Precise measurements of W, the mean energy expended per ion pair formed in alkane vapors and in argon

Ionization currents were measured for the complete stopping of Ni-63 beta rays in nitrogen, in argon, in the normal alkanes through heptane and in the branched chain alkanes through hexane. Values of W, the mean energy expended per ion pair formed, were determined from the ratio of ion current in nitrogen to that in the sample, relative to the W for nitrogen, 34.6 eV [G. N. Whyte, Radiation Res. 18, 265 (1963)]. W(beta) values for alkanes were measured with a precision of ca. 0.1% and that of argon within 0.2%. No variation of W was experienced as pressures were increased by a factor of two, up to 2700 torr. There is also no temperature dependency in argon, methane, ethane and propane in the 25[degrees] to 200[degrees] C range. Energy balance considerations reveal that at least half of the energy absorbed in the complete stopping of electrons in vapors of normal alkanes C[lowered 1] to C[lowered 5] is expended in ionization, while about one third goes into excitation and the rest remains in subexcitation electrons. W(beta) correlates well with molecular structure in the normal and branched alkanes. Ionization potentials (I[lowered O]) as well as W's may be predicted empirically by extrapolating values from tables of W, I[lowered O], and W/I[lowered O].