DIFFUSION OF ION-IMPLANTED HELIUM IN VANADIUM AND NIOBIUM

Helium ion implantation at 200 keV has been carried out on targets of pure vanadium and niobium held at elevated temperatures up to 1064 K. Helium concentration versus depth profiles was then measured for these targets using the method of nuclear reaction analysis. For target temperatures below about 800 K, there appears to be little migration of the implanted helium from the end of the ion range. At higher temperatures, significant migration of the implanted helium is evident. The migration of the helium during the implantation was analyzed by assuming that each implanted helium atom became trapped in a vacancy created by the implantation process. The subsequent migration rate was then dependent upon the mechanism for detrapping from such vacant sites. Of the two mechanisms investigated, i. e. , substitutional diffusion and dissociative diffusion by thermal detrapping, the data favored the dissociative mechanism. A similar conclusion was drawn for previously published data on titanium, iron, nickel, and zirconium.