The diffusion coefficient of helium in diamond was determined from the rate of evolution of helium upon heating boron-doped crystals after slow neutron irradiation: D = 7.0 x 10-4exp[-23.4(kcal)/RT], 1510 < T < 2470K. The activation energies were calculated theoretically from the differences in interaction energies of the helium in the interstitial site and at the saddle point between two such sites. An exp-6 potential was used with parameters estimated from inert-gas interactions. The theoretical E value was 24.8kcal. The pre-exponential factors as calculated from the vibration frequencies of the interstitial helium were about 10 times higher than that observed.

Diffusion of Helium in Silicon, Germanium, and Diamond. Luther, L.C., Moore, W.J.: The Journal of Chemical Physics, 1964, 41[4], 1018-26