STUDY OF POSITRONIUM IN MOLECULAR SOLIDS

电子偶素存在于各种分子固体中。本文研究了这些材料中与正电子素有关的几个重要现象，首次报道了正电子在C_2中湮灭的<60>单寿命现象。用<60>能量可调的脉冲正电子束测量了石墨表面附近湮灭的正电子寿命谱，证实了石墨中存在正电子表面态。我们提出了一种新的解释来解释为什么尽管表面态的存在，石墨却没有发射热正电子偶素，并详细研究了聚四氟乙烯（聚四氟乙烯）中正电子寿命谱与湮没辐射角关联之间的相关性。我们观察到广泛使用的模型在这种材料中不能给出定量一致的结果，我们对中子辐照氧化铝单晶进行了等时退火研究，这是未来原子反应堆材料的重要组成部分。通过观察空位中Ps的动量分布，我们测量了在900K以上退火产生的空位的直径。我们还证实了存在的一个组件与Al沉淀的空隙表面。

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.