Selective determination of extremely low-levels of the thorium series in environmental samples by a new delayed coincidence method

An attempt was made to apply a delayed coincidence method for the absolute determination of trace quantities of the thorium series. This method is based on selective counting of the relatively short lived nuclide²¹⁶Po (half-life 145 ms) in the thorium series members. For this purpose, a list mode time analyzing system combined with a liquid scintillation counter was assembled by means of a conventional microcomputer. A multiple time analysis was employed in the processing and data compilation of delayed coincidences to distinguish them from the true coincidences due to random events.From a time spectrum, the decay component of²¹⁶Po (145 ms) can be selectively measured. Absolute activities of its progenitors,²²⁴Ra and²²⁸Th as well as²²⁰Rn, can be determined even in the presence of the background radiations of the almost equivalent activity-strength of concomitant uranium series.