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

为了对从绳纹时代的贝壳土丘中出土的动物骨骼进行年代测定，研究了一种新的、简单的年代测定方法--Ra-226/U-238放射性比值非破坏性年代测定法。动物骨骼的γ射线测量是使用极低本底井型锗探测器在地下实验室（270兆瓦的深度）在原奥戈亚铜矿隧道进行的。发现铀系核素的原位掺入有三种模式：（1）在骨样品的降解阶段（鸟滨贝壳丘福井Pref.），相对于铀，镭的掺入量相当高; (2)选择性地将相当高量的铀掺入骨样品中（Tonodai shell mund，千叶Pref.）（3）早期骨样品（北海道土井贝壳丘）中掺入少量铀而不含镭，根据Ra-226/U-238活度比可计算出（2）和（3）例骨样品的年龄。由于Ra-226的浓度高于母铀，不能用非破坏性方法测定鸟滨样品的年龄，因此用α谱法测定鸟滨样品的年龄。目前的方法被发现给c.a.比考古时代早1000年。即使非破坏性测年方法不能应用于骨样品，伽马光谱测定法被发现是非常有用的，以获得资料的环境状况，Jyomon时代。

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.