THE EXISTENCE OF 3 O-PS STATES IN FINE POWDERS AT LOW-TEMPERATURES

The long lived o-Ps components (tau greater-than-or-equal-to 1 ns) in MgO fine powder were investigated as a function of temperature. At 200 K less-than-or-equal-to T less-than-or-equal-to 473 K we find one nearly-free interparticle orthopositronium component of which the decay constant varies linearly with temperature. At temperatures 77 K less-than-or-equal-to T less-than-or-equal-to 140 K we find three o-Ps lifetime components; the nearly-free o-Ps component tau(f), a surface component tau(s) congruent-to 5ns, and a new unknown component tau2 which varies between 60 and 30 ns. All three components and their intensities are time dependent due to interaction with surface defects generated by the irradiating energetic positrons. The adsorption of oxygen on the defect surfaces causes a total screening and a restoration of the lifetimes to their initial values.