-level discriminator (LLD) value was fixed at 300 keV, the structural parameters of this double-trench-type SiC neutron detector were optimized to maximize the intrinsic detection efficiency. Compared with the single trench type, the double-trench-type SiC neutron detector can significantly improve the neutron intrinsic detection efficiency within a limited trench depth, which was especially suitable for the current technological status that SiC materials can only be used for shallow trench etching. Apart from optimizing the structural parameters, the energy deposition spectra in the SiC detector region by the generated secondary charged particles upon thermal neutron interaction in neutron conversion materials ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$^{{10}}\text{B}$ </tex-math></inline-formula> and 6LiF) have also been estimated. It can be found that the double-trench-type SiC neutron detector counts significantly increased in the low-energy range, while the counts in the high-energy range are only slightly decreased. This is also the essential reason why the intrinsic detection efficiency of the double-trench-type SiC neutron detector was significantly improved compared to the single-trench-type SiC neutron detector.

Very cold and ultra cold neutron sources for ESS

The goal of the “Workshop on Very Cold and Ultra Cold Neutron Sources for ESS” was to discuss scientific cases, ideas and possibilities for the implementation of sources of Very Cold and Ultra Cold neutrons at the European Spallation Source. The ESS facility, presently under construction, offers several possibilities for in-pile UCN or VCN sources, in primis thanks to the available space below the spallation target where additional neutron sources can be placed to complement those above the target. Neutron beams can be extracted over a wide angular range with a grid of forty-two beamports with 6° average angular separation, allowing future instruments to be installed which may view either the upper or lower moderator systems. Of greatest interest for fundamental physics is the so-called Large Beamport foreseen for the NNBAR experiment. This beamport is also particularly well suited to feed a UCN source, for which several ideas were presented that employ either superfluid helium or solid deuterium as established neutron converter materials. Concepts for VCN sources make use of novel materials for VCN production and/or advanced reflectors to increase yields in the coldest part of the neutron spectrum from a cryogenic neutron source. In this paper we discuss these ideas and the possible locations of UCN and VCN sources at ESS.

Double-GEM based thermal neutron detector prototype

The Helium-3 shortage and the growing interest in neutron science constitute a driving factor in developing new neutron detection technologies. In this work, we report the development of a double-GEM detector prototype that uses a 10B4C layer as a neutron converter material. GEANT4 simulations were performed predicting an efficiency of (3.14 ± 0.10)%, agreeing within 2.7σ with the experimental and analytic detection efficiencies obtained by the detector when tested in a 41.8 meV thermal neutron beam. The detector is position sensitive, equipped with a 256+256 strip readout connected to resistive chains, and achieves a spatial resolution better than 3 mm. The gain stability over time was also measured with a fluctuation of about 0.2% h-1 of the signal amplitude. A simple data acquisition with only 5 electronic channels is sufficient to operate this detector.

Transient Current Analysis of Silicon Carbide Neutron Detector Using SRIM and TCAD

Since neutrons are electrically neutral, the methods used to detect neutrons generally rely on secondary charged particles, which are produced by the interaction of neutrons and neutron conversion materials (such as <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sup> LiF, <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">10</sup> B). In this report, the SRIM code has been used to analyze the ionization properties of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\alpha $ </tex-math></inline-formula> particles and <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> H particles traveling through the Silicon Carbide (SiC) diode, and subsequently, the final Bragg ionization distribution has been obtained. Next, according to the obtained ionization distribution of secondary particles, the linear energy transfer (LET) distribution of the secondary particles is extraced accurately. Finally, based on the distribution of LET, Technology Computer Aided Design (TCAD) simulations are utilized to analyze the transient current pulses of thin-film-coated and trench-type SiC diodes. In addition, the effects of particle energy and applied reverse bias on the output pulse characteristics of SiC diodes are also explored. Essentially, this paper augments the understanding of output response from SiC neutron detectors, and for trench-type detectors, the order of output current pulse amplitude decreases significantly and has a long-tail. Therefore, the trench-type detectors not only need a more advanced fabrication process but also require the design of dedicated readout electronics.

