海洋沉积物中^(210)Po的测定及其在地质年代学上的应用

THE DETERMINATION OF ^(210)po IN THE MARINE SEDIMENTS AND ITS APPLICATION IN GEOCHRONOLOGY

210Pb测年技术在海洋、湖泊地质年代学方面的应用日益广泛。如何准确、快速地测定沉积物中210Pb的含量成为人们所关注的问题。210Pb衰变时放出弱的β辐射(Emax=0.018Mev)和弱的γ辐射(Eγ=0.047Mev),直接测量比较困难,故人们常借助于测量210Pb的子体210Bi的β辐射(T1/2=5天,Emax=1.17Mev)或测量与210Pb处于久期平衡的第二代子体210Po的α放射性(T1/2=138天,Eα=5.3Mev)。后一种程序用208Po作示踪剂(T1/2=2.93年,Eα=5.11Mev),把208Po和210Po自沉积在银片上,制好的源不需放置即可在α谱仪上测量,从而缩短实验周期。只要知道加入的208Po的强度,测出210Po/208Po的比值后,就可求得样品中210Po的含量,无需进行化学回收率和仪器效率的校正。而且,α谱仪本底比β计数器低,灵敏度也较高。然而,在α谱仪上测量的源,要求制得薄而均匀,以减小谱线的尾拖,得到好的分辨率,否则会影响谱形,使解谱发生困难。因此,寻找一种既可制得均匀薄源,且又能保证一定的回收率的分离技术就显得特别重要。

A simple procedure for determining.210Po in marine sediments by dcpositing spontancously onto a silver disk from HCl solution is described.The effects of HCl concentration,volume of the solution,temperature,time of deposition,plating materials and romeα-cmitters were studied and the optimum conditions defined.The polonium radioactivity was detected byα-spectrometry.Using this procedure,the vertical,distributions of Pb-210 content in eight cores from tbe Changjiang Estuary and its adjacent continental shelf were determined.The sedimentation rates of five cores were calculated.The results are in good agreement with those obtained by the method of 210Pb-210Bi.