Ultra-high vacuum noble gas purification. Purification system

Noble gases are the chemical elements classified in group 18 of the periodic table. There are 7 of them: helium, neon, argon, krypton, xenon, radon and ununoctium, with respective symbols He, Ne, Ar, Kr, Xe, Rn and Uuo. Only the first five elements in this group are discussed in detail, to give readers a better understanding of the physico-chemical processes involved in extracting them from a geological sample by grinding, melting or ablation, and then purifying them in an ultra-high vacuum (UHV) chamber. This article does not comment specifically on radon, which is produced by the radioactive decay of radium and has very limited applications in the geosciences, nor on ununoctium, which is a purely synthetic element produced in certain physics laboratories specializing in fundamental research.

The five noble gases helium (He), neon (Ne), argon (Ar), krypton (Kr) and xenon (Xe) each have several natural isotopes. There are 2 (He) and 3 (Ne, Ar) for the light rare gases, and 6 and 9 for Kr and Xe. All these isotopes are stable, but some have been produced by radiogenic, nucleogenic or fissiogenic reactions ( ⁴ He, ²¹ Ne, ⁴⁰ Ar, ⁸⁴ - ⁸⁶ Kr, ¹²⁹ - ¹³⁶ Xe) resulting in variations in the isotopic composition of noble gases. Studies on father/son pairs are at the origin of dating methods (K/Ar, U-Th-Sm/He, etc.) and provide temporal indications on the formation of the rocks studied. In the course of many physical processes, noble gases also undergo elementary fractionation. These may have a kinetic origin, since the diffusion of light noble gases in a rock is, compared to that of heavy noble gases, faster for given temperature and pressure conditions. The adsorption of noble gases on solids is also a source of fractionation. This is due to Van der Waals-type bonds, which preferentially bind heavy noble gases to a solid to the detriment of light ones. Finally, the solubility of noble gases, in relation to the medium in which they are found, also leads to elemental fractionation. This is proportional or inversely proportional to the mass of the elements if the carrier phase is water or a silicate liquid respectively. Understanding and explaining the processes behind these isotopic and/or elemental variations (outgassing or differentiation of the mantle, volcanism, etc.) in a sample whose geological history began, for the oldest, 4.56 billion years ago, is a real geoscience challenge and requires the development of rare gas purification and separation chambers, in addition to vacuum...