3D Microstructured Inorganic Perovskite Materials for Thermal Neutron Detection

AbstractA combination of novel techniques such as a solvent‐free thin‐film deposition, perovskite patterning, and 10B back‐fill technique enables the high neutron detection efficiency in a perovskite‐based microstructured thermal neutron detector. High‐efficiency cesium lead bromide (CsPbBr3) perovskite‐based microstructured detectors are demonstrated here. Trenches up to 10 µm deep are etched into the CsPbBr3 thin films using a novel dry etching process involving a combination of HBr and Ar plasma. The microstructured diodes are then backfilled with isotopically enriched boron as neutron conversion material via a sedimentation process to preserve the perovskite integrity. The fabricated microstructured CsPbBr3 thermal neutron detectors show an efficiency of 4.3%. This represents &gt;1.2x efficiency improvement over planar silicon (3.5%) and &gt;2x efficiency improvement over planar CsPbBr3 (2.1%) detectors, respectively. More importantly, gamma‐ray discrimination of 107 is measured in CsPbBr3‐based microstructured neutron detectors.

4H-SiC Schottky Barrier Diodes for Efficient Thermal Neutron Detection.

In this work, we present the improved efficiency of 4H-SiC Schottky barrier diodes-based detectors equipped with the thermal neutron converters. This is achieved by optimizing the thermal neutron converter thicknesses. Simulations of the optimal thickness of thermal neutron converters have been performed using two Monte Carlo codes (Monte Carlo N–Particle Transport Code and Stopping and Range of Ions in Matter). We have used 6LiF and 10B4C for the thermal neutron converter material. We have achieved the thermal neutron efficiency of 4.67% and 2.24% with 6LiF and 10B4C thermal neutron converters, respectively.

Performance Testing of Dysprosium-Based Scintillation Screens and Demonstration of Digital Transfer Method Neutron Radiography of Highly Radioactive Samples

Scintillator screens consisting of a dysprosium neutron converter and various scintillator materials were tested in the Heinz Maier-Leibnitz Zentrum Forschungsreaktor München II (FRM II) ANTARES cold neutron beam with the goal of finding a suitable screen for digital transfer method neutron radiography. This work explores the cold neutron response of 16 scintillator screens, 7 of which were previously tested with thermal neutrons. Light yield, signal-to-noise ratio (SNR), and spatial resolution were measured to compare the scintillator screens and determine which were best suited for digital transfer method neutron radiography. Screens with a zinc sulfide (ZnS:Cu) scintillator were most suitable for digital transfer method radiography based on light output, spatial resolution, SNR, and gamma-ray insensitivity. Spatial resolutions between 65 and 220 μm were measured. The top-performing screens were then used to demonstrate the feasibility of a new digital transfer method neutron radiography to image highly radioactive (8.84 Sv/h at ≈1 cm) nuclear fuel at Idaho National Laboratory’s Neutron Radiography reactor (NRAD). These results suggest that digital transfer method neutron radiography can be used to indirectly image highly radioactive objects and/or use neutron beams with a large gamma-ray content on a timescale of ~10 min/image (~144 images/day), much faster than the >10 h required using the current transfer method with film (limited to ~14 radiographs/day at NRAD).

Silicon carbide detectors for high flux neutron monitoring at near-core locations

Silicon carbide (SiC) and gallium nitride (GaN) semiconductors are two detector candidates for high flux neutron monitoring entailing high temperature and high radiation environments owing to their wide band gap and high resistance to radiation damage. While the neutron elastic scattering with carbon (C) and silicon (Si) makes SiC intrinsically sensitive to neutrons, neutron interaction with nitrogen (N) by way of 14N(n,p)14C reaction provides neutron sensitivity to GaN. In this study, we investigated reactor-based high flux neutron monitoring with in-house fabricated SiC detectors and studied the feasibility of neutron detection using GaN. As proof-of-concept for GaN neutron sensitivity, we evaluated SiC detectors coupled with nitrogen based (N-based) neutron converter materials, and commercially purchased AlGaN photo sensors. Spectral response as well as raw waveform data from SiC, SiC coupled with N-based converter layers, and AlGaN sensors were acquired while exposing the detectors to a mixed neutron–gamma field of a research reactor at ex-core locations at various power levels. The experimental SiC detector spectra were compared against Geant4 Monte Carlo (MC) simulations, which agreed with the measurement results. The reactor power determined using SiC detector raw data correlated well with that indicated by a standard compensated ion chamber (CIC). The AlGaN sensors showed promising results with a correlation between sensor response and reactor power. The current study demonstrates SiC as a suitable detector for high flux neutron monitoring with good radiation tolerance based on near-core irradiation.

