It was shown that the specific heat at very low temperatures established the mass of helium which was precipitated, and chromatography gave the amount of helium desorbed during annealing. Electron microscopy determined the radius and mean separation of helium bubbles. In this way the diffusion of helium could be followed accurately. Precipitation of helium began during irradiation. Bubble nucleation began at 600C, and peaked at 1200C. The diffusion of helium satisfied:

D(cm2/s) = 2.5 x 10-3 exp[-3.45(eV)/kT]

between 1080 and 1400C.

E.Gmelin: Journal of Nuclear Materials, 1971, 38[2], 150-62