N5A3  Neutron Detectors: Thermal Neutron Convertors

Friday, Nov. 6  08:30-10:10  Golden West

Session Chair:  Richard Kouzes, Pacific Northwest National Laboratory, United States; Marek Flaska, Pennsylvania State University, United States

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(08:30) N5A3-1, Multi-Grid Boron-10 Detector for Time-of-Flight Spectrometers in Neutron Scattering Science

A. Khaplanov1,2, J. Birch3, J.-C. Buffet2, P. van Esch2, M. Etxegarai1, M. Ferraton2, B. Guerard2, R. Hall-Wilton1,4, L. Hultman3, C. Hoglung1, F. Piscitelli1,2, I. Stefanescu1

1European Spallation Source, Lund, Sweden
2Institute Laue Langevin, Grenoble, France
3Linkoping University, Linkoping, Sweden
4Mid-Sweden University, Sundsvall, Sweden
The Multi-Grid (MG) detector has been introduced at ILL and developed by a collaboration between ILL, ESS and Linkoping University. This detector design addresses the severely decreased availability of He3, in particular for neutron scattering instruments with large-area detectors, such as time-of-flight neutron spectrometers at ESS and other facilities. The MG detector is based on thin converter films of boron-10 carbide arranged in layers orthogonal to the incoming neutrons. The design of the detector provides position resolution, efficiency competitive with He3 and a strong gamma rejection capability. This paper presents the MG large-area (2.4m2) demonstrator and the progress made in order to meet the needs of production of B4C-coated layers, mechanical parts and assembly on a scale similar to that of the final detectors for ESS. A particular effort was made to produce aluminium detector parts with a low alpha background, successfully reducing the background rate to acceptable levels. Following the IN5 demonstrator, a compact prototype has been designed in order to finalise the electronic readout to be used at the ESS instruments equipped with the MG.

(08:50) N5A3-2, 10B4C Coating Development for Future High Performance Neutron Detectors

S. Schmidt1,2, M. Imam1,2, C. Höglund1,2, H. Pedersen1, J. Jensen1, K. Zeitelhack3, L. Hultman1, J. Birch1, R. Hall-Wilton2

1Department of Physics (IFM), Linköping University, Department of Physics (IFM), Linköping, Sweden
2Detector group, European Spallation Source ESS AB, Lund, Sweden
3FRM-II, T, Garching, Germany
During the past years it was shown that 10B based detectors are practicable alternatives to 3He detectors. Here, the neutron absorbing element, 10B, is deposited in form of a thin film on various substrate types. For this purpose, 10B4C coatings are considered to be favorable due to their stability and the scalability of the production process. In this study, we investigated a wide range of chemical vapor deposition (CVD) and magnetron sputtering production processes in order to deposit 10B4C coatings. The deposition techniques are assessed by means of the coating properties that are regarded as most important for neutron detector coatings. Thus, the resulting coatings are characterized by scanning electron microscopy, X-ray diffraction, and time-of-flight elastic recoil detection to determine their deposition rate, density, stresses and composition, respectively. In addition, the radiation hardness of 10B4C coatings was studied. Both deposition techniques show confined process windows for the production of high quality 10B4C coatings. CVD processes yielded best results for coatings deposited in triethylborane/Ar and triethylborane/H2 at temperatures > 600 ºC and < 900 ºC. Here, coatings featuring high B/C ratios of up to 4.5, H contents < 0.3 at% and densities of up to 2.43 g/cm3 were obtained. Magnetron sputter processes were carried out using B4C compound targets in Ar atmosphere in direct-current and high power impulse magnetron sputter mode. Direct-current magnetron sputtering offers optimum conditions for the production of high quality coatings when low process pressures of up to 300 mPa and substrate temperatures between 300 ºC and 400 ºC are used. B4C coatings deposited within this parameter window showed B/C ratios of up to 4.1, H-contents < 1 at% and densities of 2.45 g/cm3.

(09:10) N5A3-3, The Research of High Detection Efficiency Boron Lined Detector with a Honeycomb Neutron Converter

Z. Fang1,2, Y. Yang1,2, Y. Li1,2

1Department of Engineering Physics, Tsinghua University, Beijing, P. R. China
2Key Laboratory of Particle & Radiation Imaging (Tsinghua University), Ministry of Education, Beijing, P. R. China
We proposed a new design of a boron lined neutron detector with separated neutron convertor and electron multiplier to simplify the manufacturing process. The comprehensive simulations about the detection efficiency have been performed with the aid of MCNP5, Maxwell and Garfield. The neutron detection efficiency with one row of detector unit could achieve 47%@25.3meV with the natB layer and can reach up to 80%@25.3meV with five rows of detector units, implying that this detector could be used in the applications of small-angle neutron scattering for scientific research or radiation portal monitor for homeland security.

