Muon diffusion and trapping by defects in electron-irradiated Nb and Ta

The transverse spin relaxation of positive muons (μ+) has been measured on Nb and Ta after irradiation with 3 MeV electrons. In high-purity Nb theμ+ diffusivity derived from the trapping at irradiation-induced defects above 100 K is explained in terms of adiabatic hopping. At lower temperatures there is evidence for the dominating processes to be fewphonon incoherent tunnelling and coherent hopping. Annealing results in the formation of new defects capable of trapping theμ+. In Ta at least two types of irradiation-induced defects capable of trappingμ+ survive up to annealing temperatures of 400 K.