Average energy expended per ion pair in liquid xenon

Measurements were made of the average energy $W$ per ion pair formed in liquid xenon by internal-conversion electrons from ${}_{}{}^{207}{}_{}{}^{}\mathrm{Bi}$. We observed voltage pulses resulting from electron collection either in liquid xenon or in a gaseous mixture of argon (95%) and methane (5%). The relative pulse heights for the two materials determine the ratio of the $W$ values. Using the known $W$ for the gaseous mixture, we obtained a liquid-xenon $W$ of 15.6 ± 0.3 eV. This value is considerably smaller than the gas-phase values, 21.5 or 21.9 eV. For interpretation, we adapted Platzman's energy-balance equation to liquids, assuming a conduction-band picture. Theoretical values thus calculated agree well with experiment.