Alpha-Particle Ionization in Polyatomic Gases and the Energy Dependence ofW

An extended series of measurements by three different methods has been carried out to determine the variation of $W$, the average energy to make an ion pair in the gases ${\mathrm{N}}_2$ and ${\mathrm{C}}_2$ ${\mathrm{H}}_4$ as a function of the energy of the ionizing alpha particle. In one method, the ionization ratios were determined in the two gases for single alpha particles from two collimated polonium sources, the particles from one source being reduced in energy by passage through a succession of interchangable mica windows. The corresponding energy ratios were determined by auxiliary measurements in pure argon, the $W$ values in argon being assumed constant. $W$ values for alphas of initial energy up to 9 Mev were also obtained by a comparison in ${\mathrm{C}}_2$ ${\mathrm{H}}_4$ and ${\mathrm{N}}_2$ of the relative ionization from Po and ThC′ single alpha particles. Data from all these experiments indicate a continuous decrease in $W$ values in ${\mathrm{C}}_2$ ${\mathrm{H}}_4$ and ${\mathrm{N}}_2$ with increasing alpha energy over a range from 1-9 Mev. The differential $w$ (defined as the ratio of energy increment $\Delta E$ to ionization increment $\Delta I$ at any point on the alpha path) also decreases similarly and seems to approach but never quite reach, within the limits of alpha energy so far investigated, the corresponding $W$ value for beta particles. No explanation of this continuous decrease with energy of the $W$ values can at present be advanced.