Determination of effective atomic number of composite materials using backscattered gamma photons – A novel method

Highlights: • Authors measured the backscattered photons to determine Z_eff of composite samples. • Backscattered photons generated by 661.6 keV photons from weak Cs-137 gamma source. • Ratio of thickness to intensity of backscattered saturated peak used to infer Z_eff. • Present experimental technique is used to determine Z_eff ≤ 30 of composite sample. • Method can be used to determine the Z_eff of biological samples using weak source. The effective atomic number (Z_eff) of composite materials has been determined by measuring the backscattered gamma photons at 180°. A 661.6 keV gamma photons from ¹³⁷Cs radioactive source are allowed to scatter at an angle of 180° from the sample. The backscattered photons at 180° from the sample are detected with "2 × 2″ NaI(Tl) scintillation gamma ray spectrometer coupled to 16 K Multi Channel Analyzer (MCA). It is observed experimentally that the intensity of backscattered photons increases initially with increasing the thickness of the target and then it becomes saturated above a certain thickness. By knowing the saturated thickness values for known atomic number of the elemental targets, the Z_eff of composite materials has been determined. The experimentally measured Z_eff values have been compared with theoretical values predicted by direct method, power law method and AutoZ_eff code.