Z dependance of bremsstrahlung of electrons between 2.5 and 4.0 MeV

Abstract

TheZ-dependance of the high frequency part of the bremsstrahlung spektrum in the forward direction is investigated with a 4.3 MeV electron linear accelerator for electron kinetic energies ranging from 2.5 to 4.0 MeV. We used thin beryllium, carbon, aluminium, titanium, copper, zirconium, silver and gold foils as targets. The relative photon yields are measured by counting the number of neutrons from the Be9 (γ, n) Be8 reaction, the neutrons being detected by a long counter. An adequate correction factor to the bremsstrahlung yield for multiple scattering of the electrons before radiating is presented and checked at 3.5 MeV with a number of Al foils ranging in thickness from 0.27×10−3 to 1.61×10−3 radiation lenghts. The results are compared with five different theoretical expressions for the bremsstrahlung cross section. The presence of electron-electron bremsstrahlung is very clear. For each used theoretical cross section the amount of photons due to this effect was determined supposing the theoretical nuclear cross section be exact for lowZ elements. It is found that theoretical yields obtained with the Elwert corrected Born approximation formulas agree with experiment for all used elements except for gold. The experimental results for gold as compared with the Born approximation formulas are in agreement with the curves given byKoch andMotz.