Low Gamma-ray Sensitivity Research Articles

Feasibility of neutron coincidence counting for spent TRISO fuel

High-temperature gas reactors rely on TRIstructural-ISOtropic (TRISO) fuel for enhanced fission product retention. Accurate fuel characterization would improve monitoring of efficient fuel usage and accountability. We developed a new neutron multiplicity counter (NMC) based on boron coated straw (BCS) detectors and used it in coincidence mode for 235U assay in TRISO fuel. In this work, we demonstrate that a high-efficiency version of the NMC encompassing 396 straws is able to estimate the 235U in used TRISO-fueled pebbles or compacts with a relative uncertainty below 2.5% in 100s. We performed neutronics and fuel depletion calculation of the HTR-10 pebble bed reactor to estimate the neutron and gamma-ray source strengths of used TRISO-fueled pebbles with burnup between 9 and 90GWd/t. Then, we measured a gamma-ray intrinsic efficiency of 10−12 at an exposure rate of 340.87R/h. The low gamma-ray sensitivity and high neutron detection efficiency enable the inspection of used fuel.

Neutron detection efficiency and response functions of a thin neutron silicon sensor with low gamma-ray sensitivity

Response functions of a 60-μm thick neutron silicon sensor to 8-MeV neutrons were obtained experimentally and simulated using a Monte Carlo code. The simulation reproduced the experimental results well, however it was found that the discrepancy was due to insufficient treatment of energy deposition process of both alpha particles and 25Mg nuclei emitted simultaneously from the 28 Si(n,α)25Mg reaction. Here, experimental detection efficiencies were well simulated, within 14% difference for neutron energies of 1.1, 2.5, 5, 8, and 15 MeV. An optimized neutron detection efficiency curve of the neutron sensor by weighting high energy contribution was in good agreement with an ambient-dose-equivalent curve, within a factor of 2 at neutron energies from 2.5 to 100 MeV.

Microstructured boron foil scintillating G-GEM detector for neutron imaging

In this study, a new simple neutron imaging gaseous detector was successfully developed by combining a micro-structured 10B foil, a glass gas electron multiplier (G-GEM), and a mirror–lens–charge-coupled device (CCD)–camera system. The neutron imaging system consists of a chamber filled with Ar/CF4 scintillating gas mixture. Inside this system, the G-GEM is mounted for gas multiplication. The neutron detection in this system is based on the reaction between 10B and neutrons. A micro-structured 10B is developed to overcome the issue of low detection efficiency. Secondary electrons excite Ar/CF4 gas molecules, and high-yield visible photons are emitted from those excited gas molecules during the gas electron multiplication process in the G-GEM holes. These photons are easily detected by a mirror–lens–CCD–camera system. A neutron radiograph is then simply formed. We obtain the neutron images of different materials with a compact accelerator-driven neutron source. We confirm that the new scintillating G-GEM-based neutron imager works properly with low gamma ray sensitivity and exhibits a good performance as a new simple digital neutron imaging device.

Development of a wavelength-shifting-fibre-based scintillator neutron detector as an alternative to 3He at J-PARC/MLF

A wavelength-shifting-fibre-based scintillator detector has been developed as an alternative detector to 3He gas. The detector is intended for use in an inelastic neutronscattering instrument at J-PARC. The detector being developed, which is based on the one made for a SENJU diffractometer, is designed to cover a large scattering angle with a moderate pixel size as well as exhibiting a high detection efficiency, low gamma-ray sensitivity and low background count rate. A prototype detector with a pixel size of 20 x 20 mm and a neutron- sensitive area of 320 x 320 mm2 has been constructed. This paper describes the design of the detector and its performance relative to the original SENJU detector.

Detection of Fast Neutron by Storage Phosphors With Low Gamma-Ray Sensitivity

Storage phosphors such as SrBPO5:Eu2+ with low gamma-ray sensitivity were applied to fast neutron detection using polyethylene (PE) sheets as a proton generator that were set in front of the phosphors. The photostimulated luminescence (PSL) yields were measured by changing PE thickness when neutron energy was 5 MeV. The experimental results were compared with those by Monte Carlo simulation using PHITS code. The PSL yields by gamma-ray associated with fast neutron fields were estimated for both SrBPO5:Eu2+ sheets and commercial available imaging plates. The results showed that the imaging methods using such storage phosphors with low gamma-ray sensitivity had negligible gamma-ray influences and will be used for fast neutron imaging.

Development of Two-Dimensional Micro-Strip Gas Detector With Individual Readouts for Neutron Scattering Experiments

We have been developing a Micro-Strip Gas Chamber (MSGC) with individual readouts for neutron scattering experiments. Performance requirements of a two-dimensional position sensitive neutron imaging detector for detecting an accurate Bragg-peak from a sample target include high spatial resolution, high detection efficiency, fast temporal response, and low gamma-ray sensitivity. To meet these specifications in addition to achieving a compact size in spite of the multi-channels in the gas based detector, we constructed a detector system that has a gas vessel with 541 channel feedthrough directly connecting to the data processing circuits. The developed detector system exhibited a pulse pair resolution of less than 1 mus, a spatial resolution of about 0.8 mm in full width at half maximum and good agreement between the position of incident neutron and measured peak position

RBPO/sub 5/ (R=Ca, Sr)-based storage phosphors for neutron detection

Storage characteristics of RBPO/sub 5/ (R=Ca, Sr)-based phosphors have been investigated for neutron detection. To increase the photostimulated luminescence (PSL) yields of CaBPO/sub 5/:Ce/sup 3+/ and SrBPO/sub 5/:Ce/sup 3+/ phosphors, fluorine atoms were introduced and the molar ratio of the fluorine atoms was optimized for the PSL yields. In addition, the authors found that the PSL yields of SrBPO/sub 5/:Eu/sup 2+/ phosphors were significantly increased by adding Li/sub 2/B/sub 4/O/sub 7/, though introducing fluorine and bromine atoms in the phosphor was not effective for the PSL yields. These phosphors will be usable as new neutron storage phosphors with low gamma-ray sensitivity because they consist of light materials.