Analysis of 81Kr in groundwater using laser resonance ionization spectroscopy

A new analytical technique based on resonant ionization of krypton with a vacuum ultraviolet (VUV) laser source was used to determine low-level 81Kr concentrations in groundwater. The long half-life (210,000 yr) and low concentration (1.3 × 103 81Kr atoms/liter of modern water) make the detection of 81Kr by radioactive counting techniques extremely difficult. In this method, krypton gas was removed from water taken from an underground Swiss aquifer using standard cryogenic and chromatographic techniques. Stable krypton isotopes were then reduced by a factor of 107 by a two-stage isotopic enrichment cycle using commercial mass spectrometers. The enriched gas containing ~108 stable krypton atoms and ~103 atoms of 81Kr was implanted into a silicon disk. This disk was then placed in the high vacuum final counting chamber and the krypton released by laser annealing. This chamber contained a quadruple mass spectrometer which used a pulsed VUV laser source as the ionizer.1,2 The measured signal indicated that the sample contained 1200 (±300) atoms of 81Kr.