Direct Observation of Delocalized Positronium in KCl

Abstract

The angular correlation of two γ-rays from positrons annihilating in KCl has been measured at 4.2 K. Obtained data clearly show a narrow component which was not evident in a previous measurement at liquid N 2 temperature. This narrow component is a direct evidence for delocalized positronium formation in KCl. Comparison of the data with the theoretical angular correlation curve calculated by Nieminen and with the results of magnetic quenching experiments indicates that more localized positronium-like states also exist.