Effect of composition on helium diffusion in fluoroapatites investigated with nuclear reaction analysis

In the context of nuclear waste disposal, the knowledge of the diffusion of helium produced by α-decays is an important issue, in order to assess the long-term behavior of the containment matrices. Apatite is one of these potential hosts, which can incorporate many radionuclides as trivalent minor actinides and fission products. In order to determine the influence of the double (cationic/anionic) substitution on helium diffusion, we have studied two different apatite compositions (Ca10(PO4)6F2 and Ca4Nd6(SiO4)6F2). For that purpose, the helium bulk diffusion constants were derived from non-destructive 3He depth profiling using the 3He(d,p)4He nuclear reaction. Results have been obtained on sintered ceramics implanted with 3-MeV 3He+ ions at a depth around 9 μm with a fluence of 1016 ions cm−2 then annealed in air at temperatures between 250 and 400 °C. We show that the activation energy for helium diffusion determined by two different models decreases with substitution, i.e., 1.27 eV for Ca10(PO4)6F2 and 0.89 eV for Ca4Nd6(SiO4)6F2. Moreover, the activation energy in sintered Ca10(PO4)6F2 ceramics is similar to that determined by other authors on a single crystal of the same composition. It leads us to think that the diffusion mechanism involves the tunnels of the structure, the size of which increases with substitution.