Ultra-high vacuum noble gas purification. Purification methods

Our planet, during its accretion 4.56 billion years ago, trapped rare gases from the solar nebula. These, of primitive composition (little or poorly known, ³He/⁴He > ≈ 10^–4, ⁴⁰Ar/³⁶Ar ≈ 10^–4) then differentiated at both elemental and isotopic levels following crust formation, mantle outgassing and differentiation. Noble gases are currently present in all three of the Earth's major reservoirs, namely the atmosphere, crust and mantle, and their compositions are constantly evolving as a result :

• interactions between these reservoirs (subduction zones, volcanism, etc.);

• exhaust losses in space ;

• nuclear reactions.

These compositions have a :

• radiogenic, generated by fission and decay reactions certain isotopes (⁴He, ⁴⁰Ar, ¹²⁹Xe, ¹³⁶Xe) from radionuclides (²³⁵-²³⁸U, ²³²Th, ⁴⁰K, ¹²⁹I, ²⁴⁴Pu, etc.);

• nucleogenic, produced as a result of reactions between the nuclei of certain elements (⁶Li, ¹⁷-¹⁸O, ²⁴-²⁵Mg, ³⁵-³⁷Cl, etc.) and neutrons or α particles (⁴He) of rare gas isotopes (³He, ²⁰-²¹-²²Ne, ³⁶Ar, ³⁸Ar, etc.) ;

• cosmogenic, the result of interaction between target elements (O, Mg, Si, Al, Fe, Ca, Rb, Sr, Te, Ba, La, etc.) with cosmic radiation (spallation reaction) and the capture of muons to produce a large number of rare gas isotopes (³He, ²⁰-²¹-²²Ne, ³⁶-³⁸Ar, ⁷⁸-⁸³Kr, ¹²⁴-¹³²Xe).

A final component, of human origin this time, has altered the ratio ³He/ ⁴He in the water cycle as a result of ³He produced from tritium (³H) emitted into the atmosphere by nuclear testing.

The study of the elemental and isotopic composition of these reservoirs is based on rock and/or fluid (liquid or gaseous) samples collected in the field by the geologist, whose aim is to understand the geological history of a region or, more globally, that of the Earth or the solar system. To process these samples, the scientist, in particular the geochemist specializing in rare gas analysis, relies on one or more extraction techniques