Muon diffusion in niobium in the presence of traps

The first CERN experiments on Nb metal showed a behaviour of muon depolarisation rate Lambda (T) similar to that in Cu, but with indications of some structure in the 'flat' low-T region. Later measurements on more purified samples, containing less than 100 ppm substitutional impurities and varying concentrations of interstitial nitrogen, showed a very strong dependence of the depolarization rate on the nitrogen content. In the region above T=20K, the Lambda -values could be interpreted in terms of a trapping of muons near nitrogen sites and the subsequent release from these traps. The diffusion of the muons from interstitial sites in the 'pure' Nb lattice was then supposed to start above 15K. Some unsolved questions remained, however; one being the absolute value of Lambda (or the Gaussian parameter sigma ) at maximum (for about 14K); and another, the tendency of a decrease of Lambda when going below 14K. It was decided to 1) dope the existing Nb metal with a substitutional impurity, and 2) try to obtain the purest possible Nb samples for studies of the low-temperature region. (6 refs)