Year
2003
Abstract
The isotope 233U is produced in reactors by neutron irradiation of 232Th. It has nuclear properties that can be exploited for nondestructive assay (NDA) for accountability and waste measurements. These properties are thermal power and gamma-ray emissions. The 233U half-life is short enough, 159,200 y, so that its specific power is high enough to be measured by currently available calorimeters. 233U gamma rays can be measured with a Ge detector to determine the effective specific power, Peff, of an item. The same methodology for calorimetric assay of Pu can be used for calorimetric assay of 233U. However, aged 233U is more difficult to assay because there is always the presence of a high flux of gamma rays from 232U progeny. The 232U is produced by other nuclear reactions at the same time that the 233U is produced. To obtain good counting statistics for 233U, data collection may take days because the high flux leads to a high Compton background under the 233U peaks. The long count times can be reduced if (1) the age of the 233U is known and (2) the item is old enough so that significant 229Th ingrowth has occurred. The 229Th half-life is short enough, 7340 years, so that there is measurable 1567–keV gamma-ray activity from its 209Tl progeny. In this region of the gamma-ray spectrum measured by the Ge detector, the Compton background is much reduced compared to lower energies. If the age of the 233U is known, the 209Tl gamma-ray intensity can be used with those of other 232U progeny high-energy gamma rays to calculate the 232U /233U mass ratio and thus determine Peff for calorimetric assay with a shorter count time. The 209Tl activity also can be used to directly determine 233U content. The high penetrability of the 1567 keV–radiation (1/e thickness = 1.6 cm for Pb) enables segmented- or tomographic-gamma scanning techniques to be used for 233U assay in shielded containers.