Long Lifetime of Positrons in Ionic Crystals

The time-annihilation spectrum of positrons in some ionic crystals was found to be complex. In alkali chlorides, besides the short-lifetime component typical of free-positron annihilation, the spectrum displays a long-lifetime component having a relative intensity of about 25% and a mean life ranging from 4.3×${10}^{-10\mathrm{}}$ sec (LiCl) to 6.4×${10}^{-10\mathrm{}}$ sec (CsCl). A linear dependence appears to occur between the long-lifetime annihilation rate and the molecular density. In lithium hydride the long-lifetime component exhibits a relative intensity of 12% and a mean life of 1.5×${10}^{-9\mathrm{}}$ sec. Taking into account the difficulties faced by assuming positronium formation in ionic crystals, an attempt is made to interpret our results on the basis of the existence of positron compounds, like that of Wheeler (${\mathrm{Cl}}^{-}{e}^{+}$) and that of Ore (${\mathrm{H}}^{-}{e}^{+}$).