Pion decay site spectroscopy in copper

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. The implications of low-lying metastable interstitial sites for the μ⁺ stopping sites of low temperatures are discussed.