α辐射效率对ESR-铀系化石测年结果影响

Effects of alpha-efficiency variation on combined ESR/U-series dating of tooth fossil

在电子自旋共振（Electronspinresonance，ESR）．铀系化石测年中，需要考虑a、β和γ辐射对年剂量的贡献。由于仪射线产生顺磁中心的效率明显低于相同能量的β和γ射线，所以在牙化石ESR．铀系测年的年剂量计算中需要对a辐射贡献的剂量率乘以一个校正参数他值）。但不同学者通过实验测量给出的k值各不相同，本文通过对5颗来自重庆巫山龙骨坡遗址的动物牙化石测年样品的分析，探讨了k值变化及采用两种常用k值对化石样品年代计算结果的影响。结果显示，当k值从0增加到0．3时样品年龄随之递减，对于铀含量较低的样品，年龄变化范围不大（-2％），而对铀含量较高的样品年龄随k值有较大变化（～50％），其中k值从0．13增加到0．23的过程中，高铀含量样品的年龄减小了～20％；对铀含量相近的样品，^230Th／^234U比高的样品其年龄计算结果随k值的变化更大。

In combined Electron spin resonance （ESR）/U-series dating of tooth fossil, annual dose is comprised of the dose rate contributed by a -rays, β-rays and γ -rays. A correction factor （called alpha-efficiency or k-value） is applied to the measured or calculated alpha dose rate. However, due to the experimental difficulty, k-value could not be measured very precisely. Purpose： In order to investigate the effects of the two commonly used k-values and the variation on the dating results, we analyzed the tooth enamel samples from Longgupo, a hominid archaeological site in Chongqing. Methods： Five tooth fossils from Longgupo site were analyzed using combined ESR/U-series method, and their ages were calculated with DATA program. Results： The study showed that the sample ages decreased as k-value increased from 0 to 0.3. For the samples of low uranium concentration, the ages only had small changes （-2%）, while for those of high uranium concentration, the ages decreased obviously （up to 50%）, and 20% decrement appeared when k-value increased from 0.13 to 0.23. For dating results of samples of similar uranium concentration, those of higher 230Th/234u ratio were affected more obviously with the k-value variation. Conclusion： The difficulty of measurement makes it hard to obtain a very precise k-value, and different k-values indeed cause considerable uncertainty to the fossil dating results. In addition, k-value may vary according to the uranium content of the tooth tissues, which will be investigated in further study.