Z-dependence of spectral photon energy distributions of thick target bremsstrahlung

The Z-dependence of spectral photon energy distributions of external bremsstrahlung (EB) in thick targets of Al, Cu, Sn and Pb, produced by 35S and 89Sr, were studied. Theoretical EB photon distributions were obtained both from Elwert corrected Bethe-Heitler, and Tseng and Pratt theories. The experimental EB spectra were measured using a thick NaI(Tl) detector. It is found that the spectral shapes of EB spectra as a function of photon energy k displays a Zm dependence on the atomic number Z of the target material. The proportionality factor shows an exponentially decaying dependency on photon energy k that is inversely proportional to the end point energy of the beta particles coming from a particular beta emitter. The results agree well with theoretical calculations based on different models.