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 E₀. Each spectrum profile has a broad but well-defined maximum at E=Ew which progressively shifts toward higher energy as the incident-electron energy E₀ is increased but which eventually reaches an upper limit determined by the target atomic number Z. For $Z=6$ the upper limit of Ew is 0.55 MeV, while for $Z=92$ it is 3.0 MeV. An elastically scattered component which is strongly dependent upon both Z and E₀ also can be identified.