J. Phys. IV France 03 (1993) C4-161-C4-167
The existence of three o-Ps states in fine powders at low temperaturesC. DAUWE1 and G. CONSOLATI2
1 Sub-Atomic and Radiation Physics, University of Gent, Proeftuinstraat 86, 9000 Gent, Belgium
2 Dipartamento di Fisica, Politecnico di Milano, Piazza Leonardo da Vinci 32, 20133 Milano, Italy
The long lived o-Ps components (τ ≥ 1 ns) in MgO fine powder were investigated as a function of temperature. At 200 ≤ T ≤ 437 K we find one nearly-free interparticle orthopositronium component of which the decay constant varies linearly with temperature. At temperatures 77 ≤ T ≤ 140 K we find three o-Ps lifetime components ; the nearly-free o-Ps component τf, a surface component τs≈ 5ns, and a new unknown component τ2 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.
© EDP Sciences 1993