Microstructure of bismuth centers in silicon before and after irradiation with 15 MeV protons.

A decrease of two-gamma annihilation rate of a positron in a strong spin-orbit field of the annihilation site of bismuth impurity centerBi (= 9/2) in silicon with natural isotope composition was revealed (is the nuclear spin). This decrease was observed along with increasing occupancy of Bi donor states (binding energy{Bi} ≈ 69 meV). Atoms ofSi (= 1/2) isotope are involved in spin interactions of positron with Bi impurity centers.