Precise Measurements of W, the Mean Energy Required for Ion-Pair Formation in Gases I. Alkanes

Ionization currents were measured for the complete stopping of 63Ni beta rays in N2 and in alkane isomers at temperatures 25–200°C and pressures between 400 and 2700 torr. The mean energy expended per ion pair formed, W (beta), was determined from the ratio of ion current in nitrogen to that in the sample at equal electron densities. No dependency of W upon temperature or pressure was observed within the experimental precision of ± 0.1%. Using W (N2) = 34.6 eV/ion pair for reference, relative values of W in electron volts/ion pair were found to be 26.94 for methane, 24.18 for ethane, 23.69 for propane, 23.19 for n-butane, 22.82 for n-pentane, 22.62 for n-hexane, 22.47 for n-heptane, 23.52 for 2-methylpropane, 23.07 for 2-methylbutane, 23.42 for 2,2-dimethylpropane, 22.80 for 2-methylpentane, 22.81 for 3-methylpentane, 22.81 for 2,3-dimethylbutane, and 22.88 for 2,2-dimethylbutane.