The Energy Losses of Fast Electrons

Abstract

The energy losses experienced by electrons of energies from 1.5 to 4.5 Mev in traversing thin laminae of carbon and of lead were investigated with a hydrogen-filled cloud chamber. The carbon absorber had almost the same calculated stopping power as the lead and was used to provide a check on the method in regard to such factors as straggling, and scattering of the electrons by the gas of the chamber. As a source of electrons activated chlorine was used, the electrons being admitted through a thin mica window mounted on the periphery of the chamber. A comparison was made between the average energy loss per g/${\mathrm{cm}}^2$ of absorber found as the result of the measurements and the values for this quantity predicted on the basis of current theory. As the experimental rate of loss in lead was markedly greater than that theoretically expected, the attempt was made to account for this difference on the basis of the obliquity of the electron paths in the absorber; this attempt, however, was unsuccessful, so that the results indicate a higher rate of absorption in lead of electrons with the energies studied than is predicted by theory. A further comparison was made with theory in regard to the absolute probability of large energy losses—losses of approximately half the initial energy. Here a very large discrepancy was found, which indicates that the difference found in the computation of the average rate of energy loss is due to the probability of large (radiative?) losses being considerably greater in lead than is expected from theory.