Investigations of multiple scattering of 320 keV γ rays: A new technique for assigning effective atomic number to composite material

The multiple scattering of 320 keV γ photons from targets of different atomic numbers and of various thicknesses is studied here experimentally. We observe that with increasing the target thickness, the number of multiply scattered photons increases, and saturates for a particular thickness of the target, known as the saturation depth (thickness). The present measurements show the effect of the atomic number of the target on the saturation depth of 320 keV incident gamma photons multiply scattered from targets of various thicknesses. The scattered photons are detected by a properly shielded NaI(Tl) detector placed at 90° to the incident beam direction whose detector response unfolding, converting the observed pulse-height distribution to a photon energy spectrum, is obtained with the help of an inverse response matrix technique. It is observed that the saturation depth decreases with increasing atomic number of the target. The best-fit curve of saturation thickness as a function of the atomic number of the target provides a new technique to measure the 'effective atomic number' of composite materials. Monte Carlo calculations also support the present experimental results. Copyright © 2009 The Royal Swedish Academy of Sciences.