Resonance trapping of nonthermal positrons

The positron trapping rate at vacancies in metals is calculated as a function of positron energy within a square-well model. A low-lying positron $p$-wave scattering resonance is found for vacancies and small vacancy clusters, and a large increase in the trapping rate occurs near this resonance. Consequently, the trapping rate for nonthermal positrons can be more than an order of magnitude greater than for thermalized positrons. Other implications for positron studies of defects are discussed.