Hope for slow positron 2D-ACAR

Positron is trapped by the hole type defect (localized) and vanished by pair annihilation with the electron. Atomic hole, cluster of atomic hole and complex materials of impurity can be detected by using this property of positron. The positron annihilation method determined the fine structure of hole and the electron structure. 2D-ACAR of positron trapped in defect gives the detailed distribution of momentum (the two-dimension map integrated in the direction of γ-annihilation) of positron and electron localized in the hole. It makes possible the detailed comparison with the calculation results of the first principle theory. The results of 2D-ACAR of cluster and hole in Si showed that about 60% positrons were trapped and annihilated by divacancy and 40% of it were vanished from the perfect crystal unless trapping. Very interest results were found that 2D-ACAR of neutral divacancy was very isotropic and that of negative divacancy (-1 or -2) was isotropic, too. (M.N.)