Atom Trap Trace Analysis Determining The Abundance Of⁸⁵Kr And⁸¹Kr In The Environmental Sample

Noble gas radioisotopes mainly reside in the atmosphere as gaseous form and their contents are stable and/or traceable.81Kr (t1/2=2.29×105years, isotopic abundance5×10-13) is produced by cosmic ray-induced reactions with stable isotopes in the upper atmosphere.85Kr (ti/2=10.74years, isotopic abundance2.5×10-11) is mainly released during the reprocessing of spent nuclear fuel rods. Due to their unique chem-ical inertness, relative short gas-water dissolution equilibrium time, and well recorded atmosphere input function,81Kr and85Kr are ideal isotope tracers for dating environ-mental samples with the ages of50-1500kyr and2-50yr, respectively. Atom Trap Trace Analysis (ATTA) is a method based on laser cooling, decelerating, trapping and detect-ing particular isotopes, and it has advantages of ultra-high sensitivity, selectivity and zero background.The present dissertation is contributed to increase the atom counting efficien-cy, to test the quantitative performance of the ATTA instrument, and applications in groundwater dating. The resonance laser quenching method of calibration using sta-ble isotopes is proposed and implemented successfully to normalize the single-atom counting efficiency which may drift under different experimental conditions. The re-liability and reproducibility is tested by an inter-comparison with a series of standard samples measured independently in three laboratories:low-level counting laboratory in Bern (Switzerland), and two ATTA laboratories in Hefei (China), and Argonne (USA). Different uncertainties in the ATTA measurement have been analyzed, including the statistical uncertainty in single-atom counting, calibration using stable isotopes, and sample contamination. Applications of ATTA instrument in environmental studies are presented, including the determination of the atmospheric85Kr concentrations in differ-ent regions, dating deep groundwater in North China Plain, and shallow groundwater in urban Zhengding City, Leizhou peninsula and northwest Xinjiang.This dissertation is organized as following:Chapter1summarizes the properties of the long live noble gas radioisotopes and their applications.Chapter2describes the details of principle and ATTA apparatus built in Hefei. S- ingle atom identification and its simulation are presented. The method of laser quench-ing is presented to normalize the efficiency of single-atom counting. Test of quantita-tive capability is implemented by measuring a series of samples independently in three laboratories. A model considering the contamination effect is also presented.Chapter3introduces the sampling systems:water sampling and krypton purifi-cation. Measurements of environmental samples are presented, including atmospher-ic samples from different places, groundwater sampled in North China Plain, urban Zhengding City, Leizhou peninsula and northwest Xinjiang.Finally, in chapter4, future improvements and prospects of ATTA are present.