Positron trapping rate into small vacancy clusters and light substitutional impurities

The trapping rate of positrons into small vacancy clusters and light substitutional impurities in metals is calculated. The host metal is modelled by a uniform jellium with a spherical cavity describing the vacancy of the vacancy cluster. In the case of a substitutional impurity a point charge is placed at the centre of the vacancy cavity. Self-consistent electron densities are solved in the local density approximation for exchange and correlation. The corresponding localised (trapped) and delocalized positron states are calculated using a local form for the electron-positron correlation. The trapping rate is calculated as a function of the initial positron energy and for thermalized positrons as a function of temperature. The model predicts that due to scattering resonances the thermal trapping rate for some small vacancy clusters may exceed that for a single vacancy by more than an order of magnitude. The effect of light impurities H and He is seen to be rather small. The calculations show that the trapping rate is a quantity sensitive to the details of the trapping potential and predict that the experimentally observed trapping rates and their temperature dependence can be very different in different materials.