IXTH MILLENIUM BC CERAMICS FROM NIGER - DETECTION OF A U-SERIES DISEQUILIBRIUM AND TL DATING

A set of pottery sherds collected from two ancient neolithic sites in Niger (Tagalagal and Adrar Bous 10) has been dated by thermoluminescence. The natural radioactivity of these ceramics and of their surrounding sediments was measured using low background gamma spectrometry and atomic emission plasma spectrometry. With gamma spectrometry, the comparison between the activity of U-238 (deduced from the U-235 and Th-234 gamma emissions) and that of Ra-226 (deduced from Pb-214 and Bi-214 gamma emissions in equilibrium with Rn-222) shows a significant disequilibrium of the U-series. The activity ratio U-238/Ra-226, which is greater than unity in this case, is interpreted as a result of either uranium enrichment (U-234, U-235, U-238) Or radium impoverishment that has been occurring since the burying of the archaeological artefacts. The effects of the changes in radiochemical composition on the annual dose are discussed and various determinations of the annual dose are analysed according to different hypotheses of disequilibrium (either permanent state or recent occurrence). The TL results: Tagalagal - 9820 +/- 780-10,180 +/- 780 years/1993, Adrar Bous 10 - 9530 +/- 750-10,500 +/- 730 years/1993, are consistent with the radiocarbon dates obtained from charcoals collected at the same locations (the uncalibrated C-14 dates belong to the 9100-9370 BP range). These results are of fundamental interest for the chronology of early neolithic cultures.