激光囚禁放射性氪同位素技术测量地下水年代

RADIOACTIVE KRYPTON DATING WITH ATOM TRAP AND ITS APPLICATION IN GROUNDWATER CHRONOLOGY

地下水年代测定对于了解地下水循环具有十分重要的意义。放射性氪同位素^(85)Kr(半衰期10.8a)、^(81)Kr(半衰期0.23Ma)具有十分理想的地球物理化学特性,十分适于对溶解了大气的水或冰样品进行绝对年代测定。由于样品中的放射性氪原子量极少,利用^(81)Kr含量进行地下水测年时需要数吨水样品才能完成(Earth andPlanetary Science Letters,2000,182:103～113;Geophysical Research Letters,2004,31:L05503)。本文主要介绍我们在中国科学技术大学建立的激光冷却氪原子测量装置,该系统已经成功地对氪原子进行了激光冷却和囚禁,实现了单个放射性氪原子^(81)Kr和^(85)Kr的计数测量。氪原子的冷却捕获速率被大幅提高,预计测年应用中所需样品量可减小一个量级以上,这将使得我们可以开展新的地下水年代测定应用。

Groundwater dating is essential to understand the groundwater cycling. Radioactive krypton isotopes ^85Kr and ^81Kr, with half life time 10. 8a and 0.23 Ma, respectively,have excellent geological physical and chemical properties very suitable for dating air-bearing water or ice samples. However, because the concentration of radioactive krypton atoms is extremely low in sample, several tons of groundwater are necessary to process a ^81Kr dating sample （Earth and Planetary Science Letters, 2000,182 ： 103 - 113 ; Geophysical Research Letter, 2004,31 ： L05503 ）. Here we introduced the instruments under construction in University of Science and Technology of China for detecting krypton isotopes with laser cooling method. At the present stage, we have successfully carried out laser cooling and trapping of krypton isotopes. Single atom counting has been realized for the radioactive isotopes ^85r and ^81Kr. With a much higher loading rate of cold krypton atoms,the necessary sample volume in the krypton dating is estimated to be over ten times smaller. The improvements will allow new groundwater dating applications.