Retrofugal Electron Flux from Massive Targets Irradiated with a Monoenergetic Primary Beam

Nearly monoenergetic electrons in a collimated beam have been directed at normal incidence onto a variety of thick targets ranging in atomic number from Be to U. The ratio r(θ,E₀,Z) of the number of retrofugal (i.e., energetic secondary plus backscattered primary) electrons per steradian to the number of incident primaries has been measured for a total of 11 targets. The mean energy E₀ of the incident electrons was varied from 0.5 to 10 MeV and the dependence of r upon θ explored for angles between 90^∘{} and 180^∘{} with respect to the forward beam axis. For targets having Z≥29 the angular distributions are independent of E₀ and are described by cos1.35θ; but for targets with Z≤13, r(θ,E₀,Z) depends upon all three parameters. Typical values obtained for the retrofugal flux coefficient range from r(180^∘,10 MeV, 4)=1.2 milliunits/steradian for Be to r(180^∘,0.5 MeV, 92)=274 for U. These generally are higher than those reported previously.