Polarization and thickness effects in Mössbauer spectroscopy

The lattice sites of positive pions in gold have been determined as a function of temperature by means of π⁺/μ ⁺ channelling experiments. A blocking pattern in the <100> direction at low temperatures indicates that only octahedral interstitial sites are occupied. The vanishing of this pattern and the reduction of muon flux enhancement observed in <110> ifT is raised above the Debye temperature (165 K) may be interpreted in terms of muon dechannelling owing to the thermal vibrations of the lattice atoms. Time-differential π⁺/μ ⁺ channelling studies on α-iron showed between 20 K and 70 K a strong dependence of the channelling profiles on the time the pions spent in the sample. The ‘young’ pions are well localized on O- or T-sites whereas the ‘old’ pions either become delocalized or occupy sites with strongly reduced symmetry. A model accounting for these observations is suggested. It is argued that there is a close relationship to earlierμ ⁺ SR experiments.