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Title: Optimizing moderation of He-3 neutron detectors for shielded fission sources

Abstract: The response of 3-He neutron detectors is highly dependent on the amount of moderator incorporated into the detector system. If there is too little moderation, neutrons will not react with the 3-He. If there is too much moderation, neutrons will not reach the 3-He. In applications for portal or border monitors where 3He detectors are used to interdict illicit Importation of plutonium, the fission source is always shielded to some extent. Since the energy distribution of neutrons emitted from the source depends on the amount and type of shielding present, the optimum placement of moderating material around 3-He tubes is a function of shielding. In this paper, we use Monte Carlo techniques to model the response of 3-He tubes placed in polyethylene boxes for moderation. To model the shielded fission neutron source, we use a 252-Cf source placed in the center of spheres of water of varying radius. Detector efficiency as a function of box geometry and shielding are explored. We find that increasing the amount of moderator behind and to the sides of the detector generally improves the detector response, but that benefits are limited if the thickness of the polyethylene moderator is greater than about 5-7 cm.more » The thickness of the moderator in front of the 3He tubes, however, is very important. For bare sources, about 5-6 cm of moderator is optimum, but as the shielding increases, the optimum thickness of this moderator decreases to 0-1 cm. A two-tube box with a moderator thickness of 5 cm in front of the first tube and a thickness of 1 cm in front of the second tube is proposed to improve the detector's sensitivity to lower-energy neutrons.« less
Authors:
 [1] ;  [1]
  1. Brigham Young University, Provo, UT (United States)
Publication Date:
OSTI Identifier:
1178530
Grant/Contract Number:
FG52-10NA29655
Type:
Accepted Manuscript
Journal Name:
Nuclear Instruments and Methods in Physics Research. Section A, Accelerators, Spectrometers, Detectors and Associated Equipment
Additional Journal Information:
Journal Volume: 691; Journal ID: ISSN 0168-9002
Publisher:
Elsevier
Research Org:
Brigham Young Univ., Provo, UT (United States)
Sponsoring Org:
USDOE National Nuclear Security Administration (NNSA), Office of Defense Science (NA-113)
Country of Publication:
United States
Language:
English
Subject:
73 NUCLEAR PHYSICS AND RADIATION PHYSICS Neutron detection; Portal monitor; Radiation detection; Homeland security; Detection of illicit materials; Monte Carlo modeling