Evaluation of Th series disequilibrium in Western Australian monazite

Estimation of inhaled radiation doses associated with mineral sands processing is commonly performed by gross alpha-counting dust collected on air filters. The technique requires knowledge of the extent of disequilibrium in Th-bearing minerals. The daughters which can be expected to give rise to disequilibrium, viz. 228Ra and 220Rn (also designated thoron in the paper), were investigated in a typical Western Australia monazite. The thoron flux from a dry, "infinitely thick" layer of monazite was found to be 41 Bq m-2 s-1. The depth of monazite from which thoron is exhaled is limited to 40-50 mm, and within the first 10 mm is a linear function of bed thickness. The relative loss within the linear region is approximately 0.02% of the equilibrium concentration and progressively less for layers beyond 10 mm. The sample investigated gave no indication of disequilibrium involving 228Ra. The results indicate that secular equilibrium may be assumed when calculating 232Th daughter concentrations in monazite from the gross alpha activity. More extensive work on monazite samples from a number of sites will be necessary before this can be stated as a general conclusion applicable to all Western Australian monazite deposits.