Positron annihilation at divacancies in metals

The influence of the shape of a positron trap on some annihilation parameters has been calculated. Using an aspherical model for divacancies in aluminium, the positron lifetime is estimated to be 1.1 times that for monovacancies, and the divacancy trapping rate is estimated to be 2.0 times the monovacancy trapping rate. In comparison, a comparable spherical model gives nearly the same lifetime and a trapping rate which is about one-fifth larger.