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The annihilation of positrons in methane gas at room temperature has been measured at pressures below 2 atm. The annihilation rate for the free positron component is proportional to the gas density and corresponds to an effective number of electrons per molecule taking part in annihilation, Zeff = 139.6 ± 1.0 (standard error). The quenching of orthopositronium corresponds to 1Zeff = 0.57 ± 0.07. Resonance annihilation is discussed in a general way.

The time taken for positrons from a radioactive source to reach a velocity distribution in the near-thermal range has been measured for mixtures of nitrogen gas with argon, and of methane with argon. An analysis of the shoulder breadths for the free positron components leads to slowing down times of 16 ns for nitrogen and 0.25 ns for methane at densities of 1 amagat. The long slowing time for nitrogen must occur during the last eV or so of slowing and the dominant mode of energy loss is attributable to rotational excitation of the molecules. For methane the slowing to the threshold for vibrational excitation must be extremely rapid, but there is no clearly established mechanism which could account for the remaining energy loss taking place so rapidly.

The total cross sections for positrons on neon and argon atoms have been measured in the energy ranges 15 eV to 272.5 eV and 25 eV to 300 eV respectively. The cross sections indicate clearly that Born values will not be reached until at least 3 KeV. Interpolating between the measured and the valid Born regions has allowed an application of the sum rule which connects scattering length. Born forward scattering amplitude, and the momentum-integral over the total cross section. This procedure gives scattering lengths as = −0.53 ± 0.15 Bohr radii for neon and as = −2.8 ± 0.7 Bohr radii for argon; the errors include maximum credible uncertainties in the interpolations.

We have made new measurements of the lifetimes at 4.29 K and 2.92 K in He I, and at 2.14 K and 1.67 K in He II, without and with an electric field. At 4.19 K we have a new value of the lifetime, 2.01+or-0.002 ns, leading to a Zeff of 3.53 at zero field. In contrast to earlier reports, Zeff decreases very slightly as the temperature is lowered and the density increases. The orthopositronium lifetime at 4.19 K is 98.1+or-0.2 ns, at zero field, independent of field up to 18 kV cm-1. At 4.19 K the free positron decay rate falls exponentially with increasing field to a value 5% below its zero-field value, the drop-off rate being 0.36+or-0.04 cm amagat V-1. From 28 through 50 V cm-1 amagat-1 the decay rate of the free component rises slowly as expected because of the increasing positronium fraction over the same range of electric field. In liquid argon there is positronium enhancement beginning at or below 3 V cm-1 amagat-1, which contrasts with 160 V cm-1 amagat-1 for argon gas. At least two lifetimes make up the long triplet component, suggesting different degrees of thermalization, or different states of positronium within the liquid. The effect of the electric field is to enhance the longer-lived, or more thermal component, in strong contrast with the findings of Charlton et al. (1992) for the gas. The argon results suggest positron behaviour similar to that of electrons in the liquid phase, namely, such particles move in a conduction band subject to small effective scattering and diffusion cross sections. An analytic model in which the scattering length is an adjustable parameter yields the onset of positronium enhancement just below 3 V cm-1 amagat-1 when as=-0.043 a0, corresponding to an elastic scattering and diffusion cross section of 0.034 pi a02. Consequences for positron thermalization are discussed. A model of positronium losing energy slowly in the liquid without bubble formation is suggested.

The total cross sections for elastic scattering of positrons in the energy range from 4 to 19 eV have been measured by the method of transmission. By varying a magnetic field applied along the axis of the scattering chamber the transmitted fraction of the beam is altered, from which individual phase shifts can be extracted. s-, p-, and d-wave phase shifts are given over the entire energy range. The s-wave phase shifts are in agreement with values published by Drachman in 1968, while the p- and d-wave phase shifts are intermediate between values calculated by the same author in 1966 and 1971. The experimental results agree with those of Costello et al., and marginally with our own 1972 results, but are significantly different from those of Canter et al. We compute that the Ramsauer minimum in the diffusion cross section must be 0.04πa02 at 1.6 eV while the minimum in the total cross section is 0.11πa02 at 2.1 eV. The shoulder breadth observed in annihilation experiments is in nice agreement with what one would predict from our phase shifts.

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