Study on absolute age determination for inland clastic sediments, especially on dosimetric dating

本研究的主要目的是建立一种确定晚第四纪中晚更新世陆相碎屑沉积年代的方法，该方法没有任何年代标志化石。主要测试了火山灰、风成黄土、淡水硅藻土、河流和湖泊沉积等典型内陆沉积物。通过热释光(TL)和电子自旋共振(ESR)技术观察到的剂量信号，测量了沉积后累积的古剂量。剂量学：样品经石英或石英玻璃净化后，用6-12 MeV直线加速器的电子束或X射线照射。一些样品在辐照前进行了退火热处理，以评估剂量的饱和度。采用热释光探头原位剂量法和伽玛能谱活度估算两种方法测量年剂量率，并对23个样品进行了剂量测年实验。其中有一半的标本是Succes…较完整的年代为10Ka至700Ka B.P.(Ka=10^3年)。由于剂量几乎完全饱和，有几个样品未能获得其沉积年龄。因此，提出了以下几个问题和评论：(1)含石英沉积物最适合于基于太阳光突破(时间归零)机制和结晶机制的剂量学测年。(2)无定形样品如火山玻璃和硅藻土在液氮温度下可用低功率ESR测年，但在辐照过程中避免了饱和现象。(3)高于4.5Gray/min的高剂量率照射会改变敏感度和/或敏感度，特别是在大多数样品超过100kA的情况下。(4)利用直线电子束(或X射线)源进行照射是非常有用的，因为它具有很高的剂量控制精度。(5)精密伽马射线探测器可用于天然放射性元素(U、Th、K)的剂量率直接测量和定量分析。较少

Main aim of this study is to establish a method for the Mid to Late Pleistocene age determination of non-marine clastic sediments without any chronological marker fossils for the later Quaternary. Typical inland sediments including volcanic ash, eolian loess, fresh water diatomite, and river and lake deposits are mainly tested. Paleodose accumulated after sedimentation were measured by means of dosimetric signals observed by using thermoluminescence (TL) and electron spin resonance (ESR) techniques.Dosimetry: After purification for quartz or silica glass, those samples were irradiated by electron beam or X-ray from 6-12MeV linear accelerator. Some samples were annealed before irradiation in order to evaluate saturation in dose. Annual dose rate were measured by using two methods, in-situ dosimetry using TL probes and activity estimation from chemical data of gamma-ray spectrometry.Experiments for dosimetric dating of 23 specimens was conducted. Among these, a half specimens were succes … More sfully dated ranging from 10Ka to 700Ka B.P. (Ka =10^3 years). Because of almost complete saturation in dose, several samples were failed to get their ages of sedimentation.Consequently, the following topics and comments are pointed out: (1) Quartz bearing sediments would be most suitable for the dosimetric dating based on a mechanism of sun-light breaching (time zeroing) as well as that of crystallization. (2) Amorphous specimens such as volcanic glass and diatomite could be dated by using low power ESR measurement at liquid nitrogen temperature, if saturation were avoidable during irradiation. (3) High dose rate irradiation over 4.5 Gray/min would change susceptibility and/or sensitivity especially in cases of most sample older than 100Ka. (4) Irradiation by means of lineac electron beam (or X-ray) source is very useful because of its high accuracy of dose control. (5) Precise gamma-ray detector can be put to practical use for direct measurement of dose-rate as well as quantitative analysis for natural radioactive elements (U,Th,K). Less