  The kinetics of helium gas sorption by a C60 powder and subsequent desorption
of the 4He impurity from the saturated powder has been investigated in the
temperature interval T = 2-292 K. Evidence is obtained that supports the
existence of two stages in the temperature dependences of sorption and
desorption. The stages account for the different times taken by helium to
occupy the octahedral and tetrahedral interstices in the C60 lattice. The
characteristic times of sorption and desorption coincide. It is found that the
temperature dependences of the characteristic times of occupying the octahedral
and tetrahedral interstices are nonmonotonic. As the temperature is lowered
from 292 K to 79.3 K, the characteristic times increase, which indicates a
predominance of thermally activated diffusion of helium in C60. On a further
decrease to T = 10 K the characteristic times reduce over an order of
magnitude. Below 8 K the characteristic times of sorption and desorption are
temperature-independent. This suggests a tunnel character of 4He diffusion in
C60.