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.

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.

Development of the “GP2” Detector: Modification of the PImMS CMOS Sensor for Energy-Resolved Neutron Radiography

This paper reports on the development and commissioning of the GP2 detector. GP2 was developed to address the requirement for a high-resolution event-mode imaging detector, for application in energy-resolved neutron radiography. The name GP2 derives from the use of gadolinium as a neutron conversion material, combined with a second-generation mass spectrometry sensor known as PImMS2. Theoretical and measured characteristics of GP2 are compared, with emphasis on the usability and functionality of the detector. The development of the detector has been steered by a design philosophy which was established to ensure that the detector has unique and novel impact. The motivation and consequences of the design philosophy are discussed. The key parameters reported are the neutron detection efficiency (7.5% at 2.5 A), gamma sensitivity ( $1.5 \times 10^{-3})$ , and a spatial resolution (modulation transfer function at 10%) of 6.4 lp/mm for a 4- $\mu \text{m}$ -thick natural gadolinium neutron converter film.

Analysis of an indirect neutron signature for enhanced UF6 cylinder verification

The International Atomic Energy Agency (IAEA) currently uses handheld gamma-ray spectrometers combined with ultrasonic wall-thickness gauges to verify the declared enrichment of uranium hexafluoride (UF6) cylinders. The current method provides relatively low accuracy for the assay of 235U enrichment, especially for natural and depleted UF6. Furthermore, the current method provides no capability to assay the absolute mass of 235U in the cylinder due to the localized instrument geometry and limited penetration of the 186-keV gamma-ray signature from 235U. Also, the current verification process is a time-consuming component of on-site inspections at uranium enrichment plants. Toward the goal of a more-capable cylinder assay method, the Pacific Northwest National Laboratory has developed the hybrid enrichment verification array (HEVA). HEVA measures both the traditional 186-keV direct signature and a non-traditional, high-energy neutron-induced signature (HEVANT). HEVANT enables full-volume assay of UF6 cylinders by exploiting the relatively larger mean free paths of the neutrons emitted from the UF6. In this work, Monte Carlo modeling is used as the basis for characterizing HEVANT in terms of the individual contributions to HEVANT from nuclides and hardware components. Monte Carlo modeling is also used to quantify the intrinsic efficiency of HEVA for neutron detection in a cylinder-assay geometry. Modeling predictions are validated against neutron-induced gamma-ray spectra from laboratory measurements and a relatively large population of Type 30B cylinders spanning a range of enrichments. Implications of the analysis and findings on the viability of HEVA for cylinder verification are discussed, such as the resistance of the HEVANT signature to manipulation by the nearby placement of neutron-conversion materials.

Initial results from new 3D neutron detectors

In this paper we report the initial results from our second generation of 3D silicon detectors for neutrons. The devices are briefly described and the first functional characterization tests carried out in laboratory before coupling to neutron converter material are reported. Particular emphasis is given to the read-out system used for the suppression of signals induced by γ-rays, that is one of the main issues in neutron detection. Experimental results are discussed with the aid of TCAD simulations.

Direct comparison between solid state Silicon+6LiF and 3He gas tube neutron detectors

During the last decade the worldwide lack of 3He for neutron detection has triggered research and development on alternative technologies and methods. One of the promising techniques consists of assembling sandwiches of fully depleted double sided Silicon detectors between thin layers of 6LiF neutron converter material. In this paper we show the very promising result of a comparative test of such a detector with respect to an elliptical section 3He filled proportional counter for application at spallation neutron source facilities.

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.

