  Ion-implantation is a useful technique to study irradiation damage in nuclear
materials. To study He effects in nuclear fusion conditions, He is co-implanted
with damage ions to reproduce the correct He/dpa ratios in the desired or
available depth range. However, the short-term fate of these He ions, i.e over
the time scales of their own collisional phase, has not been yet unequivocally
established. Here we present an atomistic study of the short-term evolution of
He implantation in an Fe substrate to approximate the conditions encountered in
dual ion-implantation studies in ferritic materials. Specifically, we calculate
the fraction of He atoms that end up in substitutional sites shortly after
implantation, i.e. before they contribute to long-term miscrostructural
evolution. We find that fractions of at most 3% should be expected for most
implantation studies. In addition, we carry out an exhaustive calculation of
interstitial He migration energy barriers in the vicinity of matrix vacancies
and find that they vary from approximately 20 to 60 meV depending on the
separation and orientation of the He-vacancy pair.
