Detection of concealed fissionable material by delayed neutron counting
We performed measurements with a small, light-weight neutron generator, which can be carried by one person. The time structure of delayed neutrons was measured in order to detect and identify hidden or shielded nuclear material in geometrical configurations whereof only the outer shape is known but little or no information is available on the inner structure. A small block of depleted uranium was irradiated repeatedly by a sealed neutron tube of a 14 MeV neutron generator for different time intervals. The delayed neutrons were measured by a neutron "slab" counter consisting of 6 He-3 tubes moderated by high density polyethylene. After the end of each interrogating neutron pulse we analyzed the delayed neutrons in different time intervals, ranging from 3 s to 300 s and thus recorded the "decay curves" of the delayed neutrons. We optimized neutron irradiation and measuring time to gain information on the existence of fissionable material in a short time. These experiments show that fissionable material can be detected clearly and easily in a suspicious object without any information on the inner geometry and the surrounding moderating material within a very short time (several minutes).