A New Nondestructive Technique Employing Multiple Scattering of Photons for Measurement of Effective Atomic Numbers of Composite Materials

The objective of this work is to present a method/technique for the determination of the effective atomic number (Z_eff) of composite materials [mixed materials of many atomic numbers (Z's)]. In the present measurements, an intense beam of gamma-ray photons irradiates targets of different elements and composite materials and of varying thicknesses. The scattered radiations are detected by a properly shielded NaI(Tl) scintillation detector whose response unfolding, converting the observed pulse-height distribution to a true photon spectrum, is obtained with the help of an inverse matrix approach. This also results in the extraction of the numbers of multiple-scattered events from the thick targets. We observe that the numbers of multiple-scattered events, having the same energy as in single-scattered distribution, increase with an increase in target thickness and then saturate for a particular target thickness known as saturation thickness (depth). The saturation thickness is found to decrease when the Z of pure elements increases. A calibration curve (saturation depth versus Z of pure elements) and the measured saturation thickness values for composite materials are used to assign the respective Z_eff values of these composite materials. Monte Carlo calculations also support the present experimental results.