An investigation of energy dependence on saturation thickness for 59.54, 123, 279, 360, 511, 662, 1115 and 1250 keV gamma photons in carbon and aluminium

•Multiple scattered photons from carbon and aluminium were observed at 135°.•Eight different sources were used to find saturation thicknesses.•Energy of gamma photons was related to saturation thickness.•Results were verified by the Monte Carlo N-Particle simulation code.

Multiple back-scattering of gamma photons from carbon and aluminium targets is studied with a view to estimate energy dependence on saturation thickness. The number of multiple backscattered photons increases with an increase in the target thickness and then saturates at a particular thickness called the saturation thickness. An intense collimated beam, obtained from 241Am, 57Co, 203Hg, 133Ba, 22Na, 137Cs, 65Zn and 60Co sources, is allowed to impinge on rectangular carbon and aluminium targets of varying thickness. The scattered photons from the samples are detected by a 76 mm×76 mm NaI (Tl) scintillation detector placed at a backscattering angle of 135°. The measured saturation thickness for multiple backscattering of gamma photons is found to be increasing with an increase in the energy of incident gamma photons. The experimental results are compared with Monte Carlo calculations carried out using the Monte Carlo N-Particle (MCNP) simulation code.