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

The response of a 3He neutron detector 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 3He. If there is too much moderation, neutrons will not reach the 3He. 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 3He tubes is a function of shielding. In this paper, we use Monte Carlo techniques to model the response of 3He tubes placed in polyethylene boxes for moderation. To model the shielded fission neutron source, we use a point 252Cf source placed in the center of polyethylene spheres of varying radius. Detector efficiency as a function of box geometry and shielding is explored. We find that increasing the amount of moderator behind and to the sides of the detector generally improves the detector response, but that incremental benefits are minimal if the thickness of the polyethylene moderator is greater than about 5–7 cm. The thickness of the moderator in front of the 3He tubes, however, is very important. For bare sources, about 4–5 cm of moderator is optimum, but as the shielding increases, the optimum thickness of this moderator decreases to 0.5–1 cm. Similar conclusions can be applied to polyethylene boxes employing two 3He tubes. Two-tube boxes with front moderators of non-uniform thickness may be useful for detecting neutrons over a wide energy range.