Positrons and positronium in grainy and porous solids

Systematic investigations have been performed using positron annihilation t echniques on micrograins of crystalline powders (e.g., MgO, Al2O3, SiO2) an d on various zeolites (e.g., silicalite 1, Na-X, Na-Y, ZSM-5, mordenite) ai ming at a better understanding of tile formation and decay of the long-live d ortho-positronium states. The techniques were lifetime spectroscopy, Dopp ler-effect measurements and the recording of the full energy distribution o f the annihilation radiation. The influences of heat treatment, sample evac uation and presence of gases were investigated. Lifetimes over 130 ns and o -Ps fractions of about 30% were found. A strong correlation of positron ann ihilation technique data with a water content of the samples was evidenced. Possible pictures of the formation and decay of ortho-positronium in porou s media are discussed.