Electric-field dependence of positronium formation in liquids and polymers

Positron annihilation lifetime measurements have been performed to study the effect of an external electric field on the probability of positronium (Ps) formation in several liquids (isooctane, hexane, cyclohexane) and polymers [polystyrene, poly(4-methyl-1-pentene), low- and high-density polyethylene, polypropylene, polycarbonate]. The ortho-positronium intensity $I_3$ for all the samples except polar polycarbonate decreased with increasing external electric-field strength in the range of 0–60 kV/cm. Based on the Onsager equation, the average initial spatial separation between a positron and an electron in the terminal positron spur has been deduced to be 10–44 nm for the nonpolar polymers, 21–52 nm for the degassed liquids, and 19–37 nm for the air-saturated liquids, indicating that the average thermalization distance of the positron is larger than that of an excess electron. It is shown that the time required for Ps formation is on the order of 10 ps.