Neutron-chamber detectors and applications

Detector applications in nuclear safeguards and waste management have included measuring neutrons from fission and (α, n) reactions with well-moderated neutron proportional counters, often embedded in a slab of polyethylene. Other less-moderated geometries are useful for detecting both bare and moderated fission-source neutrons with good efficiency. The neutron chamber is an undermoderated detector design comprising a large, hollow, polyethylene-walled chamber containing one or more proportional counters. Neutron-chamber detectors are relatively inexpensive, can have large apertures, usually through a thin chamber wall, and offer very good detection efficiency per dollar. For 10% of the cost, a neutron-chamber detector may give one-half the total detection efficiency of a slab detector for detecting neutrons from a distant, bare source. A nuclear-safeguards pedestrian monitor detects concealed plutonium using two such chamber detectors to form a walk-through portal. Neutron-chamber detectors have also been used for monitoring vehicles and for assaying large crates of transuranic waste. Our Monte Carlo calculations for a new application (monitoring low-density waste for concealed plutonium) illustrate the advantages of the hollow-chamber design for detecting moderated fission sources.