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 radiation from radionuclides in the uranium and thorium decay chains. In luminescence dating of sediment samples, it is usually assumed that the dose rate does not change over the period of burial, implying that the uranium and thorium decay series are in secular equilibrium. For the Th-232 decay chain there is little iterature available on the equilibrium 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, disequilibrium is commonplace in the surficial environment and the half-lives of several members of this decay chain (U-234, Th-230, Ra-226) are sufficiently long that any disequilibrium, once established, may persist for millennia. In these circumstances, the dose rate will vary with time unless the decay rate is matched by the transport and deposition of the relevant (unsupported) nuclide. We present data from a variety of fluvial and lacustrine depositional environments, and demonstrate that disequilibria is common in these Australian surficial sediments. The origins of the disequilibria and their likely evolution in time are discussed. The effect on the dose rate is assessed and, in the majority of cases, is found to be comparable with other luminescence dating uncertainties of typically 5-10%. Copyright (C) 1996 Elsevier Science Ltd