THERMAL-DESORPTION OF HELIUM IRRADIATED BERYLLIUM STUDIED BY THDS, NDP, PA AND SEM

Helium interaction with beryllium was investigated by means of thermal helium desorption spectrometry (THDS), neutron depth profiling (NDP), scanning electron microscopy (SEM), and positron annihilation (PA). From the experiments, it could be concluded that at low dose (10(17) He-4 m(-2)) and at low energy (<3 keV) helium is released by a thermal vacancy assisted diffusion mechanism. When helium is implanted at higher energies (30 keV He-3(+)), small helium filled vacancy clusters are created. Upon heating, PA and NDP results indicated that helium bubbles nucleated and increased in size, starting at 500 K, before helium was released at T> 975 K. At the highest dose (7.7 x 10(20) m(-2), 30 keV), helium started to release at 100 K lower temperature. Scanning electron micrographs of the samples heated to 1075 K revealed extensive swelling of the beryllium surface, ten times the original thickness. Rather than to go into resolution and to permeate to the surface the helium remained trapped inside the growing bubbles until these penetrate the surface.