| Co-Investigator(Kenkyū-buntansha) |
NINAGAWA Kiyotaka Okayama University of Science, Department of Applied Physics, Professor, 理学部, 教授 (80098590)
SUZUKI Takehiko Tokyo Metropolitan University, Department of Urban Environmental Sciences, Associate Professor, 都市環境学部, 助教授 (60240941)
TSUKAMOTO Sumiko Tokyo Metropolitan University, Department of Urban Environmental Sciences, Research Associate, 都市環境学部, 助手 (30275010)
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| Research Abstract |
ESR(electron spin resonance), TL(thermoluminescence), and OSL(optically stimulated luminescence) dating methods, based on the same principle, have wide applications of age determination in quaternary in the range beyond that of radiocarbon method. However, these have still been considered to be viable methods. In the present project, these three methods were applied to the same tephra samples of various known ages so that the obtained ages can be compared with each other, also with the know ages. Tepharas are also appropriate samples in the present project in the aspect that they are important key layers in many geological contexts.
Quartz grains were extracted from Zenigame-Menagawa, Shibahara, Shirakawa-Ten'ei, Shirakawa-Saigo, Nantai-Shichihonzakura, Ikedako, Irito Aira-Tanzawa (Irito Ignimbrite), Aira-Shiko, Aira-Iwato, Kikai-Kuzuhara, Ata-Torihama, Kuju-1, Kuju-Handa, and AT tephras. Natural accumulated doses were obtained by ESR and isothermal TL methods. It was found in the present study that OSL is not appropriate for measurement of Japanese tephras because the rate of recupelation is too large to obtain reliable accumulated doses. Natural dose rates were obtained from the concentrations of U, Th and K concentrations in the samples which were measured by the low background gamma ray spectroscopy.
Isothermal TL ages were consistent well with the known ages while ESR ages are not so much inconsistent in the range younger than 100ka, but sometimes the ages were so much scattered in the samples of older ages. The problem should probably be the additive dose method where the gamma ray dose were applied to the samples to see the increase of the signal intensities which are extrapolated to the zero ordinate to obtain natural accumulated doses. Preliminary experiments on signal regeneration were performed to find the statistical errors become much smaller with this protocol.
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