Innovating methodological development for groundwaters dating with 81Kr - 85Kr radioactive chronometers

In order to answer to the current demand of characterization of the hydrogeological systems of the deep geological repositories (radioactive wastes, CO2), the 'CNAB' laboratory has begun in 2003 to develop instruments for carrying out the innovating method of underground water dating by radioactive chronometers 81Kr (t1/2=229 Ky) and 85Kr (t1/2=10.73 years). The isotopic ratios Kr/81Kr and Kr/85Kr allow to obtain an absolute dating of the waters in the ranges 100 Ky-1 My and 0-50 years respectively. The low abundance of 81Kr and 85Kr in natural waters (1200 at.81Kr/liter of water, 19500 at.85Kr/liter of water) and the available analysis techniques required until now very important volumes samples (16000 liters for an analysis by AMS (Collon and al. (2000)) and 2000 liters for analysis by ATTA fluorescence (Sturchio and al. (2005)). The resonance ionization spectroscopy time of flight mass spectrometers (RIS-TOF) developed at the 'CNAB' laboratory will allow the analysis of some thousands krypton atoms, lowering the water volume to treat at 20 liters. The main obtained experimental results are described. (O.M.)