Experimental determination of gamma-ray discrimination in pillar-structured thermal neutron detectors under high gamma-ray flux

In this paper, we demonstrate a detector that has a high neutron-to-gamma discrimination of 8.5×105 with a high thermal neutron detection efficiency of 39% when exposed to a high gamma-ray field of 109 photons/cm2s. The detector is based on a silicon pillar structure filled with a neutron converter material (10B) designed to have high thermal neutron detection efficiency. The pillar dimensions are 50µm pillar height, 2µm pillar diameter and 2µm spacing between adjacent pillars.

스퍼터링 코팅기법을 이용한 중성자 검출용 B4C 박막 개발

gas has been used for neutron monitors as the neutron converter owing to its advantages such as high sensitivity, good -discrimination capability, and long-term stability. However, is becoming more difficult to obtain in last few years due to a global shortage of gas. Accordingly, the cost of a neutron monitor using gas as a neutron converter is becoming more expensive. Demand on a neutron monitor using an alternative neutron conversion material is widely increased. has many advantages among various alternative materials, as a neutron converter. In order to develop a neutron converter using (actually ), we calculated the optimal thickness of a neutron converter with a Monte Carlo simulation using MCNP6. In addition, a neutron converter was fabricated by the Ar sputtering method and the neutron signal detection efficiencies were measured with respect to various thicknesses of fabricated a neutron converter. Also, we developed a 2-dimensional multi-wire proportional chamber (MWPC) for neutron beam profile monitoring using the fabricated a neutron converter, and performed experiments for neutron response of the neutron monitor at the 30 MW research reactor HANARO at the Korea Atomic Energy Research Institute. The 2-dimensional MWPC with boron () neutron converter was proved to be useful for neutron beam monitoring, and can be applied to other types of neutron imaging.

Characterization of 3D and planar Si diodes with different neutron converter materials

In this paper, we report on the characterization of silicon 3D and planar sensors, coupled with different neutron converter materials, such as 10B, B410C and 6LiF, with different deposition thickness. Selected results from the electrical and functional characterization of the devices are shown and comparatively discussed with the aid of SRIM and Geant4 simulations.The limited neutron detection efficiency, on the order of ≃ 1% (planar) and ≃ 8% (3D) from simulations, is understood, and hints for the optimization of the devices have been derived.

Stability of 10B4C thin films under neutron radiation

Thin films of 10B4C have shown to be very suitable as neutron-converting material in the next generation of neutron detectors, replacing the previous predominantly used 3He. In this contribution we show under realistic conditions that 10B4C films are not damaged by the neutron irradiation and interactions, which they will be exposed to under many years in a neutron detector. 1 μm thick 10B4C thin films were deposited onto Al or Si substrates using dc magnetron sputtering. As-deposited films were exposed to a cold neutron beam with fluences of up to 1.1×1014 cm−2 and a mean wavelength of 6.9 Å. Both irradiated and as-deposited reference samples were characterized with time-of-flight elastic recoil detection analysis, scanning electron microscopy, transmission electron microscopy, X-ray photoemission spectroscopy, and X-ray diffraction. We show that only 1.8 ppm of the 10B atoms were consumed and that the film composition does not change by the neutron interaction within the measurement accuracy. The irradiation does not deteriorate the film adhesion and there is no indication that it results in increased residual stress values of the as-deposited films of 0.095 GPa. From what is visible with the naked eye and down to atomic level studies, no change from the irradiation could be found using the above-mentioned characterization techniques.

Thin film cadmium telluride charged particle sensors for large area neutron detectors

Thin film semiconductor neutron detectors are an attractive candidate to replace 3He neutron detectors, due to the possibility of low cost manufacturing and the potential for large areas. Polycrystalline CdTe is found to be an excellent material for thin film charged particle detectors—an integral component of a thin film neutron detector. The devices presented here are characterized in terms of their response to alpha and gamma radiation. Individual alpha particles are detected with an intrinsic efficiency of >80%, while the devices are largely insensitive to gamma rays, which is desirable so that the detector does not give false positive counts from gamma rays. The capacitance-voltage behavior of the devices is studied and correlated to the response due to alpha radiation. When coupled with a boron-based neutron converting material, the CdTe detectors are capable of detecting thermal neutrons.