Hyperfme Interactions 89 (1994) 425-434 425 Positron annihilation in helium, argon and nitrogen liquids under an electric field I. Pepe, D.A.L. Paul 1, F. Gimeno-Nogues, J. Deutsch and R. Prieels UniversitkCatholiquede Louvain, Louvain-la-Neuve, Belgium We have made new measurements of the lifetimes at 4.19 and 2.92 K in HeI, and at 2.14 and 1.67 K in HelI, without and with electric field. The ortho-positronium lifetime is indepen- dent of field up to 20 kV/cm. At 4.19 K, there is no increase observed in the positronium frac- tion from zero field to 20 kV/em, in strong disagreement with Manuzio and Rizzuto. Some additional positronium is, however, formed above 20 kV/cm, with a corresponding decrease in lifetime of the free component. At 4.19 K the free positron mean life rises sharply with increas- ing electric field to a constant value of about 2.14 ns above 5 kV/cm. The increase in the free positron lifetime with field is less marked at 2.92 K. In liquid argon measurements were made at the normal boiling point only, at fields up to 20 kV/em, above which we experienced electri- cal breakdown. The ortlao-positronium intensity is 7% at zero field and be~n~ to increase at 2.5 kV/cm, reaching 13% at 20 kV/cm at which field value it was still increasing. The ortho- positronium lifetime displayed an 8% increase over this field range. For liquid nitrogen we repeated some of our earlier measurements at 77.3 K and made some measurements at 64.5 K so as to test models for positronium formation in the spur. A good fit to a simple exponential formula is obtained at both temperatures, yielding a single drop-offrate of 0.21 em/kV. 1. Experiments with liquid helium Annihilation lifetimes in liquid helium have been studied since the 1950s [1-9], and there is one report [10] on lifetime experiments with liquid helium subjected to an electric field. In liquid helium there is a triplet positronium component of almost 100 ns lifetime and roughly 15% intensity [8] and a component correspond- ing to free positron annihilations of 2 ns and about 80% intensity. The 2 ns compo- nent obscures the short-lived para-positronium component that is expected to have a lifetime of 0.125 ns and an intensity of 4%, or one third of the ortho compo- nent. The para-positronium component has, however, been identified in experi- ments on the angular correlation of the two-quantum annihilations (ACAR), in which a very narrow component is observed [11,12]. The narrow component arises from the low zero-point momentum of positronium trapped in a submicroscopic bubble in the liquid helium, of radius about 1.5 nm [13]. The bubble model also I On leave fr~ the Physics Department' University ~176176176 Canada" 9 J.C. Baltzer AG, Science Publishers