The replacement of 3He implanted in Nb by subsequent 4He bombardment and vice versa

The exchange of helium isotopes 3 He, 4 He implanted into polycrystalline niobium at 30 and 50 keV was investigated by measuring the decrease of 3 He during postimplantation of 4 He and the increase of 3 He implanted after preimplantation of 4 He using the 3 He(d, α)H reaction. At low doses both isotopes are trapped with nearly 100% efficiency until a saturation concentration of the isotope mixture has been obtained. Thereafter the exchange proceeds to a first approximation according to a simple mixing law. From this we conclude that at saturation concentration the binding energy of the helium to traps is low and the helium diffuses readily within the implanted layer.