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加速器質量分析法による古文書および古経典の<14>^C年代測定

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Abstract

History is a reconstruction of past human activity, evidence of which is remained in the form of documents or relics. The radiocarbon dating of ancient document and sutra provides important information for the reconstruction of historic period. But radiocarbon age as another expression of ^<14>C concentration in the sample is different from the historical age when the document was written. The difference between radiocarbon age and historical age becomes more serious problem for recent sample which requires more accurate age determination. There is discrepancy between radiocarbon age and calendar age owing to variability of ^<14>C concentration in the atmosphere. Although radiocarbon age is converted into calender age with the calibration curve, the calibrated radiocarbon age is still different from the historical age. It is known as "old wood effect" for wooden cultural property that the calibrated radiocarbon age is older than the historical age when it was produced as a tool. The purpose of this study is to clarify the relation between calibrated radiocarbon age and historical age of ancient Japanese document by AMS radiocarbon dating of Japanese ancient documents and sutras written dates of which are clarified from the paleographic standpoint. We measured radiocarbon ages of 15 sutras and 7 documents written dates of which are known. Alpha-cellulose was extracted from each paper sample by chemical treatment. The cellulose was burnt to CO_2 at 850℃ with CuO. We prepared graphite targets for AMS measurements, reducing CO_2 with H_2 and Fe catalyst at 650℃ in sealed glass tube. We measured radiocarbon ages of the graphite samples with the Tandetron accelerator mass spectrometer at Nagoya University. The isotopic fractionation is corrected by δ ^<13>C values measured with the Finnigan MAT-252 mass spectrometer. The calibrated radiocarbon ages of samples taken from the obverse-side sheets on which Chinese characters were written with Indian ink are in good agreement with the corresponding historical ages. Japanese paper had been made mainly from bast fiber of deciduous trees : Kozo, Ganpi and Mitsumata. Because the fiber of old branches is unsuitable for paper manufacture and yields paper of poor quality, fresh branches grown within a few years were harvested selectivity. In addition, for Kozo paper, the interval from trimming off branches to writing document on manufactured paper is usually within one year, because paper changes in quality by preservation and becomes unsuitable to write with Indian ink. The good agreement between calibrated radiocarbon age and historical age is supposed by such characteristics of Japanese paper. The calibrated radiocarbon age of a paper sample stuck on the back side of a sutra for reinforcement is much younger than the historical age of the sutra. Although the calibrated radiocarbon age could indicate the age when the sutra was repaired, it leaves room for further investigation. It is likely that older paper was used for repairing, since fresh paper would make creased on the original paper. Radiocarbon age of another reinforcing paper was older than that of obverse-side sheet. Therefore, calibrated radiocarbon age of reinforcing paper has no distinct relevance to the historical age. It was shown in this study that the radiocarbon age of the obverse-side paper of document which was used for writing characters and transmitting some information has little gap by "old wood effect" ; accordingly, such paper of ancient Japanese document or sutra is a suitable sample for radiocarbon dating of recent historic period. Japanese paper used for reinforcement has no clear relationship between the calibrated radiocarbon age and the historical age when the sutra was written. The radiocarbon dating of such kind of paper would, however, provide us information about recycling or repair of ancient document.

タンデトロン加速器質量分析計業績報告 Summaries of Researches Using AMS 1999 (平成11)年度

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