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1999
Conference Paper
Titel
Measurement of the time structure of delayed neutrons after induced fission in special nuclear material
Abstract
Nondestructive measuring capabilities were investigated for the in situ detection and analysis of special nuclear material, which is surrounded by additional shielding thus impeding direct detection. We performed measurements of the time structure of delayed neutrons in order to detect and identify hidden or shielded nuclear material in geometrical configurations, which are not known in detail. A block of depleted uranium was irradiated repeatedly with our 14 MeV neutron generator for different time intervals. After the end of the interrogating neutron pulse we integrated the delayed neutrons in different time intervals and thus recorded the "decay curves". The distance from the neutron tube to the object was varied to simulate measurement under difficult spatial conditions. Detection within a distance of several meters is possible. To enhance the detection efficiency we optimized the thickness of the polyethylene (PE) moderator of the neutron detector for delayed neutrons. The experimental data were compared with MCNP calculations. To obtain information on the existence of fissionable material in a short time we optimized neutron irradiation and integration time. These experiments show that fissionable material can be detected clearly and easily in a suspicious box without information on the geometry within a very short time (several minutes).