Energy Spectra of the Retrofugal Electron Flux from Massive Targets

Electrons from a linear accelerator have been directed in a collimated beam at normal incidence upon thick targets of C, Al, Cu, Ag, W, Pb, and U. Energy spectra of the retrofugal electrons have been obtained at 155° for seven distinct values of incident-electron energy between 1 and 10 MeV. For all targets, the measured spectra extend from 0.16 MeV to the incident energy $E_0$. Each spectrum profile has a broad but well-defined maximum at $E=E_w$ which progressively shifts toward higher energy as the incident-electron energy $E_0$ is increased but which eventually reaches an upper limit determined by the target atomic number $Z$. For $Z=6\mathrm{}$ the upper limit of $E_w$ is 0.55 MeV, while for $Z=92\mathrm{}$ it is 3.0 MeV. An elastically scattered component which is strongly dependent upon both $Z$ and $E_0$ also can be identified.