Abstract
With the aim of obtaining direct information on the sites occupied by positive pions (π+), channelling experiments have been performed on the decay muons (μ+) from π+ implanted into high-purity copper crystals. At 100 K virtually exclusive occupancy of octahedral interstitial sites (0) is found in good accordance with earlier μSR results. At room temperature a significant fraction of the implanted pions decay on metastable tetrahedral interstitial sites (T). The data suggest strongly that above about 100 K both μ+ and π+ diffuse by phonon-assisted tunnelling between stable 0 sites and metastable T sites. The enthalpy difference between these two sites is estimated to be about 50 meV for μ+ and of the same order of magnitude for π+. These values are much smaller than the available theoretical predictions.
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