The temperature dependence of the two positronium bubble states in liquid SF 6

Abstract

Positron lifetime measurements have been performed on liquid SF 6 in the temperature range from −45°C to 71°C ( T c = 45.65°C). The positron lifetime spectra were resolved into four lifetime components. In the order of increasing lifetimes the four lifetime components are associated with the decay of para-positronium ( p-Ps), free positrons, ortho-positronium ( o-Ps) from a small bubble state, and o-Ps from a large bubble state. The lifetime of o-Ps annihilating from the large bubble state τ 4 increases from 5.7 ns at −45°C to 19.5 ns at 53°C. The lifetime of o-Ps annihilating from the small bubble state τ 3 was found to be 2–2.5 ns in the main part of the temperature range studied. Apparently, this is the first observation of two different o-Ps states in a liquid. The intensity I 4 ( I 3) increases (decreases) from 16.9% (16%) at −45°C to 47.2% (6.4%) at the critical point while above I 3 and I 4 are essentially temperature independent. The large Ps bubble state seems to be similar to the Ps bubble state found in most liquids.