First simultaneous detection of helium and tritium inside bubbles in beryllium.

Electron energy loss spectroscopy (EELS) was applied to detect and analyze quantitatively helium (He) and tritium (H) enclosed inside bubbles in irradiated beryllium. Both gases were formed in beryllium under neutron irradiation as a consequence of neutron-induced transmutation reactions. They were detected for the first time as pronounced peaks at 13.0 eV for H and 22.4 eV for He in EELS spectra collected from flat hexagonal bubbles. An adhesion of H or formation of thin beryllium hydride layers on the internal basal surfaces was observed. The number densities of both gases were estimated using electron scattering cross-section and intensities obtained from EELS spectra. The number density values estimated for various bubbles fluctuate from 4 to 15 at/nm for He and from 4 to 10 molecules/nm for H. Lower gas number density was measured inside large bubbles. The observed higher density of tritium at inner walls of bubbles seems to confirm very recent ab initio calculations of the interaction of hydrogen isotopes with beryllium surfaces.