Correlation of Positronium Yield with Excess Electron Mobility in Liquid Neopentane

The positron lifetime technique has been used to study the positronium (Ps) yield as a function of temperature in liquid neopentane. Positron lifetime spectra have been recorded in the temperature range: 23-190°C; the critical temperature of neopentane is Tc = 160.3°C. The results display a clear correlation between the measured Ps yield and the excess electron mobility, i.e. the Ps yield increases (decreases) with increasing (decreasing) excess electron mobility. The ortho-Ps yield, I3, (3/4 of the total Ps yield) increases from 52% at 23°C to a maximum of 58.5% at 120°C. Above 120°C I3 decreases to a minimum of 57% at the critical temperature. Beyond the critical point, I3 increases with rising temperature. In the present paper we give a rather detailed discussion of the influence of the properties of secondary/excess electrons on the measured Ps yield. Furthermore, we give a short discussion of the state of Ps in liquid neopentane.