Helium desorption from Fe and V by atomic diffusion and bubble migration

Abstract

Thermal helium desorption from homogeneously implanted iron and vanadium foils was investigated during linear heating and isothermal annealing, respectively. The isothermal experiments showed an initial period with a square-root dependence of the desorbed helium on time, characteristic of atomic helium diffusion, followed by a reduction of desorption due to clustering. The substitutional helium atoms migrate by a dissociative mechanism, with dissociation energies of 1.4\ifmmode\pm\else\textpm\fi{}0.3 eV for both metals. Up to temperatures of 600 K (Fe) and 700 K (V), retrapping was dominated by irradiation-induced vacancies. At higher temperatures diffusion times became unmeasurably small. Retention of helium in the specimens indicated that di-helium clusters are stable up to 673 K in Fe and up to 773 K in V. Desorption after clustering was analyzed by comparison with models of bubble coarsening. The vanadium results were best described by bubble migration via surface diffusion at all temperatures investigated (573--1173 K), while the iron data from 900 to 1184 K indicated bubble migration by volume diffusion as the prevalent coarsening mechanism.