study of the effect of thickness scattered material on the backscattering photons in the energy spectrum of the NaI detector

Two radioactive sources [Co-60 and Cs-137] were used, also lead material with thickness [1-5 mm], was used to study the effective back scattered photons from shielding material. The ratio of scattering area to the total increase, until saturation thickness, and then these ratio will fix, while the ratio of photo peak to the total net photo peak to the total decreases. Until saturation thickness, and then these ratio will fix, for all distance used between radioactive sources and the NaI[T1] detector. But by increase the energy the saturation thickness increase