  We report a methodology for measuring 85Kr/Kr isotopic abundances using Atom
Trap Trace Analysis (ATTA) that increases sample measurement throughput by over
an order of magnitude to 6 samples per 24 hours. The noble gas isotope 85Kr
(half-life = 10.7 yr) is a useful tracer for young groundwater in the age range
of 5-50 years. ATTA, an efficient and selective laser-based atom counting
method, has recently been applied to 85Kr/Kr isotopic abundance measurements,
requiring 5-10 microliters of krypton gas at STP extracted from 50-100 L of
water. Previously a single such measurement required 48 hours. Our new method
demonstrates that we can measure 85Kr/Kr ratios with 3-5% relative uncertainty
every 4 hours, on average, with the same sample requirements.
