Positron-surface sticking rates

The probability that positrons are trapped into the image-potential-induced state on a metal surface, i.e., ‘‘stick’’ to the surface, is discussed. Our model assumes that the positrons couple weakly to the surface and, hence, that the sticking coefficient goes to zero as the temperature tends to zero. This assumption is validated by the order-of-magnitude agreement with experimental data that we obtain for trapping rates for thermalized and epithermal positrons. We have also predicted that the trapping rate varies strongly with the positron work function and that trapping rates into helium bubbles are significantly smaller than for empty cavities.