Neutron Converter Material Research Articles

High aspect ratio composite structures with 48.5% thermal neutron detection efficiency

The pillar structured thermal neutron detector is based on the combination of high aspect ratio silicon p-i-n pillars surrounded by the neutron converter material 10B. By etching high aspect ratio pillar structures into silicon, the result is a device that efficiently absorbs the thermal neutron flux by accommodating a large volume fraction of 10B within the silicon pillar array. Here, we report a thermal neutron detection efficiency of 48.5% using a 50 μm pillar array with an aspect ratio of 25:1.

New Type of Neutron Image Scintillator based on H310BO3/ZnS(Ag)

Abstract In this contribution we present a system of converter (H3 10BO3) and phosphor (ZnS:Ag) which were fabricated by spraying method. The particle sizes and the mass ratio of the neutron converter and phosphor materials were optimized by simulation and experimental methods, respectively. Furthermore, the spatial resolution of scintillators at thicknesses of 0.40 nm and 0.35 mm were1.11 lp/mm (MTF = 0.1) and 1.33 lp/mm (MTF = 0.1), respectively. The results indicated that H3 10BO3/ZnS(Ag) scintillator have potential application in thermal neutron radiography.

Optimizing diode thickness for thin-film solid state thermal neutron detectors

In this work, we investigate the optimal thickness of a semiconductor diode for thin-film solid state thermal neutron detectors. We evaluate several diode materials, Si, CdTe, GaAs, C (diamond), and ZnO, and two neutron converter materials, 10B and 6LiF. Investigating a coplanar diode/converter geometry, we determine the minimum semiconductor thickness needed to achieve maximum neutron detection efficiency. By keeping the semiconductor thickness to a minimum, gamma rejection is kept as high as possible. In this way, we optimize detector performance for different thin-film semiconductor materials.

Gamma-ray rejection, or detection, with gadolinium as a converter

Gadolinium is a competent neutron conversion material for neutron detection due to its extremely high neutron capture cross section. It differs from the other neutron reactive materials by emitting large amounts of low-energy electrons for the consequent signal generation in a detector. Such low-energy electrons, though abundant, are prone to be contaminated by internal and/or external gamma rays, such as the activated 43.0 keV K-X rays, given the high atomic number of gadolinium. While the 43.0 keV K-X ray ought to be rejected as it originates in part from the external gamma rays when neutron detection is concerned, the ability to separate this energy line from other signals points to a practical mode of gamma-ray detection by a thin-film semiconductor with gadolinium as a converter. In this paper, a gamma-ray discrimination scheme for neutron detection is studied, which also provides insight into gamma-ray detection with a small semiconductor device with gadolinium as a converter, in line with the same principle of isolating the K-X rays activated by high- or medium-energy gamma rays.

Investigation of solid [formula omitted], [formula omitted] and [formula omitted] for ultracold neutron production

We have investigated the properties of the ultracold neutron converter materials deuterium D 2 , oxygen O 2 and heavy methane CD 4 in the temperature range between 8 K and room temperature. The experimental program was performed at the FUNSPIN beamline of the Swiss Spallation Neutron Source (SINQ) at Paul Scherrer Institut (PSI). In this paper the measured cold neutron total cross-sections for D 2 , O 2 and CD 4 are presented.

High Spatial and Temporal Resolution Neutron Imaging With Microchannel Plate Detectors

Special microchannel plates (MCPs) developed by Nova Scientific Inc. incorporate high efficiency neutron conversion materials into the MCP to provide a high neutron stopping power. <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">10</sup> B and <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">nat</sup> Gd have high interaction cross sections for thermal neutrons and their incorporation into MCP glass is a convenient way to make efficient MCPs for neutron detection with high spatial resolution. We have evaluated neutron event counting 2D imaging detectors using these MCPs with a cross delay line readout, cross strip readout, and a Medipix2 readout. Tests at several reactors with the cross delay line and cross strip readouts have established spatial resolution with neutrons as good as ~30 microns FWHM over a 27 mm diameter detector, with event rates approaching 1 MHz, low fixed pattern noise, event time tagging of 25 ns and intrinsic background rates of < 0.05 events cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-2</sup> sec <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> . Evaluation of neutron sensing MCP detector with Medipix2 readout (14 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ) has allowed operation at high counting rates (500 MHz) with the spatial resolution limited by the 55 micron pixel size of the Medipix2 readout. We have also used the Medipix2 for centroiding of neutron events to sub pixel resolution to obtain better spatial resolution (< 15 mum) for neutrons at reduced event rates (100 kHz). Initial measurements of thermal neutron detection efficiencies give values of 20% to 25% for thermal neutrons and 45% for cold neutrons without optimization of the detection geometry. Preliminary tests with shielding and a LaBr scintillator to gate neutron detections in coincidence with gamma rays produced by neutron interactions has enabled gamma ray rejection factors of 3 times 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sup> to be achieved. Further improvements in the neutron detection efficiency and gamma ray discrimination efficiency can be gained by optimization of the geometrical and electronic configurations.

