Analysis Of Krypton Gas Dissolved In Groundwater For Radiokrypton Dating

Noble gas radioisotopes are homogeneously distributed in the atmosphere and are considered to be ideal environmental tracers.85Kr (t1/2=10.8 yr, isotopic abundance 2.5 ×10-11) is a fission product that can be used to calibrate atmospheric transport models and date young groundwater with an age range of 2-50 yr.81Kr (t1/2=2.29 ×105 yr, isotopic abundance 5 x 10"13) is a cosmogenic nuclide and a desired tracer for old groundwater and ice on the time scale of 50-1500 kyr. Atmospheric 39Ar (t1/2 =269 yr, isotopic abundance 8 x 10-16) is also mainly produced by cosmic ray-induced reactions and is of particular interest for dating groundwater with an age range of 50-1000 yr and studies of deep ocean mixing and circulation. Employing laser cooling, trapping and atom counting techniques, Atom Trap Trace Analysis (ATTA) is currently the most practical method dating for environmental samples using 81Kr.The present dissertation mainly describes the setting up of an efficient sampling and purification system for separating noble gases from groundwater samples and the the subsequent analysis process, including field sampling, krypton purification and ra-diokrypton atom counting with ATTA. Using vacuum extraction chamber and mem-brane contactor methods, we recovered dissolved gases from groundwater samples ef-ficiently with a high extraction yield. An efficient krypton purification system, using a combined process of cryogenic distillation and gas chromatography, was then devel-oped for detecting radioactive noble gases (85Kr and 81Kr). Based on a similar separa-tion method, a combined purification system has also been built to recover both argon and krypton gases from air-like samples for future 39 Ar detection. Using these systems, the ages of groundwater samples from various regions in China have been determined, including young groundwater (by 85Kr detection) with an age of a few decades and so-called "fossil water" (by 81Kr detection) with an age of almost one million years.This dissertation is organized as follows:In chapter 1, we summarize the applications of noble gas radioisotopes on envi-ronmental dating and briefly introduce the principle of ATTA built in Hefei. The sizes of various environmental samples for ATTA measurement are listed.In chapter 2, some reports on the systems of noble gas separation from ground- water samples are summarized. Next, the principles and categories of three sampling systems are presented.In chapter 3, several different noble gas purification systems are introduced in detail, including the krypton system at USTC, the combined krypton-argon system at USTC and the krypton system at the University of Chicago. Potential improvements and prospects for the whole analysis process are given.In chapter 4, the sampling, detection and basic age results of ground water samples obtained by using the whole analysis process are presented.