Measurement of ^<14>C content in 4^<th> century BC Japanese camphor tree rings(Proceedings of the 19^<th> Symposium on Chronological Studies at the Nagoya University Center for Chronological Research in 2006,Part 2)

Sunspot numbers, which reflect solar activity, have presented clear 11-year periodicity since the beginning of the 18^<th> century. But in the period around 1645 to 1715 AD sunspots were nearly absent, so we can not know if 11-year periodicity of solar activity was continued or not during the period by the records of sunspot numbers. This period is called as the Maunder Minimum. Radiocarbon ^<14>C, which is one of cosmogenic radioisotopes, is a good indicator of solar activity too. By determining the concentration of the ^<14>C in each tree ring, we can get information on the change of solar activity in grand solar minima like Maunder Minimum. Variation of decadal ^<14>C content measured by Stuiver et al. shows that grand solar minima, a period solar activity was extremely weak for decades or more, occurred several times in the last 10000 years. Stuiver classifies grand solar minima into 3 categories depending on the length of their durations. The Maunder type minima have a period of 〜80 yr and the Sporer type minima last 〜40 yr longer. We have already obtained the records of ^<14>C content in Japanese cedar tree rings of 1413 to 1745 AD including the Spoerer Minimum and the Maunder Minimum with annual time resolution. As a result of frequency analysis of these ^<14>C records, we found that the Sun maintained its periodic activity even if during the minima. We found, however, the cycle length of the "11-year cycle" during the Maunder Minimum was around 14 years and that during the Spoerer Minimum was around 11 years. This suggests that a pattern of the "11-year cycle" cycle length variation depends on a type of minima. In order to verify this hypothesis, we have measured ^<14>C content in Japanese camphor tree rings of 431 to 323 BC. The age, when this tree was growing has been determined by comparing ^<14>C records measured by us with world typical ^<14>C records called as the IntCal04. The period 431 to 323 BC covers start of ^<14>C increase to around its peak in the 4^<th> century BC solar minimum. We will measure and analyze the ^<14>C content during the last half of the 4^<th> century BC solar minimum and find out the actual cycle length of the "11-year cycle".