海洋沉积物光释光测年中环境剂量率测量方法对比分析

Comparations of environmental dose rate measurements for optically stimulated luminescence dating on marine sediments

光释光测年作为一种沉积物绝对测年方法已在第四纪地质研究中得到了广泛应用,提高光释光测年精度和准确度一直是学术界关注的热点,长期以来人们对等效剂量测量程序和计算方法的研究十分丰富,而对环境剂量率的研究则相对较少。本文以琼西南海域ZBW钻孔岩心样品为例,利用电感耦合等离子质谱/发射光谱法(ICP-MS/OES)、中子活化分析(NAA)、γ谱仪法对海洋沉积物环境剂量率进行了对比测量。结果表明,在部分层段3种方法测量的结果存在差异,其原因为:(1)ICP-MS/OES前处理过程中重矿物中的U不完全溶解;(2)ICP-MS/OES测量结果为U、Th、K元素含量,包含了部分非放射性核素含量;(3)NAA测量用的样品量小,存在偶然性误差,样品中非放射性物质的稀释作用影响测量结果;(4)样品中存在铀系不平衡现象,结合研究区沉积环境和高纯锗γ谱仪测量结果进行对比分析表明,铀系不平衡主要来源于自生铀吸收,对环境剂量率的影响约为11%。

Optically stimulated luminescence(OSL)dating as a absolute dating method for sediments has been widely used in Quaternary studies.Improving the accuracy and precision of OSL dating has been a hot spot in academic research.However,for a long time,there have been numerous studies on equivalent dose measurement procedures and calculation methods,and relatively few studies on environmental dose rates.In this paper,we conducted OSL chronology study on marine sediment samples obtained from the Borehole ZBW in the southwestern of Hainan Province.The contents of uranium,thorium and potassium in the samples were measured by inductively coupled plasma mass spectrometry/optical emission spectrometry(ICP-MS/OES),neutron activation analysis(NAA)andγ-ray spectrometer,respectively.The results show that there are differences in the results measured by the three methods in some strata,and the reasons could be:(1)incomplete dissolution of U in heavy minerals during the ICP-MS/OES pretreatment process;(2)the ICP-MS/OES measurement results are the elemental contents of U,Th,and K,which include some non-radionuclide contents;(3)the small sample size used for NAA,which was subject to chance errors,and the dilution effect of non-radioactive material in the sample affects the measurement results;(4)there is an imbalance of uranium system in the sample.Combined with the sedimentary environment in the study area and the data measured by HPGeγ-ray spectrometer,the results show that the uranium imbalance originates from the autogenous uranium absorption phenomenon,and the effect on the environmental dose rate is about 11%.