High spatial resolution neutron sensing microchannel plate detectors

Neutron sensing microchannel plates (MCPs) developed by Nova Scientific Inc. incorporate high-efficiency neutron conversion materials in the MCP glass to provide a high neutron stopping power. We have developed and tested neutron event counting 2D-imaging detectors using these MCPs with three different imaging systems, a cross delay line readout, a cross strip readout, and a MEDIPIX readout. Tests at MNRC and NIST with the cross delay line and cross strip readouts have established spatial resolution with neutrons as good as 17 μm RMS over 27 mm, with event rates of 1 MHz, event time tagging of 25 ns and intrinsic background rates of <0.05 events cm −2 s −1. MEDIPIX readouts for neutron imaging have poorer spatial resolution (55 μm) and event timing, but are capable of even higher rates (GHz).

Luminescent detectors of ionising radiation

At present in slow neutron imaging an active layer of an imaging plate IP contains a mixture of storage phosphors, usually BaFBr:Eu2+ used for imaging of X-rays, and a neutron converter material, usually Gd2O3, LiF.A novel Li-containing luminescent material perspective for a direct neutron conversion and storage is discussed. Irradiation of LiBaF3 crystals results in generation of Frenkel defect pairs and creation of F-type centres responsible for three absorption bands in UV-and visible spectral region. Because photo-stimulation in each of these absorption bands leads to bleaching of induced absorption, the F-type colour centres are convenient for storage of radiation dose. Photo-stimulated decay of F-centres causes recombination luminescence of impurity centres. Thermoactivated decay of F-type centres is governed by ionic process. The thermal stability of F-centres at RT and consequent material storage characteristics can be improved by doping of the LiBaF3 with heterovalent oxygen impurities. The obtained radiation energy storage, photo- and thermostimulated read-out characteristics justify that LIBaF3 is a suitable active media for imaging of slow neutrons.

Development of neutron image plate for low-flux measurements

The results of an experimental test for the performance of a commercially available neutron-sensitive image-plate detector are presented. The particular aim of the experiments is to study the nature of noise in a neutron image obtained under the real conditions of a reactor hall. The most significant component of noise was found to be an external γ-ray background. The γ-ray dose of 1 μS/h leads to a noise in a neutron image with a neutron equivalent of about 1.2 n/cm2 s , which is comparable to magnitudes of weak diffraction peaks and diffuse scattering at thermal neutron scattering experiments. The high sensitivity of image plates to γ-rays results from the high charge density of storage-phosphor (BaFBr:Eu2+) and neutron-converter (Gd2O3) materials used in commercially available neutron image plates. The possibility of the use of 6LiF as a neutron converter instead of Gd2O3 to reduce the γ-ray sensitivity is discussed.

Feasibility of a neutron detector-dosemeter based on single-event upsets in dynamic random-access memories.

The feasibility was investigated of a solid-state neutron detector/dosemeter based on single-event upset (SEU) effects in dynamic random-access memories (DRAMs), commonly used in computer memories. Such a device, which uses a neutron converter material to produce a charged particle capable of causing an upset, would be light-weight, low-power, and could be read simply by polling the memory for bit flips. It would have significant advantages over standard solid-state neutron dosemeters which require off-line processing for track etching and analysis. Previous efforts at developing an SEU neutron detector/dosemeter have suffered from poor response, which can be greatly enhanced by selecting a modern high-density DRAM chip for SEU sensitivity and by using a thin 10B film as a converter. Past attempts to use 10B were not successful because the average alpha particle energy was insufficient to penetrate to the sensitive region of the memory. This can be overcome by removing the surface passivation layer before depositing the 10B film or by implanting 10B directly into the chip. Previous experimental data show a 10(3) increase in neutron sensitivity by chips containing borosilicate glass, which could be used in an SEU detector. The results are presented of simulations showing that the absolute efficiency of an SEU neutron dosemeter can be increased by at least a factor of 1000 over earlier designs.

