Bremsstrahlung Differential Cross-Section Measurements for 0.5- and 1.0-Mev Electrons

The bremsstrahlung cross section, differential in photon energy and angle, has been determined by measuring the x-rays emitted from thin targets of beryllium, aluminum, and gold, for incident electron energies of 0.5 and 1.0 Mev, with a 5-inch diameter, 4-inch long NaI(Tl) scintillation spectrometer at angles of 0, 10, 20, 30, 60, 90, and 120 degrees. The results show that in this energy region, the Bethe-Heitler-Sauter (Born approximation) theory underestimates the cross section. A comparison of the cross section for beryllium and aluminum gives evidence for electron-electron bremsstrahlung. The experimental values for the Heitler parameter $\frac{{\phi }_{\mathrm{rad}}}{\overline{\phi }}$ with a gold target are 10 at 1.0 Mev and 9.5 at 0.5 Mev. The low-$Z$ values for $\frac{{\phi }_{\mathrm{rad}}}{\overline{\phi }(1+\frac{1}{Z})}$ are 8.8 for beryllium and 8.4 for aluminum at 1.0 Mev and 6.8 for aluminum at 0.5 Mev. The accuracy of these values is estimated to be better than 20%. The corresponding Bethe-Heitler values for $\frac{{\phi }_{\mathrm{rad}}}{\overline{\phi }}$ are 6.5 at 1.0 Mev and 5.5 at 0.5 Mev.