ATTA-3: The Next-Generation Instrument for $^{81}$Kr-Dating

Atom Trap Trace Analysis (ATTA) has been used to analyze two rare isotopes: $^{81}$Kr (half-life = 230,000 yr, isotopic abundance $\sim $ 10$^{-12})$ and $^{85}$Kr ($\sim $10$^{-11})$, in environmental samples. Radiokrypton dating enabled by the ATTA method can now be used to determine the ages of old groundwater in the range of 50,000--1,000,000 years. The present apparatus (ATTA-2) has an overall counting efficiency of 0.01{\%} and, for $^{81}$Kr dating, requires a water sample of at least 1,000 liters. We are developing a new apparatus (ATTA-3) to trap and count $^{81}$Kr atoms with the goal of reaching a counting efficiency of 1{\%}. The required sample size could be reduced down to 10 liters of water or ice. ATTA-3 would enable a wide range of applications in the earth sciences. This work is supported by NSF, Division of Earth Sciences, under Award No. EAR-0651161, and by DOE, Office of Nuclear Physics, under Contract No. DE-AC02-06CH11357