Backscattering of Electrons from 3.2 to 14 MeV

Monoenergetic electrons of energies from 3.2 to 14 MeV provided by a linear accelerator have impinged normally on thick solid targets. Backscattered electrons have been detected by an ionization chamber, the multiplication factor of which was calibrated with a Faraday chamber as a function of average energy per backscattered electron. Angular distributions of backscattered electrons measured for a total of seven targets, effectively semi-infinite and ranging in atomic numbers from 4 to 92, show a trend similar to Dressel's result. However, the backscattering coefficients obtained are lower than his values and are consistent with those reported by other previous authors. Variations of angular distribution and backscattering coefficient with target thickness also have been investigated for Cu, Ag, and Au targets at an incident energy of 6.1 MeV. Some of the angular distributions observed were compared with results of a simple calculation proposed by the author, and an interpretation has been given that the relative contribution of sidescattering compared with that of diffusion increases with increasing energy in the region considered. Backscattering coefficients for the thinnest targets of lower atomic numbers level off toward a nonzero intercept similar to Cohen and Koral's lower-energy result. This tendency is considered to be caused by the contribution of energetic secondary electrons. The backscattering coefficient η(E₀, Z, ∞) of electrons incident on the semi-infinite target of atomic number Z with kinetic energy E₀ above 1 MeV is expressed by an empirical equation η(E₀, Z, ∞)=1.28exp[-11.9Z^-0.65(1+0.103Z0.37E₀0.65)] (E₀ in MeV), appreciable deviations from experimental data occurring only for Z≤6 and ZE₀2 MeV^-1.