International Topical Meeting on Nuclear Research Applications and Utilization of Accelerators
|
|
|
SM/EN-09
Detection of Concealed Fissionable Material by Delayed Neutron Counting W. Rosenstock Fraunhofer – INT, Euskirchen, Germany Corresponding Author: wolfgang.rosenstock@int.fraunhofer.de One of the greatest challenges in detecting concealed nuclear material by non-contact means is the measuring of uranium, especially highly enriched uranium, particularly if it is surrounded by additional shielding material. Hence, γ detection is not a promising option. However, neutrons provide a high probability to penetrate typical shielding materials well. Whereas plutonium emits enough neutrons from spontaneous fission to be measured by passive methods in most of the cases, highly enriched uranium, particularly in casings, does not emit sufficient neutrons to be detected passively. Therefore, we investigated in the active interrogation with a neutron generator in particular. A clear proof of the existence of special nuclear material is the emission of delayed neutrons after induced fission, e.g., by a neutronn generator. 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 andidentify 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). The authors have assigned to the International Atomic Energy Agency a non-exclusive, royalty-free licence to publish this paper. |
