Unexpectedly high activity of ²²⁸Th in excretion samples following consumption of Brazil nuts

A worker provided a routine faecal sample for plutonium and americium analysis. In the course of this analysis 500 mBq of ²²⁸Th was discovered. There seemed no credible occupational route for intake of thorium. Further investigation revealed that the worker consumed ∼25 g d^-1 of nuts, including Brazil nuts. A sample of these nuts was analysed and found to contain activities of ²²⁸Th in sufficient quantity to account for the faecal activity. However, follow-up urine samples taken from the worker showed 0.6-0.7 mBq of ²²⁸Th. The intake of ²²⁸Th via nuts is insufficient to account for this activity in urine. However, it is likely that the intake of ²²⁸Th was accompanied by similar activity of the parent ²²⁸Ra, and biokinetic calculations show that decay of ²²⁸Ra in vivo would produce sufficient ²²⁸Th to account for the observed urine activity.