This proposal addresses the development of a new technique, based on the Atom Trap Trace Analysis (ATTA) method used for radioactive dating with rare Kr isotopes, to measure the Kr content of other rare gases at levels as low as 3x10^(-14). A new generation of detectors based on liquid rare gases offers a promising avenue for exciting new discoveries. These detectors include the XENON dark matter search project, and the XMASS and CLEAN neutrino detection projects, all of which rely on the use of liquid xenon or neon as a scintillation medium to detect infrequent events at low energies. In all of these systems, the beta decay of krypton-85 is a significant and problematic source of background events. 85Kr is chemically inert, and has a half-life of 10.76 years, making it extremely difficult to separate from other rare gases. The XENON detector will require Kr/Xe abundances of at most one part in 10^10, and the XMASS and CLEAN detectors will require Kr/Xe or Kr/Ne of at most one part in 10^14. This level of purity cannot be guaranteed with conventional commercial techniques.
In the ATTA method, a sample of the gas to be analyzed is introduced to an apparatus that uses laser cooling techniques to cool and trap Kr atoms in a metastable state from an atomic beam. Single trapped atoms can be counted with high signal-to-noise by detecting the laser fluorescence of the trapped atoms with a sensitive photodetector, directly measuring the trap loading rate. The Kr abundance can be determined by comparing the loading rate for an ultra-pure sample to that of a sample with known Kr content.
Broader Impact The proposed measurements will aid in the XENON, CLEAN and XMASS efforts by helping characterize the background levels in these new detectors. Additionally, the ATTA technique may be of some commercial interest to manufacturers of research grade gases. The technique can be extended to detect elements other than Kr, and to measure impurity gases at levels well below those possible with conventional techniques. This may enable the manufacture of gases of higher purity than is currently possible, which would be useful to researchers in many fields.
