SOME ASPECTS OF THE INTERACTIONS OF ORTHOPOSITRONIUM WITH PERFECT ANDDEFECT SURFACES OF INSULATING MATERIALS

ETLA (Elastic Thermalisation Lifetime Analysis) is a new model functio n for the fitting of lifetime spectra of slowly thermalizing orthoposi tronium in the free space between the grains of insulating oxide powde rs. From this model one can obtain three fundamental parameters; the m ass of the cluster of atoms scatters the o-Ps, the probability mu that the o-Ps is forced into two-quantum decay at each collision and the e mission energy E(0) of the o-Ps from the powder surface. As a conseque nce of full thermalisation of the Ps it is shown that the o-Ps quenchi ng constant lambda(q)(infinity) follows a T-1/2 dependency for MgO, Al 2O3 and SiO2 and for temperatures between 77 and 500 K. In MgO powder at low temperatures the irradiation by the positrons induces surface p aramagnetic defects which are identified as Mg+ sites, These surface d efects do not affect the thermalisation, but they induce paramagnetic o-p conversion i.e., an increase in mu, and they are also responsible for an increase in the emission energy E(0) of the o-Ps.