Helium apparent diffusion coefficient and trapping mechanisms in implanted B4C boron carbide

Abstract

When boron carbide is irradiated in nuclear reactors, large helium quantities are produced due to the (n,α) neutron absorption reactions. In a recent paper, we have identified the trapping sites for helium, grain boundaries and damaged zones. In this paper, we propose the determination of an apparent diffusion coefficient for helium. 3He implantations then annealing were performed in B4C samples of different grain sizes. The 3He(2H,α)1H nuclear reaction was used to profile helium before and after annealing. Helium profiles with two superimposed components were observed. The narrow component is attributed to helium trapped in the implanted, damaged zone, either in clusters too small to be seen by TEM or as helium-defect complexes. The large component evolution is bounded by the distribution of the grain boundaries surrounding the implanted zone, confirming the high trapping efficiency of the grain boundaries. From an Arrhenius plot, an activation energy for helium diffusion in grains of 2.0 ± 0.2 eV was deduced in a large grain size material.