Kinetics of the sorption of $^3$He by C$_{60}$ fullerite. The quantum diffusion of $^3$He and $^4$He in fullerite

The kinetics of the sorption and subsequent desorption of gaseous $^3$He in a C$_{60}$ fullerite powder has been studied in the temperature range of 2–292 K. The temperature dependences of the diffusion coefficients of $^3$He and $^4$He impurities in fullerite have been plotted using the measured characteristic times of filling of octahedral and tetrahedral interstices, as well as previous data. These temperature dependences of the diffusion coefficients of $^3$He and $^4$He impurities in fullerite are qualitatively similar. A decrease in the temperature from 292 to 79 K is accompanied by a decrease in the diffusion coefficients, which corresponds to the dominance of the thermally activated diffusion of helium isotopes in fullerite. A further decrease in the temperature to 8–10 K leads to an increase in the diffusion coefficients by more than an order of magnitude. The diffusion coefficients of $^3$He and $^4$He are independent of the temperature below 8 K, indicating the tunnel character of the diffusion of helium in C$_{60}$ fullerite. The isotope effect is manifested in the difference between the absolute values of the diffusion coefficients of $^3$He and $^4$He atoms at the same temperatures.