Since Barnes et al. (1956) proposed the uraniumionium (230Th) method of dating, many works have been performed for dating of fossil corals (Komura and Sakanoue, 1967; Bloom etal., 1974; Ku et al., 1974; and see the references therein). The 230Th method has a potential to estimate the absolute age back to about 300000 years. This method is one of the most reliable methods for a sample older than a few ten thousand years, since the half-life of 14C limits the applicable age back to about 40000 years. In order to determine 230Th age, activity ratios of 230Th/234U and 234U/238U must be measured. Alpha spectrometry has usually been performed to measure these isotopes. On the other hand, measurement of 230Th can be made indirectly by its daughter nuclide 222Rn by assuming radioactive equilibrium between 230Th and 226Ra (222Rn) (Nydal, 1975). If this assumption is acceptable, the 230Th can also be measured by 214Pb, 214Bi and 210Pb, the amount of which can be determined by y-spectrometry. Furthermore, 238U concentration can be determined by measuring low energy r-rays from its daughter nuclide 234Th. In this report, the non-destructive 226Ra/238U method is proposed for the dating of fossil corals by -spectrometry using a germanium detector Ge (Li) and a low energy photon spectrometer (LEPS). The non-destructive dating method has advantages over the conventional ones in that the elaborate and timeconsuming chemical procedures are unnecessary and no change in the chemical state occurs in the sample analyzed, hence, the same sample can be used for further investigation after the non-destructive measurement.
Komura et al. (Sun,) studied this question.