Experimental evaluation of effective atomic number of composite materials using back-scattering of gamma photons

A method has been presented for calculation of effective atomic number (Z_eff) of composite materials, by using back-scattering of 662?keV gamma photons obtained from a ¹³⁷Cs mono-energetic radioactive source. The present technique is a non-destructive approach, and is employed to evaluate Z_eff of different composite materials, by interacting gamma photons with semi-infinite material in a back-scattering geometry, using a 3^″?×?3^″ NaI(Tl) scintillation detector. The present work is undertaken to study the effect of target thickness on intensity distribution of gamma photons which are multiply back-scattered from targets (pure elements) and composites (mixtures of different elements). The intensity of multiply back-scattered events increases with increasing target thickness and finally saturates. The saturation thickness for multiply back-scattered events is used to assign a number (Z_eff) for multi-element materials. Response function of the 3^″?×?3^″ NaI(Tl) scintillation detector is applied on observed pulse-height distribution to include the contribution of partially absorbed photons. The reduced value of signal-to-noise ratio interprets the increase in multiply back-scattered data of a response corrected spectrum. Data obtained from Monte Carlo simulations and literature also support the present experimental results.