226Ra/238U dates by non-destructive γ-ray spectrometry of late Pleistocene corals in Minami- and Kita-Daito Island, Okinawa, Japan

The Ra-226/U-238 method based on nondestructive gamma ray spectrometry is discussed for dating 18 specimens of Pleistocene autochthonous corals in Minami- and Kita-Daito Islands. The U-238 and Ra-226 contents were indirectly determined by their daughter nuclides of Th-234 and Pb-214, respectively, assuming that the radioactive equilibrium between Th-238 and Th-234 and between Ra-226 and Pb-214 has been maintained. All the specimens, initially aragonitic mineral, were powdered, compressed into the disk shape of 50 mm diameter, and made airtight. These specimens were measured by a hyperpure germanium detector for 2 to 3 days. The Ra-226/U-238 dates of 15 specimens in Minami- and 3 specimens in Kita-Daito Island were obtained. The date of highest frequency was 120 to 130 ka, and the mean value was 128±27 ka. The nondestructive Ra-226/U-238 dating by gamma spectrometry has the advantages that elaborate and time-consuming chemical procedure is unnecessary, and the change in the chemical state does not occur in the samples analyzed, therefore, the same sample can be used for further investigation. (K.I.)