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Technical Memoranda 41
Roberts RG, Uren CJ and Murray AS
Supervising Scientist, 1993
ISSN 0810-9532
The Alligator Rivers Region Research Institute housed one of five thermoluminescence (TL) dating laboratories operating in Australia. The laboratory was established in 1985 to assist geomorphological research into uranium mining activities in the Region. Dating ceased in 1990 after the TL component of two geomorphological consultancies had been completed (Nanson et al 1990, Roberts et al 1991).
Techniques for dating Quaternary sediments have been developed, with specific application to fluvial and colluvial sand deposits in tropical northern Australia. In TL dating, the age of the deposit is determined as a function of the 'equivalent dose' (ED, the quantity of ionizing radiation required to produce the observed natural TL intensity) and the dose rate (the rate of supply of ionizing radiation at the depositional locale). To determine the ED, we used the 90-125 µm diameter quartz fraction of each deposit (following conventional quartz-inclusion pretreatment procedures) and adopted a combined additive-dose and regenerative methodology, as proposed by Readhead (1984, 1988). For unheated sediments, the TL clock is reset by exposure to sunlight, but an unbleachable (residual) TL signal remains even after prolonged exposure. The residual TL signal at the time of sediment deposition was estimated from ED determinations on modern (surface and near-surface) deposits, again following Readhead (1984, 1988).
The laboratory was at the forefront of TL dating in two respects. First, it was among the first in Australia to examine the potential of dating water-lain deposits by TL. This provided the opportunity to obtain ages for a range of deposits that are widespread across northern Australia and are of particular relevance in assessing the long-term geomorphological stability of uranium mining waste sites. Second, the dose rate was determined by state-of-the-art techniques that were not readily accessible to other TL dating laboratories. Radionuclide concentrations were deduced from high-resolution gamma and alpha spectrometry, which enabled disequilibrium in the uranium decay series to be identified and the time-dependent correction in the dose rate to be applied. The latter was performed using the computer program listed in Appendix A.