(09:30) N5A3-4, Current Status of Aerogel as a Neutron Converting Material

N. S. Edwards1, K. A. Nelson1, N. J. Hinson1, R. G. Fronk1, S. Steiner2, A. Visentin2, R. Nelson2, J. Griffin2, D. S. McGregor1

1Dept. of Mechanical and Nuclear Engineering, Kansas State University, Manhattan, Kansas, United States of America
2Aerogel Technologies, LLC, South Boston, Massachusetts, United States of America
Aerogel samples with compositions containing boron and/or lithium content were investigated as a candidate material for neutron-conversion. After studying the ability for reaction products, created from the reaction of a thermal neutron with either boron or lithium, to propagate through the bulk of a solid sample, it was determined that aerogel materials can be approximated as solid samples as the size of the nano-pores are too small to cause sufficient ionization in the gas-filled volume of each nano-pore. Therefore, Aerogel Technologies, Inc. produced aerogel samples with so-called “macrochannels” incorporated, allowing for reaction products created within the bulk of the aerogel to enter the macrochannels and produce measurable reaction product ionization. The samples with densities of 0.1 or 0.2 g cc-1, 1.5 mm diameter macrochannels, and macrochannel spacing of 2.0, 2.5, or 3.0 mm were tested for neutron sensitivity using a moderated-252Cf source. Additionally, the thermal neutron detection efficiency as a function of the aerogel wall thickness between neighboring macrochannels for sample densities of 0.01, 0.05, 0.10, 0.15, and 0.20 g cc-1 was simulated using MCNP6. The sample composition was 50% 6Li, 5% 11B, 31% Si, and 14% O for all simulations. An optimal thermal neutron detection efficiency of approximately 43% for a 0.10 g cc-1 sample with a wall thickness of 0.2 mm (200 µm) was determined for an LLD setting of 300 keV. MCNP6 was also used to simulate the thermal neutron detection efficiency as a function of neutron reactive content (either 6Li or 10B) ranging between 5 – 75%. The remaining compositional content was Si and O at a ratio of 2:1. The density was maintained at 0.10 g cc-1 and 2.0 mm diameter macrochannels with a 2.8mm spacing between neighboring macrochannels were implemented. An optimal thermal neutron detection efficiency of approximately 47% for an LLD setting of 0 keV was simulated for a composition containing 5% 11B and 55% 6Li content.

(09:50) N5A3-5, Charge Propagation Through- and Neutron Sensitivity of- Reticulated Vitreous Carbon Foam

N. S. Edwards, K. A. Nelson, C. N. Tiner, N. J. Hinson, P. B. Ugorowski, R. G. Fronk, M. A. Reichenberger, D. S. McGregor

Dept. of Mechanical and Nuclear Engineering, Kansas State University, Manhattan, Kansas, United States of America
Reticulated vitreous carbon (RVC) foam, coated with a neutron-reactive layer typically consisting of boron, has been previously studied in a limited capacity as a neutron-conversion material. One major assumption in previous neutron sensitivity studies was that charge-carriers, born in the bulk of the RVC foam due to ionization of reaction products, could not be extracted from the bulk of the RVC foam sample and fully induce a charge on the readout electronics. Thus, the propagation of charge-carriers, both electrons and ions, through the bulk of RVC foam samples was studied for linear pore densities of 5, 10, 20, 30, 45, 80, and 100 pore per linear inch (PPI). Determination of charge-carrier propagation capabilities was achieved by comparing the measured count rates from a collimated 241Am alpha particle source with and without a sample present. Resulting average count rates with a sample present were 2.48 ± 0.10 and 2.27 ± 0.10 counts/second, and without a sample present were 2.63 ± 0.06 and 2.14 ± 0.02 for electron and ion propagation, respectively. The results indicate that both electrons and ions can propagate through a RVC foam sample and suggests that, when using RVC foam with a neutron converting layer applied, charge-carriers created within the bulk of the RVC foam sample can be extracted from the bulk and thus induce charge on the readout electronics. In parallel with investigating the propagation of charge-carriers through the bulk of RVC foam, neutron sensitivity tests were performed using a moderated-252Cf source for 10, 20, 30, 45, 80, and 100 PPI RVC foam samples that were PECVD-coated with natural orthocarborane.