Specific Ionization by High-Energy Electrons

The average specific ionization produced by electrons of energies ranging from 1 to 34 Mev was measured in ${\mathrm{H}}_2$, He, and ${\mathrm{N}}_2$ at atmospheric pressure. The measurements were made by sending a collimated beam of electrons through an ionization chamber into a Faraday cup. At minimum ionization the number of ion pairs per cm (probable specific ionization) was (7.56±0.09), (6.15±0.08), and (53.2±0.7) at N.T.P. in the three gases, respectively. If these results are compared with the theory of energy loss, the values of $W$ (the average number of electron volts to produce an ion pair) are calculated to be (37.8±0.7), (44.5±0.9), and (${34.8}_{-0.7\mathrm{}}^{+0.9\mathrm{}}$) for the respective gases. The corresponding minimum values of the total specific ionization are calculated to be (9.19±0.18), (7.55±0.16), and (${61.6}_{-1.5\mathrm{}}^{+1.2\mathrm{}}$) ion pairs per cm. The relativistic increase in ionization from minimum to 34 Mev agrees with the calculated increase in energy loss to within 1 percent for ${\mathrm{N}}_2$, but in He and ${\mathrm{H}}_2$ the increase in ionization is less than the predicted increase in energy loss. The effect is more certain in ${\mathrm{H}}_2$ where an increase of about 3.3±0.7 percent in $W$ is observed. Calculations indicate that this effect may be due to the production of Čerenkov radiation which fails to produce ions in the gas.