Effect of an Electric Field on Positronium Formation in Gases: Experimental

The increase in positronium formation due to a static electric field has been measured in various gases including He, Ne, A, H₂, D₂, N₂, and several more complex molecular gases by a study of the energy spectrum of the annihilation γ radiation. For the rare gases the increase has a sigmoid dependence on the ratio of electric field to pressure. The fraction of the positrons that form positronium increases by a maximum factor of 1.5, 1.4, and 2.1 for He, Ne, and A, respectively. A similar increase was observed for H₂, D₂, and N₂. No effect of the electric field was found in the polyatomic gases CO₂, CH₄, C₂{H}₆$, and C${Cl}₂F₂, but a small anomalous decrease in positronium formation was seen in SF₆. Some data are given using a microwave field at 2460 Mc/sec. The detailed theory of the increase given in the following paper provides a basis for obtaining from the observed data values of the elastic scattering cross section of positrons by the rare gas atoms, which are 0.023πa₀², 0.12πa₀², and 1.2πa₀²±25% for He, Ne, and A, respectively. Quantitative interpretation for the polyatomic gases is more difficult. At the high electric fields for which positronium formation has its maximum value, the fraction of positrons forming positronium is between 50% and 80% for the rare gases and for H₂ and N₂; the mode of decay of the remaining positrons is not understood.