Hopping diffusion of helium isotopes from samples of lunar soil

Abstract

This paper reports on a detailed study of diffusion of helium isotopes from a sample of lunar soil (weight, 3.3 mg; bulk grain size, <74 μm; sampling depth, 118 cm in a 1.6-m-long core of lunar soil brought from the Moon by the Soviet automatic station Luna-24). The studies have been performed using step heating in the temperature range 300–1000°C in combination with a mass spectrometric isotope analysis of helium extracted at each temperature step. It has been demonstrated that the diffusion does not obey Fick’s law, which should be attributed to a large number of radiation damages in crystals of lunar soil minerals and can be described in terms of the formalism accepted for jump diffusion. The diffusion activation energy for both helium isotopes (⁴He, ³He) has been found to be identical and equal to 0.5 eV, and the frequency factors amount to 0.51 and 0.59 s⁻¹, respectively. The random errors σ in the determination of these parameters are approximately equal to 5%. The lunar soil delivered to the Earth loses helium during the storage. At the beginning of the storage at room temperature, one gram of the lunar material under investigation loses approximately 3 × 10⁹ helium atoms every second. It has been revealed that the jump diffusion of helium exhibits a strong isotopic effect: the light isotope ³He escapes at substantially higher rates. In order to prevent helium losses accompanied by isotope fractionation, the brought lunar soil should be stored at a low temperature.