Detailed Design of High-Resolution Gadolinium-GEM neutron detectors for the NMX instrument at ESS (#4067)
D. Pfeiffer1, 2, M. Christensen1, R. Hall-Wilton1, 3, I. Llamas-Jansa4, M. Lupberger2, E. Oksanen1, E. Oliveri2, L. Ropelewski2, M. Shetty1, P. Thuiner2
1 European Spallation Source, Lund, Sweden
The detector solution for NMX consists of Gas Electron Multiplier (GEM) detectors with natural Gadolinium converter, read out by VMM ASICs. This contribution presents the achievements made with regard to the detector design, readout and data analysis. To manufacture a large area neutron converter, ultrasonic welding has been successfully used as a way to efficiently join Gd foils. The design of the NMX detector has been optimized with the goal of minimizing the dead space inside and at the side of the detector. The VMM2 ASIC has been successfully integrated into the SRS readout system, and tools have been developed to monitor and analyze the data. Measurements with the VMM2 with gammas and neutrons show promising rate capabilities and a double hit resolution of 600 ns.
Keywords: Neutron detector, GEM, VMM2, VMM3, Gadolinium
Detection Efficiency and Spatial Resolution Analysis of Neutron Energy Selective Imaging with Neutron Sensitive Microchannel Plates (#3279)
Y. Zhao1, 2, Y. Yang1, 2, Z. Zhang1, 2
1 Tsinghua University, Department of Engineering Physics, Beijing, Beijing, China
Neutron sensitive microchannel plates (nMCP), as key components of image intensifier based neutron energy selective imaging systems, have played a major role in neutron energy selective imaging and have been applied in many areas owning its high detection efficiency and spatial resolution. Doping microchannel plate with neutron sensitive nuclides, proposed by G.W. Fraser et al., is a typical method to make normal MCP become neutron sensitive. Inspired by this approach, microchannel plates doped with 3 mol% natGd2O3 have been developed in this study, both in 50 mm and 106 mm diameter. Tests show this nMCP; the detection firstname.lastname@example.org meV thermal neutrons is 34%; assembling with a delay line readout, can achieve the spatial resolution of 64 μm and 88 μm for 15 kV X-rays and 25.3 meV neutrons respectively. The spatial resolution could be affected by a variety of factors, such as the materials concentrations of nMCP; the readout methods and the particle source. The spatial resolution could be, of course, affected by readout methods, such as Vernier, Wedge-and-Stripe, Delay line, Cross-strip, and Medipix, all of which have a different typical achievable spatial resolution. In previous work, Wedge-and-Strip and Delay line readouts have been used and test showed they have a considerably different achievable spatial resolution for a 106 mm diameter nMCP-MCP V-stack.
It should be noticed that the detection efficiency and spatial resolution is intrinsically determined by some internal processes occurring in the nMCP, such as the neutron absorption, IC electron emission and ionization, the electron multiplication and the electron cloud collection, all of which will somehow determine the ‘spatial resolution limitation’ of nMCP or nMCP stacks in a certain geometry and nuclide composition. This paper aims to thoroughly describe these processes and present ways in which they affect the detection efficiency and spatial resolution of the nMCP.
Keywords: nMCP; detection efficiency; spatial resolution; energy selective imaging;
Development of accelerator-driven transportable neutron source for non-destructive inspection of concrete construct (#3724)
T. Kobayashi1, Y. Otake1, Y. Kushima2, Y. Ikeda1, N. Hayashizaki3
1 RIKEN, Neutron Beam Technology Team, Saitama, Japan
A compact neutron source by using a particle accelerator is a promising tool for actual material analysis, infrastructural diagnostics, nuclear detection, and medical treatment. We have been operating the neutron source RANS (RIKEN Accelerator-driven compact Neutron Source) with 7 MV proton LINAC and a beryllium target for 5 years and learned a lot about measurement, maintenance and safety management. The weight of the accelerator and the target station of RANS are 5 tons and 25 tons, respectively. For outdoor use of neutrons such as non-destructive inspection of old concrete constructs, developing of smaller and lighter system is required. We started research and development of a mobile neutron source “RANS2” with a newly designed proton LINAC. Based on the experimental results at RANS facility, we chose lithium for neutron generation target material and the energy of the new accelerator was set at 2.49 MV. Total neutron yield at the proton current of 100 uA is calculated to be about 1011 neutrons / sec. Although it is only equivalent to 1/10 of RANS, there are many advantages. The neutron energy distribution of RANS2 is mainly between 300 and 700 keV. Fast neutrons of several hundreds keV are suitable for transmission and reflection imaging of concretes around 30 cm thick. The shield surrounding the target can lose much weight because there are no MeV neutrons. The shield of backward direction will be simplified compared to RANS because a large portion of neutrons are emitted in a forward direction with 2.49 MeV Li (p, n) Be reaction. The present status of the R&D is as follows. The design and the fabrication of the ECR proton ion source and 2.49 MV 200 MHz RFQ LINAC were completed including vacuum and cooling test. Ion generation and acceleration test is in preparation. The lithium target including cooling system, neutron reflector and shielding is in the middle of design with using numerical simulation. The total system will be completed by 2018.
