Radioactive dating of animal bones of Jyomon-era by extremely low-level gamma-ray spectrometry.

为了估计汝门时代贝壳丘出土动物骨骼的年龄，采用Ra-226/U-238活度比的非破坏性测年方法作为一种新的、简单的测年方法进行了研究。在原大古屋铜矿隧道地下实验室（270 mwe深），利用极低本底井式锗探测器对动物骨骼进行了伽玛射线测量。发现铀系核素的原位掺入有三种模式；(1)骨样在降解阶段（鸟滨贝丘福井县）掺入的镭相对于铀的含量较高；(2)骨样选择性掺入的铀含量较高（千叶县通代贝丘）；(3)骨样在早期（北海道东伊贝丘）掺入少量不含镭的铀。情况(2)和(3)中骨样品的年龄可以通过Ra-226/U-238活性比来计算。由于Ra-226的浓度高于母体铀，因此无法用无损法测定鸟浜样品的年龄，因此用α光谱法测定了鸟浜样品的年龄。目前的方法被发现比考古年龄早了大约1000年。即使非破坏性测年法不能应用于骨样品，伽玛能谱法也被发现是非常有用的，可以获得关于jyomon时代环境情况的信息。

In order to estimate ages of animal bones excavated from shell mounds of Jyomon-era, non-destructive dating method using Ra-226/U-238 activity ratio has been examined as a new and simple dating technique. Gamma-ray measurements of animal bones were performed using extremely low background well type Ge detector set in underground laboratory (270 mwe of depth) in a tunnel of former Ogoya Copper Mine. It was found that there are three pattern of in-situ incorporation of uranium series nuclides ; (1) rather high amount of radium relative to uranium is incorporated during degradation stage of bone samples (Torihama shell mound Fukui Pref.), (2) rather high amount of uranium is selectively incorporated into bone sample (Tonodai shell mound, Chiba Pref.) and (3) small amount of uranium without radium is incorporated in bone sample in early stage (Toi shell mound, Hokkaido).Age of bone samples in case (2) and (3) can be calculated from Ra-226/U-238 activity ratios. Torihama sample could not be dated by non-destructive method because the concentration of Ra-226 is higher than parent uranium, therefore, age of Torihama sample was measured by alpha spectrometry. Present methods was found to give c.a. 1000 year older than archaeological ages. Even if non-destructive dating method cannot be applied to bone samples, gamma spectrometry was found to be very useful to get information on environmental circumstances of Jyomon-era.