Positronium Trapping in Porous Solids: Means and Limitations for Structural Studies

Zs. Kajcsosa, L. Liszkaya, G. Duplâtreb, L. Vargaa, L. Lohonyaia, F. Pásztia, E. Szilágyia, K. Lázárc, E. Kótaia, G. Pál-Borbélyd, H.K. Beyerd, P. Caullete, J. Patarine, M.E. Azenhaf, P.M. Gordog, C. Lopes Gilg, A.P. de Limag and M.F. Ferreira Marquesh aKFKI Res. Inst. for Part. Nucl. Physics, P.O.B. 49, 1525 Budapest 114, Hungary bLab. Chim. Nucléaire, IReS, IN2P3/CNRS, B.P. 28, 67037 Strasbourg Cedex 2, France cInst. of Isotope and Surface Chemistry, P.O.B. 77, 1525 Budapest, Hungary dCentr. Res. Inst. for Chemistry, P.O.B. 19, 1525 Budapest, Hungary eLab. Mat. Minéraux, URA CNRS 4283, rue A. Werner, 68093 Mulhouse Cedex, France fDept. of Chemistry, University of Coimbra, Coimbra 3004-535, Portugal gDept. of Physics, University of Coimbra, Coimbra 3004-516, Portugal hInstituto Superior de Engenharia, Coimbra 3004-535, Portugal

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Received: 20 09 2004;

Positron annihilation techniques are used for the structural investigation of solids but the interpretation of results in grainy and porous media is still unclear. A unique picture can be obtained assuming that the dominant process is Ps trapping in competing "extended free volume" sites. In samples with a large amount of free volumes near-saturation Ps trapping will rule the lifetime pattern, and very long lifetimes of over 100 ns might arise from o-Ps trapped in mesopores. It is shown that lifetime parameters must be corrected for the 3γ/2γ counting efficiency ratio. The results demonstrate the high sensitivity of Ps to mesopores in zeolites but also that Ps-trapping poses limitations on the applicability of lifetime to structural investigation in porous systems. The evolution of the lifetime spectra upon changes in the sample and measuring conditions should be considered in a complex way, observing not only changes in some selected components but in the whole lifetime pattern simultaneously.

DOI: 10.12693/APhysPolA.107.729 PACS numbers:36.10.Dr, 78.70.Bj, 61.43.Gt