Keywords: Accelerator, Neutron source, Imaging
Measurement of secondary cosmic-ray neutrons from near the geomagnetic North Pole (#1563)
R. S. Woolf1, L. Sinclair2, R. Van Brabant2, B. Harvey2, B. F. Phlips1, E. G. Jackson3
1 U. S. Naval Research Laboratory, Space Science Division, Washington, DC, United States of America
We report the results from a campaign to measure thermal and fast neutrons conducted near the geomagnetic North Pole at CFS Alert, Nunavut, Canada (82.5ºN, 62.5ºW; cutoff rigidity, RC = 0 GV) in June of 2016. The spectrum of cosmogenic neutrons at Earth’s surface covers a wide energy range, from thermal to several GeV; the flux varies with magnetic latitude, elevation, solar activity and nearby materials. We performed measurements at CFS Alert for varying elevation and ground moisture with moderated and unmoderated 3He detectors – thermal and epithermal sensitivity – and EJ-299-33 pulse shape discrimination plastic scintillators – fast neutron and gamma-ray sensitivity. For comparison, we performed a follow-on measurement campaign near Ottawa, Canada (45.4ºN, 75.7ºW; RC = 1.5 GV) at similar elevations. To correct the thermal/epithermal data sets for the aforementioned parameters affecting the local neutron flux, we used a predictive model originally developed for soft-error upset testing based on both measurements and calculations. To first order our data supports the model predictions; unaccounted for higher-order effects, such as ground moisture, could be the cause to the observed discrepancies between our uncorrected and corrected data. Additionally, we investigated the dependence of the fast neutron spectral distribution for varying elevation and ground moisture at similar RC. In our presentation we will show the fully corrected model-data comparison to include ground moisture content for the thermal/epithermal neutron measurements, the fast neutron spectral distributions as a function of elevation, Monte Carlo modeling results, and discuss the implications for future cosmogenic neutron rate modeling of our high latitude data.
Keywords: Cosmogenic neutrons, thermal neutrons, epithermal neutrons, fast neutrons, helium3, EJ-299-33, geomagnetic north pole
Characterization & Aging Studies of Lithium-Glass/Polyvinyl Toluene Composites with Varying Geometries for Neutron Detection (#2591)
A. Foster1, A. Meddeb2, K. Wilhelm2, Z. Ounaies1, 2, I. Jovanovic3
1 Pennsylvania State University, Materials Science and Engineering, State College, Pennsylvania, United States of America
Composites fabricated from lithium-glass embedded in polyvinyl toluene matrices have shown promise as neutron detectors with excellent gamma/neutron discrimination. By coupling two materials with dissimilar scintillation decay times, neutron cross-sections, and dimensions, excellent pulse shape and energy discrimination for neutron detection can be realized. Part of this article focuses on the fabrication and characterization of two lithium-glass and polyvinyl toluene composites with different glass geometries. A 1-3 (“1” being the dimensionality of the filler, i.e. glass, and “3” being the dimensionality of the matrix, i.e. polymer) glass rods composite and 0-3 glass shards composite were chosen based on previously done simulations. Challenges associated with the rods and shards geometries were overcome through careful manipulation of processing parameters. Optical and nuclear properties of the detectors were also investigated. The neutron intrinsic efficiencies of the composites were measured and found to be 4.80±0.054% and 0.097±0.036% for the rods and shards detectors, respectively, with excellent gamma rejection for both (measured to be better than 10^-5). The other part of this study focuses on the aging effects associated with the polymer matrix. Rapid clouding of the PVT occurs when the polymer is exposed to water with an increased temperature. However, aging can be reversed when annealed at a high temperature, which suggests a recovery process is possible. Progress has been made in determining the causes of the aging and possible mechanism for degradation and regeneration of the matrix.
Keywords: Composite, Neutron, Detection, Fabrication, Light Output, Pulse-shape Discrimination
Comparison between Silicon Carbide and Diamond for thermal neutron detection (#2508)
O. Obraztsova1, L. Ottaviani1, W. Vervisch1, B. Geslot2, G. de Izarra2, O. Palais1, A. Lyoussi2
1 Aix-Marseille University, IM2NP - UMR CNRS 7334, Marseille, France
The principal role of neutron detectors in nuclear applications is to get information about the actual neutron yield and reactor environment. Harsh radiation environments near the nuclear reactor core require the radiation detectors to be resistant to high temperature (up to 500°C) and radiation level. It is worth noting that a detector for industrial environment applications must have the stable response over considerable long period of use.
Silicon Carbide is one of the most attractive materials for neutron detection. Thanks to its outstanding properties, such as high displacement threshold energy (22-35 eV), wide band gap energy (3.27 eV) and high thermal conductivity (4.9 W/cm·K), SiC can operate in harsh environment (high temperature, high pressure and high dose rate) without additional cooling system.
Diamond is another semi-conductor considered as one of most promising materials for radiation detection. Diamond possesses several advantages in comparison to other semiconductors such as a wider band gap (5.5 eV), higher threshold displacement energy (40-50 eV) and thermal conductivity (22 W/cm·K), which leads to low leakage current values and make it more radiation resistant that its competitors.
The aim of this work is to compare the ability and efficiency to detect thermal neutrons of these two semi-conductors as well as to study their radiation stability under the same irradiation conditions. For this purpose, the neutron irradiation tests of detectors were implemented in MINERVE research reactor at CEA Cadarache.
Keywords: SiC neutron detector, Diamond neutron detector, thermal neutron detection, radiation stability, neutron detectors