Estimation of uranium isotope ratio by interrogation with a sealed tube neutron generator

Uranium inside containers or surrounded by shielding material does not emit sufficient radiation to determine the isotope ratio passively or even to get an estimate on the existence or quantity of the isotope U-235 by nondestructive passive measurements outside the container. Therefore, we investigated the active nondestructive interrogation of shielded nuclear material with a sealed tube 14 MeV neutron generator. To distinguish the thermal fissionable isotopes from those fissionable only with fast neutrons we have performed measurements with our 14 MeV neutron generator with and without a moderating assembly between the interrogating neutron source and the object of interest. The moderator consisted of 10 cm tungsten so that a considerable part of the neutron flux has energies below the fission threshold of the even isotopes (e.g. U-238). Discrimination between source and prompt fission neutrons is made by shielding the detector and on the basis of the high multiplicity of the fissio n neutrons by a coincidence analyzer. Monte Carlo calculations show that the coincidence count rate is enhanced with U-235 in place. The results show that this method is capable of detecting shielded uranium and to estimate enrichment. This system is designed to perform in situ interrogation of suspected boxes with small neutron generators.