The standardisation of Th-229 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 i n both the working and natural environment. In order to do this in a manner where the results of radiochemical analysis are traceable to national stan dards, a chemical yield tracer has to be employed. Th-229 has been identifi ed as a suitable tracer for such purposes and has been used in the past. Th is paper describes the absolute standardisation of Th-229 by 4 pi alpha-gam ma coincidence counting. The results of this work are presented together wi th an analysis of the standardisation uncertainties that were achieved in p ractice. (C) 2000 The National Physical Laboratory. Published by Elsevier S cience Ltd. All rights reserved.