Fast and thermal neutron detectors for radiation portal monitors

Radiation portal monitors (RPMs) can include thermal or fast neutron detectors or a combination of the two. Nearly all current RPM designs contain only thermal neutron detectors, predominantly3He proportional tubes, in order to detect the spontaneous fast neutron emissions from special nuclear material (SNM) as an elevated count rate above the low natural neutron background. The spontaneous fission neutrons emitted by SNM isotopes are emitted at fast energies on the order of MeV. Thermal neutron detector materials, such as3He, exhibit their highest interaction cross-sections at much lower thermal neutron energies, below the Cadmium cutoff energy of 0.5 eV. Therefore,3He tubes are embedded in a low Z moderator, such as high density polyethylene (HDPE), to allow for neutrons to thermalize, thus increasing the system detection efficiency. For SNM hidden in cargo containers, the surrounding environment, such as cargo or a designed shield, will act as an additional moderator.3He tubes in RPMs therefore are often slightly undermoderated to account for the presence of other moderating material. Fast neutron detectors, such as liquid or plastic organic scintillation detectors or crystalline organic scintillators like stilbene, are typically insensitive to neutrons below 400 keV in energy but are very efficient for higher energy neutrons. Through a series of measurements and simulations of a252Cf spontaneous fission neutron source with3He and organic liquid scintillation detectors and stilbene, the effects of varying thicknesses of HDPE shielding are studied. For some shielding scenarios operational benefits might exist for using fast neutron detectors instead of or in addition to thermal neutron detectors. The goals of this study are to identify shielded SNM scenarios for which RPMs might benefit from the addition of fast neutron detectors and to quantify those gains in terms of neutron detection efficiency.