Study of umklapp annihilation of positrons in copper by means of rotating-specimen method

High momentum components in the momentum distribution of annihilation photons from Cu single crystal have been investigated using the crossed-slit rotating-specimen method. The obtained data have been analyzed by using a simple model based on the Wigner-Seitz approximation. Agreement of the calculated results with the experimental data is good in high momentum region, suggesting that, away from the zone boundaries, the ''umklapp'' annihilation rate does not strongly depend on the momentum. However, the observed intensity of the ''umklapp'' annihilation relative to the ''normal'' annihilation has been considerably smaller than that predicted by the Wigner-Seitz model. (auth.)