The increase in positronium formation due to a static electric field has been measured in various gases including He, Ne, A, ${\mathrm{H}}_2$, ${\mathrm{D}}_2$, ${\mathrm{N}}_2$, and several more complex molecular gases by a study of the energy spectrum of the annihilation $\gamma$ 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 ${\mathrm{H}}_2$, ${\mathrm{D}}_2$, and ${\mathrm{N}}_2$. No effect of the electric field was found in the polyatomic gases C${\mathrm{O}}_2$, C${\mathrm{H}}_4$, ${\mathrm{C}}_2$${\mathrm{H}}_6$, and C${\mathrm{Cl}}_2$${\mathrm{F}}_2$, but a small anomalous decrease in positronium formation was seen in S${\mathrm{F}}_6$. 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 $0.023\pi {a_0}^2$, $0.12\pi {a_0}^2$, and $1.2\pi {a_0}^2\pm 25\%$ 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 ${\mathrm{H}}_2$ and ${\mathrm{N}}_2$; the mode of decay of the remaining positrons is not understood.

• Received 26 January 1956

DOI:https://doi.org/10.1103/PhysRev.103.1258