The standardisation of 229Th for an environmental yield tracer

The naturally occurring long-lived nuclides of thorium (Th-238, Th-230 and Th-232) are all alpha-emitters that are of interest both to the nuclear and non-nuclear industries and levels of these nuclides need to be monitored in both the working and natural environment. In order to do this in a manner where the results of radiochemical analysis are traceable to national standards, a chemical yield tracer has to be employed. Th-229 has been identified as a suitable tracer for such purposes and has been used in the past. This paper describes the absolute standardisation of Th-229 by 4 pi alpha-gamma coincidence counting. The results of this work are presented together with an analysis of the standardisation uncertainties that were achieved in practice. (C) 2000 The National Physical Laboratory. Published by Elsevier Science Ltd. All rights reserved.