Work function and epithermal positron emission from copper

The energy spectra for slow positrons re-emitted from a copper surface bombarded with monoenergetic positrons of energies between 10 eV and 2.2 keV, using the UEA electrostatic beam system, demonstrate the balance between thermalization and nondashthermalization of positrons prior to ejection from the exit surface into the vacuum. Although the fraction of nondashthermalized positrons passing through the surface becomes increasingly prominent as the incident energy decreases, the intensity of the sharp peak in the spectra associated with thermalized (or near-thermalized) positrons remains significant to the lowest incident energies used (∼ 10 eV). The energy dependence of the ratio of epithermal to thermal fractions is similar to experimental data obtained for silver, but diverges from the theoretical predictions of Kong and Lynn at low incident energies.