Development of a new facility for dating old ground waters and ice cores using 81Kr.

Krypton-81 is produced by nuclear reactions induced by cosmic rays in atmosphere while Krypton-85 is released from nuclear installations. With respective halflifes of 229 ky and 10.76 y, these isotopes are considered as ideal tracers for dating groundwater or ice core [1-6]. The measurement of 81Kr from a groundwater sample was first demonstrated using several isotopic enrichment steps followed by RISTOF technique [1-3]. Due to the very low abundance of 81Kr in groundwater - 1200 atoms/l in modern water - the dating generally requires sample volumes of at least several hundred liters [5,7]. Since 2003, in a collaborative work with IRIM, CNAB is developing a set of four instruments in order to measure 81Kr in groundwater using reasonable volume of water (20 l). Each instrument is dedicated to perform important steps in the dating process