Performance of a straw-based portable neutron coincidence/multiplicity counter

We propose a boron-coated straw (BCS) based fission counter that incorporates a 3×20 array of individually sealed BCS detectors, each 4.43 mm in diameter and 48 cm long. The straws are embedded inside a high-density polyethylene block with dimensions 48.3×38.1×5.08 cm
³
(H×W×D), matching the outer dimensions of a commercially available fission counter (Ortec Fission Meter). The latter consists of two folding panels, each comprised of a single row of
³
He tubes in front of a 48.3×38.1×2.54 cm
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(H×W×D) moderator slab. The
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He tubes, 15 in each panel, are 2.54 cm (1 inch) in diameter, 48.3 cm long, and pressurized to 7.5 atm. The total volume of
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He contained in the system is 55 liters. The response of the two systems - two BCS panels, and Ortec FM - was measured with a
²⁵²
Cf source at various distances. The source was configured either with no moderation, or with a 2.5-inch thick moderator around it. Measurements were repeated both indoors, in a laboratory area, and outdoors, in an open space. The two BCS panels were placed adjacent to one another, with the source centered between them, 91 cm above the ground. The FM was positioned in a similar manner, with its two panels unfolded (
³
He tubes facing the source). The results show that the two systems have similar response to the unmoderated source when the source is near. As the source moves further away, the ratio BCS-to-
³
He drops to about 0.4. This is due to the fact that the Ortec FM is more sensitive to lower energy neutrons that have scattered in the ground and eventually reach the detector. For the same reason, the straw-based system response to the moderated source is about 30% that of the FM response. Here, neither the location (outdoors/indoors), nor the distance, play a significant role, since a sufficient fraction of neutrons are moderated at the source, thus minimizing the effect of reflections. Pointing to further applications, a modeling study of a high performance multiplicity detection instrument is also presented. Through use of high straw density, a detection efficiency of 48% can be reached with a rapid die-away time of 50 μs.