THE OPTICAL-POTENTIAL RESPONSIBLE FOR THE ABSORPTION OF A POSITRON ATA VACANCY

We describe an approach to the positron trapping at vacancies based on the optical theorem. We calculate the imaginary part of the optical p otential in an ab initio model at the time when electron-hole excitati on takes place. The obtained analytical formulae for the optical poten tial are tested as a function of the positron incident energy, the pos itron binding energy, and the electron density parameter r(s). The cal culation of the optical potential range agrees well with the vacancy r adius. The approach is used for the description of observed temperatur e dependences of trapping rate at vacancies in pure metals as reported in the literature. The results are satisfactory and the obtained valu es of the optical potential are quite reasonable.