THE EFFECTS OF DISEQUILIBRIA IN THE URANIUM AND THORIUM DECAY CHAINS ON BURIAL DOSE-RATES IN FLUVIAL SEDIMENTS

Buried sediments receive about 53% of their annual dose of ionising ra diation from radionuclides in the uranium and thorium decay chains. In luminescence dating of sediment samples, it is usually assumed that t he dose rate does not change over the period of burial, implying that the uranium and thorium decay series are in secular equilibrium. For t he Th-232 decay chain there is little iterature available on the equil ibrium conditions in sediments, but given the short half-lives of the longer-lived daughters in the series, Ra-228 (5.75 years) and Th-228 ( 1.91 years), the decay chain is expected to be in secular equilibrium in most natural materials. However, for the U-238 decay chain, disequi librium is commonplace in the surficial environment and the half-lives of several members of this decay chain (U-234, Th-230, Ra-226) are su fficiently long that any disequilibrium, once established, may persist for millennia. In these circumstances, the dose rate will vary with t ime unless the decay rate is matched by the transport and deposition o f the relevant (unsupported) nuclide. We present data from a variety o f fluvial and lacustrine depositional environments, and demonstrate th at disequilibria is common in these Australian surficial sediments. Th e origins of the disequilibria and their likely evolution in time are discussed. The effect on the dose rate is assessed and, in the majorit y of cases, is found to be comparable with other luminescence dating u ncertainties of typically 5-10%. Copyright (C) 1996 Elsevier Science L td