Year
1999
Abstract
The enrichment of uranium changes the thermal power and neutron emission rate of uranium in a way that can be exploited for 235U assay. The contravariant behavior of the 234U and 238U isotopes with increasing 235U enrichment on thermal and neutron emission rates can be quantified by mass spectrometry measurements. Isotopic compositions of uranium isotopic reference materials and nondestructive analysis standards were used to calculate rations of calorimetry and nuetron coincidence counting measurements results for different enrichments. Assuming the correlation between therman power and neutron emission rate and enrichment holds true for unknown items, the calorimetry and neutron measurements can be used to determine the average enrichment of an item. In adition, once the enrichment has been established, the historical mass spectromety data also can be used to determine correction factors so that the 234U mass of the item can be related to the 238U mass, and 235U enrichment, the 235U content of an item can be estimated. Both calorimerty and neutron counting measure the radiation from the entire volume of the sample and would be able to assay photon-shielded or inhomogeneous material.