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 $r(\theta ,E_0,Z)$ 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 $E_0$ of the incident electrons was varied from 0.5 to 10 MeV and the dependence of $r$ upon $\theta$ explored for angles between 90° and 180° with respect to the forward beam axis. For targets having $Z\ge 29$ the angular distributions are independent of $E_0$ and are described by ${cos}^{1.35}\theta$; but for targets with $Z\le 13$, $r(\theta ,E_0,Z)$ depends upon all three parameters. Typical values obtained for the retrofugal flux coefficient range from $r({180}^{\circ },10\mathrm{} \mathrm{MeV}, 4)=1.2$ milliunits/steradian for Be to $r({180}^{\circ },0.5\mathrm{} \mathrm{MeV}, 92)=274$ for U. These generally are higher than those reported previously.