Use of nuclear track detector to measure very low alpha radioactivity in urine

By far the most frequent risk of contamination is via the respiratory tract. Metabolic data have demonstrated that, with the commonly handled α emitters (plutonium, uranium, americium), the faecal excretion is 10 to 100 times higher than in urine (except of course for class D compounds). The detection limit in faecal analysis is 8 mBq, thus an equivalent sensitivity can only be obtained by bringing the limit for urine below 0.1 mBq. This is the purpose of the method developed at Cadarache. The preliminary results presented pertain to 239Pu. The chemical separation is quite conventional but the final sample is a cerium fluoride deposit. A track detector was used for counting. It was first necessary to optimise both the thickness of the screen placed between the radioactive source and the detector, and the chemical treatment for transforming the tracks into holes. Three minutes were required to count the perforations by moving the detector across a spark system. Due to detector background noise and efficiency, a 35 day exposure time is required to detect 0.1 mBq of 239Pu. The same sensitivity levels should be feasible without difficulty for 238Pu and 241Am. However, the natural uranium present in urine prevents such sensitivity being realised with uranium. (author)