A detector for shielded fission sources

Abstract

Abstract In recent years, a great deal of attention has been given to the passive detection of fissile material by detecting spontaneously emitted neutrons. Neutron detection schemes generally rely on low-density detection media with high neutron-gamma discrimination capability. However, such detectors generally suffer from low efficiency for high energy (>1.00 MeV) neutrons and usually are constructed with moderating material built into the detector package to compensate for that. But when the source is highly shielded by hydrogenous materials, the moderating material surrounding the detector actually decreases the detector efficiency (Rees and Czirr, 2012). This problem is further compounded if the source is shielded with borated material, significantly reducing the number of emerging neutrons. We have built, tested, and modeled a simple neutron detector that uses a large block of plastic scintillator fitted with a removable cadmium foil. This detector is sensitive to fission gammas, gammas emitted from neutron capture in shielding, gammas emitted from neutron capture in the plastic scintillator, and gammas produced by neutron capture in the cadmium foil. We demonstrate that by applying suitable data analysis, we can detect a shielded californium source with a very small chance of false positives from typical gamma sources . When boron is added to the shielding, we also use the 477 keV gammas emitted when neutrons capture in boron. This process is enhanced by using an auxiliary NaI gamma detector . Although the detection method flies in the face of currently promoted detectors, it is a simple technique that could be applied to a variety of detection applications.