Charge Distribution Produced by 4- to 24-MeV Electrons in Elemental Materials

Charge-deposition distributions of monoenergetic electrons normally incident on thick absorbers of Be, Al, Cu, Ag, and Au have been measured with a thin collector moved through the absorber thickness. The measurements have been made at incident energies of 4.09, 7.79, 11.5, and 14.9 MeV, and also at 23.5 MeV for absorbers other than Be. Values of the most probable charge-deposition depth $x_m$ and the charge-deposition straggling $w$ have been determined from the distributions observed. The ratio of $x_m$ to the theoretical mean range $L$ of the incident electrons decreases with decreasing incident energy $E_0$ and with increasing atomic number $Z$ of the absorber, reflecting the effect of multiple-scattering detours of the primary electrons. The ratio $\frac{w}{L}$ shows an increase with increasing $E_0$ in the region of $E_0\gtrsim 11$ MeV for Al and in the entire energy region of the present experiment for Cu, Ag, and Au; this trend is considered mainly due to bremsstrahlung energy-loss straggling. The contribution to the measured distributions of electrons mediated by bremsstrahlung has been found to be less than about 1 × ${10}^{-3\mathrm{}}$ ${\mathrm{cm}}^2$/g absorbed electron. Some of the distributions are compared with the Monte Carlo results of Berger and Seltzer. Agreement between the experimental and calculated results is rather good except in the case of the Be absorber.