Research On High Intense Metastable Krypton And Argon Atomic Beam Source For Ultrasensitive Isotope Analysis

There are three long-life noble gas radioisotopes in the atmosphere(85Kr,81Kr and 39Ar).They are ideal tracers for water samples due to their chemical inertness and near single source,and have important applications in the study of groundwater,seawater,mountain and polar glacier.However,the natural abundance of 85Kr,81 Kr and 39Ar is extremely low(10-16～10-11),which is very difficult to detect and a great challenge to modern ultrasensitive isotope analysis techniques.Atom Trap Trace Analysis(ATTA)has been developed to solve the problem of detecting long-life noble gas radioisotopes.ATTA is a single-atom-level detection method based on laser cooling and trapping technology which has the advantages of superb isotopic selectivity,singleatom-level sensitivity and high efficiency,and it’s the only method that has achieved routine measurement of 85Kr,81Kr and 39Ar.In recent years,the radioactive krypton and argon dating technology based on ATTA method has been applied in a series of frontier studies,such as ice core dating,paleogroundwater dating and ocean currents tracing.With the further application,the researchers need ATTA method to reach higher analytical precision and measurement efficiency.The core of ATTA method is to manipulate neutral noble gas atoms by laser.In order to achieve this manipulation,noble gas atoms need to be prepared on the corresponding long-life metastable state.Therefore,the preparation of noble gas atomic metastable state will directly affect the key parameters of ATTA method,such as measurement efficiency,analytical precision and measurement time.Centering on this key problem,the research on high intense metastable krypton and argon beam source based on RF helical resonator is carried out in this thesis.The plasma parameters of RF helical resonator discharge were diagnosed and the transport of metastable krypton atoms in the RF source was studied via spectroscopy.According to the measurement results,we explored the method to increase the metastable atomic flux intensity by adding external magnetic field to confine plasma,and finally developed high intense beam sources compatible with Kr-ATTA and Ar-ATTA instrument respectively,which significantly improved the atomic counting rate of ATTA instrument.The intense beam sources promote high-precision 81Kr dating and large-scale application of 39Ar dating.The main contents of this thesis include:1)The electron density and temperature,and ion energy distribution are diagnosed by Langmuir probe and retarding field energy analyzer respectively which are important plasma parameters for the subsequent simulation and experimental study.The influence of different parts of RF source on krypton degas rate was quantitatively measured which confirmed that discharge tube was the main source of cross contamination.By adding magnetic field to deflect ions and xenon discharge washing,the krypton degas rate of the ATTA instrument is reduced to 3 nL(STP)/h,which effectively reduces cross contamination between samples.2)The generation and transport process of metastable krypton atom in RF helical resonator beam source is studied via spectroscopy.We found that the internal discharge tube still makes an important contribution(～70%)to the metastable krypton flux under typical working condition.Based on the experimental data,a model for the metastable krypton generation and transport in the discharge tube and formation of metastable atomic flux is established.The metastable krypton atom spatial distribution in the discharge tube and flux velocity distribution are simulated from the established model,which are in good agreement with the experimental results.Based on this model,the effects of discharge tubes with different diameters on metastable krypton atomic flux are studied,which provides a basis for optimizing beam source design and improving the metastable krypton flux intensity.3)Combining with previous studies,the methods of improving the metastable krypton and argon atomic flux intensity by utilizing static magnetic field and collimated molecular beam were explored,and high intense beam sources compatible with Kr-ATTA and Ar-ATTA instrument were developed.The metastable krypton and argon flux intensity has been increased by a factor of 4 to 5.The atomic counting rate of KrATTA and Ar-ATTA instrument has been significantly improved.The atomic counting rate of 81 Kr and 39Ar are 6000 atoms/hour and 24 atoms/hour respectively when measuring modern samples,which reach the best level in the world at present and promote high-precision 81Kr dating and large-scale application of 39Ar dating.