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The temperature and density dependence of positron annihilation in CO2 and SF6

Positron lifetime experiments have been performed on CO2 and SF6 gases at temperatures in the range 297–400 K and at densities up to 10 amagat. The ensemble-averaged ⟨Zeff⟩ parameter has been extracted at each temperature from the observed density dependence of the annihilation rates. The latter was found to be consistent with annihilation via positron interactions both with single molecules and with pairs. The three-body (positron-plus-two molecules) annihilation coefficient, ⟨b⟩, has also been obtained at each temperature. Both ⟨Zeff⟩ and ⟨b⟩ are found to be approximately independent of temperature in the range investigated. A simple three-body collision model for ⟨b⟩ is developed and discussed.