STUDY OF POSITRONIUM IN MOLECULAR SOLIDS

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Positronium forms in various molecular solids. In this research we investigated several important aspects of the positronium-related phenomena in these materials.We reported for the first time that positrons annihilate in C_<60> with a single lifetime. From this observation we concluded that positronium does not form in C_<60> and that the positrons populte in the interstitial region.We measured lifetime spetra for the positrons annihilating near the surface of graphite (HOPG) by using energy-tunable pulsed positron beam at ETL.We confirmed that a positron surface state exists in this material. We proposed a new interpretation to explain why thermal positronium is not emitted from graphite in spite of the existence of the surface state.We investigated correlation between the positron lifetime spectra and the angular correlation of annihilation radiation in PTFE (teflon) in detail. We observed that the widely used model do not give a quantitatively consistent result in this material.We performed isochronal annealing study of neutron-irradiatedalumina single crystal, which is important as a future atomic reactor material. We measured the diameter of the voids createdby annealing at temperatures above 900K by observing the momentum distribution of Ps inside the voids. We also confirmed existence of a component related with Al precipitation on the void surface.