The Total Ionization Produced in Air by Electrons of Various Energies

Range in air and total ionization by electrons of energy up to 2225 volts.—Electrons from a tungsten filament were accelerated to a cylindrical anode with a small axial hole through which a fraction passed into a hemispherical ionization chamber maintained at a definite pressure by means of an adjustable leak, while the filament chamber was connected to a diffusion pump. The number of electrons entering the ionization chamber and the number of positive ions formed by them was measured with an electrometer. From ionization-pressure curves the range of electrons of various energies was determined, and was found to obey the fourth-power velocity law $R\propto v^4$, or $\sqrt{R}=\frac{V}{15900}$, where $V$ is the energy in equivalent volts and $R$ is in cm at atmospheric pressure. The total ionization curve begins at 17 volts, and rises with minor peaks at 127, 250, 375 and 494 volts of 1.83, 2.70, 4.10 and 5.05 ions per electron, to 43.2 ions per electron at 1000 volts; then the increase is approximately linear to 55 ions per electron at 2225 volts, and the values of others for high speed electrons indicate that this increase continues at about the same rate. These results tend to confirm Bohr's theory of ionization as corrected by Fowler. The voltages at which the total ionization shows peaks are interpreted as the ionization potentials of the L-electrons of argon (250) and the K-electrons of nitrogen (375) and of oxygen (494). The ionization per cm of path at 1 mm pressure, obtained by di viding the total ionization by the range, reaches a maximum of 112 ions/cm for 40 volts and a second maximum of 13.9 ions/cm for 987 volts. The first maximum agrees with that predicted from the ionization potentials of the molecule and atom of nitrogen, and the second corresponds to twice the ionization potential for the K-electrons of oxygen and gives a value of 22.8 volts energy per ion pair produced, in agreement with theory. For electrons with 40 volts energy, Bohr's theory gives the maximum effective radius of air molecules during collision as approximately 5×${10}^{-8\mathrm{}}$ cm. The 987 volt electrons are probably those which give the "sphere" tracks in C. T. R. Wilson's photographs. Their range is.04 mm at atmospheric pressure.