The effect of positron-phonon scattering on positron trapping at defects in metal

A fully quantum mechanical theory of the influence of positron-phonon scattering on the trapping rate of positrons by defects in metals is developed. The theory is applicable to strong as well as to weak trapping, and includes the effect of trapping on the positron distribution in the vicinity of the trap. Nonequilibrium many-body theory is used to formulate the problem in order to include positron annihilation in a natural way. It is concluded that for weak trapping the golden rule is applicable if an initial distribution of thermalised positrons including the positron-phonon interaction is used. For stronger trapping the lowest-order correction to the golden rule is calculated and the corresponding positron density is examined. The positron density around the trap is altered by the trapping process as time progresses, and this is illustrated by calculations of the time evolution of the positron density near a monovacancy. The correction to the golden rule and the shape of the density change near the monovacancy is determined primarily by the time evolution of the positron wavefunction caused by trapping. The effect of positron-phonon scattering around such traps is small in comparison. (author)