  Atom Trap Trace Analysis (ATTA), a laser-based atom counting method, has been
applied to analyze atmospheric Ar-39 (half-life = 269 yr), a cosmogenic isotope
with an isotopic abundance of 8x10^-16. In addition to the superior selectivity
demonstrated in this work, counting rate and efficiency of ATTA have been
improved by two orders of magnitude over prior results. Significant
applications of this new analytical capability lie in radioisotope dating of
ice and water samples and in the development of dark matter detectors.
