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  • 1 University of Gent Department of Subatomic and Radiation Physics Proeftuinstraat 86 B 9000 Gent (Belgium)
  • 2 Charles University Department of Low Temperature Physics Holesovickach 2 CZ-18000 Prague 8 (Czech Republic)
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ETLA (Elastic Thermalisation Lifetime Analysis) is a new model function for the fitting of lifetime spectra of slowly thermalising orthopositronium in the free space between the grains of insulating oxide powders. From this model one can obtain three fundamental parameters; the mass of the cluster of atoms which scatters the o-Ps, the probability that the o-Ps is forced into two-quantum decay at each collision and the emission energyE(0) of the o-Ps from the powder surface. As a consequence of full thermalisation of the Ps it is shown that the o-Ps quenching constant q() follows atT1/2 dependency for MgO, Al2O3 and SiO2 and for temperatures between 77 and 500 K. In MgO powder at low temperatures the irradiation by the positrons induces surface paramagnetic defects which are identified as Mg+ sites. These surface defects do not affect the thermalisation, but they induce paramagnetic o-p conversion i.e., an increase in , and they are also responsible for an increase in the emission energyE(0) of the o-Ps.