Long-Slit Rotating-Specimen Measurement of the Annihilation Radiation from Copper Single Crystals

Abstract

Anisotropic distribution of the collinear pairs of photons from positron annihilation in a Cu single crystal has been measured around the ⟨100⟩ axis with a conventional long-slit system.
A method for the evaluation of the neck radius of the Fermi surface from the slope of the counting rate of the collinear photon pairs plotted against the azimuth angle around ⟨100⟩ is presented and has been applied to Cu as a test case. The neck radius of Cu thus determined has been found to agree with that given by Halse.
The observed anisotropy is larger than that expected from the topology of the Fermi surface with a Wigner-Seitz evaluation of the higher momentum components. This result suggests the anomalous enhancement of the annihilation rate of the neck electrons.