Crystal orientation dependence of surface modification in molybdenum mirror irradiated with helium ions

The crystal orientation dependence on a surface damage in molybdenum mirror has been investigated under the irradiation with 3 keV helium ions to a fluence of 10²² ions/m². A strong correlation between the surface roughening and the grain orientation was confirmed from SEM-EBSD analysis. It was found that significant surface roughening including crater-like depressions is formed on the grain surfaces with crystalline planes close to (1 0 0), while relatively smooth surface remains on grains with the other directions. In contrast, the reflectivity measurement of the single crystal molybdenum with a spectroscopic ellipsometry showed smaller reduction in (1 0 0) sample than in (1 1 0) and (1 1 1) samples. The smallest helium desorption in (1 0 0) sample was also detected with thermal desorption spectroscopy in comparison with them in (1 1 0) and (1 1 1) samples, which indicates (1 1 0) and (1 1 1) samples include the large amount of defects acting as effective trapping site for He atoms. A cross-sectional TEM observation actually showed a relatively large bubbles at sub-surface regions of (1 1 0) sample. These results indicate that the optical reflectivity should be affected by not only a top surface profile but also internal radiation damages such as high density of helium bubbles.