The standardisation of 229Th for an environmental yield tracer

The naturally occurring long-lived nuclides of thorium (228Th, 230Th and 232Th) 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. 229Th has been identified as a suitable tracer for such purposes and has been used in the past. This paper describes the absolute standardisation of 225Th by 47pi alpha-gamma coincidence counting. The results of this work are presented together with an analysis of the standardisation uncertainties that were achieved in practice.