High-precision ESR dating by the correction for radiation damage production rate
218 , 2016-03-04 , 山梨大学教育人間科学部
In electron spin resonance (ESR) dating, ESR age is obtained by diving total radiation dose by annual dose. To accomplish high precision of ESR dating, the total radiation dose needs to be determined more precisely. The scattering of ESR signal intensities obtained from individual irradiated samples is caused by the difference among radiation damage production rates which individual irradiated samples have, and leads to the lowing of precision of the total radiation dose and ESR age. I have tackled high-precision ESR dating by applying the correction for radiation damage production rate to ESR signal intensities obtained from individual irradiated samples, using fossil teeth and bones of bats collected from red clay sediments in Akiyoshi cave, Yamaguchi, Japan. A CO^-_2 radical center (g=2.003) derived from calcium phosphate minerals is detected from fossil teeth and bones of bats and regularly increases with radiation dose. After the correction for radiation damage production rate, the datermination factor (R^2) of regression line for the total radiation dose obtained from the fossil teeth increases from R^2=0.9086 to R^2=0.9897 and the high precision of ESR dating is accomplished, while that obtained from the fossil bones hardly changes and the high precision is not recognized. The difference in result is attributed to the shape of fossil bones. As a result of ESR dating, the formation ages of the fossil teeth and bones are estimated at 63±9～94±22 ka with high determination factors of R^2=0.82-1.99. These ESR ages are coincident with the radioactive age (7-9 ka) of Aso-4 tephra composing the red clay sediments.