The Primary Ionization of High Energy Electrons in Nitrogen and Neon

Abstract

A large expansion chamber in a magnetic field of about 400 gauss is used to study the tracks of high energy betarays produced by bombarding a lead screen inside the chamber by gamma-radiation from radon. Two photographs at right angles are taken, and the tracks examined by projection on to a screen in their original configuration. Measurements are made of the number of primary ions per cm of path produced in nitrogen and neon. The results in nitrogen can be expressed approximately by the relation: $I=19\mathrm{}{\beta }^{-1.15\mathrm{}\pm 0.15},$ and in neon by the relation: $I=12.6\mathrm{}{\beta }^{-1.35\mathrm{}\pm 0.15},$ where $I$ is the primary ionization in ions per cm at normal temperature and pressure, and $\beta$ is the velocity of the betaray in units of the velocity of light. Measurements on a few tracks in oxygen agree within experimental error with the results of Williams and Terroux. The primary ionization of the positive electrons observed is indistinguishable from the negative electrons. Comparisons are made with the theoretical calculations of Bethe, Moller, and Williams, and the agreement is satisfactory over the range of velocities used. The magnitude obtained for the primary ionization in nitrogen corresponds, on the basis of the formula obtained by hydrogen like wave functions, to an average ionization potential of 16.6 volts for the five outer electrons in the nitrogen atom, which is very near its minimum ionization potential. Theoretical calculations are made for neon, using ionization potentials for each electron calculated from critical absorption wave-lengths. The experimental results are somewhat greater than those calculated from the theory, but are as close as the somewhat arbitrary assumptions involved would allow one to expect. The variation of primary ionization with velocity checks well for values of less than 0.97, but the predicted increase for very high energies is not observed.