Direct gamma-spectrometric measurement of the 226Ra 186.2keV line for detecting 238U/226Ra disequilibrium in determining the environmental dose rate for the luminescence dating of sediments.

In the age determination of sediments according to the luminescence method, it is important to know whether there was secular equilibrium in the Th and U decay series during accumulation of the environmental radiation dose in geological times. This can be investigated via gamma-ray spectrometry, whereby--in the (238)U series--(214)Pb and (214)Bi are considered as the indicators for (226)Ra-mobility, and (210)Pb as the indicator for (226)Rn-emanation. In the present work, the direct measurement of (226)Ra--which can give extra information on the interpretation of the radioactive equilibrium--is investigated as to its precision and accuracy. Since there is a serious spectral interference on its 186.2keV gamma-line (by (235)U at 185.7keV), various correction procedures were outlined and their performance was checked via the analysis of (certified) reference materials, leading to the recommendation of a "method of choice".