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 years) and low concentration (1.3 x 10381Kr atoms per liter of modern water at 100C) 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 a commercially available mass spectrometer. The enriched gas containing about 108 stable krypton atoms and about 103 atoms of 81Kr was implanted into a silicon disc. This disc was then placed in the high vacuum final counting chamber and the krypton was released by laser annealing. This chamber contained a quadrupole mass spectrometer which used a pulsed VUV laser source as the ionizer. The measured signal indicated that the sample contained 1200 (+-300) atoms of 81Kr