The diffusion of He at 740 to 1250K was deduced from the rate of evolution of He upon heating B-doped crystals after slow neutron irradiation:

D(cm2/s) = 5.1 x 10-4 exp[-13.4(kcal/mol)/RT],

L.C.Luther, W.J.Moore: The Journal of Chemical Physics, 1964, 41[4], 1018-26