Low-temperature anomaly of positron trapping by defects in metals

Resonance trapping of thermalized positrons at defects in metals has been investigated for the first time using the cloudy crystal ball model, which has been used for nuclear reactions with neutrons. It is shown that not only the positron s-wave resonance but olso p and d resonances make substantial contributions to the trapping rate of thermalized positrons in the practical temperature range. Because of the resonance effect, the trapping rate (ν) strongly depends on the positron energy: In the neighbourhood of an s resonance the rate ν sharply increases toward zero energy, whereas in the vicinity of a p or d resonance the rate ν has a maximum at an intermediate energy owing to the centrifugal force. With this model, not only the prevacancy effect but also the strong temperature dependence of ν recently observed at low temperatures can be explained as the manifestation of resonance trapping. Furthermore, the wide range of sensitivity of positrons to thermal vacancies in metals can be well understood, since the specific trapping rate for vacancies in metals can extend over several orders of magnitude, i.e., 1012-1016 s-1, depending on whether or not the resonance condition is satisfied. (author) 56 refs., 7 figs