The W values for photons and electrons in mixtures of argon and alkanes.

The mean energy required to form an ion pair for 5.89-keV 55Fe and 1.49-keV Al photons in mixtures of argon-methane and argon-butane gas was measured using the proportional-counter technique. The measured W values in the mixtures are lower than what is expected for regular mixtures, i.e., mixtures without an appreciable intermolecular energy transfer between the mixture components. The additional ionization yield through the nonmetastable Penning processes reaches a maximum abruptly at very low partial pressure of polyatomic admixtures (alkanes), decreasing steadily with higher admixture partial pressure.