Thermal desorption measurements were performed on three isotropic graphite samples which were exposed to helium gas at 100 to 700C under 7 to 130kPa for 1h. Helium atoms which were desorbed from the sample during ramp heating at 10C/min in vacuum were identified with a quadrupole mass spectrometer. Helium was dissolved into graphite for a few ppm in atomic fraction. The thermal desorption curve for ISO-880U appeared to consist of two peaks (450 and 620C) after helium gas exposure at above 500C. The helium desorption could be ascribed to the diffusion-controlled process where the helium atoms may leave through bulk surface and open pores.

Dissolution and Diffusion of Helium in Graphite. Atsumi, H., Yamauchi, T., Miyake, M.: Journal of Nuclear Materials, 1991, 179-181[1], 227-30