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 HeI, and at 2.14 and 1.67 K in HeII, without and with electric field. The 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 from zero field to 20 kV/cm, 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 increasing 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/cm, above which we experienced electrical breakdown. The ortho-positronium intensity is 7% at zero field and begins 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-off rate of 0.21 cm/kV.On leave from the Physics Department, University of Toronto, Canada.