POSITRON-ANNIHILATION IN HELIUM, ARGON AND NITROGEN LIQUIDS UNDER AN ELECTRIC-FIELD

We have made new measurements of the lifetimes at 4.19 and 2.92 K in H eI, and at 2.14 and 1.67 K in HeII, without and with electric field. T he ortho-positronium lifetime is independent of field up to 20 kV/cm. At 4.19 K, there is no increase observed in the positronium fraction f rom zero field to 20 kV/cm, in strong disagreement with Manuzio and Ri zzuto. 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 increasing electr ic field to a constant value of about 2.14 ns above 5 kV/cm. The incre ase 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 on ly, at fields up to 20 kV/cm, above which we experienced electrical br eakdown. The ortho-positronium intensity is 7% at zero field and begin s to increase at 2.5 kV/cm, reaching 13% at 20 kV/cm at which field va lue it was still increasing. The orthopositronium lifetime displayed a n 8% increase over this field range. For liquid nitrogen we repeated s ome of our earlier measurements at 77.3 K and made some measurements a t 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 temperat ures,yielding a single drop-off rate of 0.21 cm/kV.