Variations isotopiques fines de l’hélium atmosphérique # 2 : développement d’un nouveau traceur atmosphérique

PI : Bernard Marty

CRPG participants : Pete Burnard

We aim to develop a new geochemical tracer : the fine variations of the isotopic composition of helium in air. This inert gas is not retained in the atmosphere because of its light mass, and the residence time of He isotopes in air is 106-107 yr. The isotope composition of air He is the result of isotope sinks and sources. The latter are mainly volcanism which permits degassing of primordial 3He from the mantle, and the continental crust which degasses 4He produced by natural radioactivity. The human activity has dramatically increased the flux of 3He from the decay of tritium produced by the nuclear industry, and the flux of 4He released from the crust by extraction of coal and hydrocarbons. He isotope variations related to the human activity could be at the per mil level according to our calculations. Existing He analysers have precisions at the % level, so a new technology needs to be developed, which involves simultaneous measurement of both isotopes and standard bracketing. We shall analyse He isotopes in air sampled every 6 months at CRPG since 10 years.

The new He isotope mass spectrometer developed in CRPG in collaboration with GV Instruments. Note the dual tube, with the Faraday collector (front) and the electrostatic filter plus the electron multiplier (back)