Penetration of Positrons in Solid Targets

The study of the interaction of positron beams with solid targets was approached by several investigators, also due to its importance for positron annihilation spectroscopy. This technique allows non destructive investigations of the structural defects of surfaces and interfaces: in particular information is provided about the nature and distribution of point defects in solid materials. The solution of the diffusion equation, necessary to obtain the fractions of incident positrons annihilated at different depths inside the target, requires the knowledge of the positron stopping profile, namely the initial depth distribution of the absorbed positrons. Also transmission of positrons is of great interest because it allows one, once backscattering is known, to establish the total fraction of particles absorbed by the target as a function of depth and primary energy. A theoretical model is proposed to compute both stopping profiles and transmission of positrons: the theory is compared with experimental data of Mills and Wilson [A.P. Mills and R.J. Wilson, Phys. Rev. A 26(1982)490] and with other models and Monte Carlo simulations.