Dissolution and diffusion of helium in graphite

Thermal desorption measurements were performed on three isotropic graphite samples which were exposed to helium gas at 100-700deg C under 8-130 kPa for 1 h. Helium atoms which were desorbed from the sample during ramp heating at 10deg C/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 appears to consist of two peaks (450 and 620deg C) after helium gas exposure at above 500deg C. The helium desorption can be ascribed to the diffusion-controlled process where the helium atoms may leave through bulk surface and open pores. (orig.)