The optimization and γ-ray effects of the neutron imaging plate

A neutron imaging plate has been optimized by considering the elementary processes of neutron detection occurring in the NIP. It is composed of neutron converter materials, and photostimulated luminescence materials which create color centers. As a function of a molar ratio of these materials and the thickness of the NIP, the optimized NIP was defined and it has been proved by the measurement of the neutron diffraction experiment from a protein crystal. The most serious problem is the γ-ray sensitivity of the NIP. The energy dependence of γ-ray sensitivity has been measured and the way how to shield from the γ-rays has been proposed.

Neutron detection with imaging plates Part I. Image storage and readout

The detection of neutrons with imaging plates is based on a sequence of physical processes, which are happening during the neutron exposure and the readout of the image information. These processes are investigated in detail starting with the neutron absorption and conversion to secondary radiation by various neutron converter materials and ending with the detection of the photostimulated luminescence, which is emitted during the readout. It is shown that these processes can be quantified by several key parameters, such as i.e. the neutron absorption cross section, the emitted secondary energy and the conversion efficiency of released secondary energy to storage centers in the storage phosphor. The resulting detector characteristics are described in the second part, namely, Neutron detection with imaging plates – part II: detector characteristics [Nucl. Instr. and Meth. A 424 (1999) 34–39].

Application of imaging plate neutron detector to neutron radiography

As an imaging plate neutron detector (IP-ND) has been available for thermal neutron radiography (TNR) which has high resolution, high sensitivity and wide range, some basic characteristics of the IP-ND system were measured at the E-2 facility of the KUR. After basic performances of the IP were studied, images with high quality were obtained at a neutron fluence of 2 to 7×10 8 n cm −2. It was found that the IP-ND system with Gd 2O 3 as a neutron converter material has a higher sensitivity to γ-ray than that of a conventional film method. As a successful example, clear radiographs of the flat view for the fuel side plates with boron burnable poison were obtained. An application of the IP-ND system to neutron radiography (NR) is presented in this paper.

Imaging performance of imaging plate neutron detectors

A new series of experimental imaging plate neutron detectors (IP-NDs) were made, where the composition of the respective IP-NDs, containing a photostimulable BaFBr:Eu 2+ phosphor and a neutron converter material, Gd 2O 3 or 6LiF, were varied systematically. The different conversion processes between natural abundant Gd and 6Li caused distinct imaging properties. The imaging steps and the factors governing the image quality are considered in the same way as X-ray radiography, and the quantum noise is estimated by the effective neutron absorption in these IP-NDs when they are read by a BAS2000 image reader. From this point of view, Gd is superior to 6Li, but 6Li is expected to be more suited to imaging than Gd in higher γ-ray to neutron ratios because of its higher amplification rate in the conversion and excitation process.

Light collection in scintillation detector composites for neutron detection

The authors have been investigating the heterogeneous combination of neutron-conversion materials in a plastic or liquid scintillation matrix with the goal of developing neutron detectors of high detection efficiency and fast response. They report on one such combination, consisting of thin-wall glass microspheres filled with high-pressure /sup 3/He gas dispersed in plastic scintillation. They have developed glass formulations that are capable of containing the helium in shells with wall thickness as low as 1 mu m with leak rates at room temperature that correspond to several years half life. The energy loss of the reaction products in the shell wall and the low light yield of plastic scintillations for low-energy protons result in low light yield per event. The authors discuss the use of an optical modeling code to investigate the incorporation of wavelength-shifting elements to aid in the collection of this light from highly scattering composite scintillation media. >

新型転換炉「ふげん」用炉内中性子検出器の開発

In the Prototype Heavy Water Reactor "Fugen", Zircaloy-2 is used as in-core structural material for neutron economy. In-core neutron detectors for the Local Power Monitor are placed in heavy water moderator of 50°C. Such "Fugen" in-core conditions bring a high level thermal neutron flux environment, which burns away the neutron conversion material (235U) in the detector more rapidly compared to that of the light water reactors. Therefore, it was strongly required to realize a neutron detector which could work for a long time under such environment.To resolve this unexperienced subject, the authors started to develop an advanced in-core neutron detector for "Fugen" by adopting various technologies, The developed neutron detectors have smart structures and excellent characteristics such as (1) light weight structure to avoid gas displacement due to γ-heating, (2) structure of assembling detectors in a Zircaloy-2 tube, (3) wide range and linear current measuring performance and (4) long life performance by the use of mixed uranium (235U+234U). This kind of in-core neutron detector is now in the reactor core of "Fugen" nuclear power plant and is contributing to the high-efficient reactor operation.