Determination of Nb-93m content in pressure vessel steel by liquid scintillation counting

The unintended presence of low niobium (Nb) impurity levels in reactor pressure vessel steels suggests the possibility of Nb's use as a fast neutron in-situ dosimeter, using the Nb⁹³ ( n, n') Nb^93m reaction.

This project investigated the use of a Liquid scintillation Counter (LSC) to measure very low Nb⁹³ activities. The first phase of this project involved the development of a UVA Nb solution standard in order to establish both counting efficiencies and "quench" for Nb in the LSC. In the second phase, Nb was radiochemically separated from "typical" U.S. pressure vessel steels that had been neutron-activated for the University of California at Santa Barbara. Finally, an enhanced Beckman LS200 LSC was used to determine the Nb^93m activity obtained from various steel samples of interest.

The Minimum Detectable Amount (MDA) (95% confidence interval) for Nb^93m was determined to be o. 04 ± o. 01 Bq/g for a counting period of 100 hours, in the presence of typical impurity levels of iron and tantalum. Unfortunately, even though this low MDA indicates that much of the Nb^93m activity data collected are valid, poor reproduction of the radiochemical separation procedure, and the minute Nb levels in U.S. pressure vessel steels did not permit elemental Nb determination. Therefore, calculation of exposed neutron fluence and use as an in-situ dosimeter are not possible.