Evidence for the existence of two kinds of orthopositronium in condensed rare gases and for irreversible transitions between them

With reference to time-differential measurements of the momentum distributi on of annihilating positron-electron pairs (AMOC) in the condensed rare gas es Ne, Ar, and Kr, the paper discusses the question whether in condensed ma tter positronium (Ps) forms only at predetermined trapping sites or "anywhe re", with subsequent processes leading to Pa trapping or to the formation o f Ps bubbles. Ne, in which Ps bubbles are formed not only in the liquid but also in the solid state, may serve as model material for more complex Ps-f ormers, such as polymers. (C) 2000 Elsevier Science Ltd. All rights reserve d.