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

利用Ra-226/U-238活度比的无损测年方法，作为一种新的、简单的测年技术，被用于测定柔门时代贝丘出土的动物骨骼的年龄。用极低本底井型Ge探测器对原Ogoya铜矿巷道中的动物骨骼进行了伽马射线测量。研究发现，铀系核素的原位掺入有三种模式：(1)在骨样的降解阶段(Torihama壳丘Fukui Pref.)，相对于铀，较高数量的镭被掺入；(2)较高数量的铀被选择性地掺入骨样(Tonodai壳丘，Chiba Pref.)(3)早期(北海道东井壳丘)骨样中含有少量不含镭的铀，根据Ra-226/U-238活度比值可推算(2)例和(3)例的骨样年龄。由于Torihama样品中Ra-226的浓度高于母质铀，不能用非破坏性方法测定样品的年龄，因此用α光谱分析方法测定了Torihama样品的年龄。目前的方法被发现可以给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骼样本，伽马能谱也被发现对于获取柔门时代环境状况的信息非常有用。

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.