The effective atomic number (Z_eff) of composite materials has been determined by measuring the backscattered gamma photons at 180^o. A 661.6 keV gamma photons from ¹³⁷Cs radioactive source are allowed to scatter at an angle of 180^o from the sample. The backscattered photons at 180^o from the sample are detected with 2'' × 2'' NaI(Tl) scintillation gamma ray spectrometer coupled to 16K 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 AutoZeff code.

复合材料的有效原子序数（Z _eff）已通过测量180 ^o处的反向散射伽马光子来确定。允许来自¹³⁷ Cs放射源的661.6 keV伽马光子以180 ^o的角度与样品散射。用耦合到16K多通道分析仪（MCA）的2''×2''NaI（Tl）闪烁伽马射线光谱仪检测样品在180 ^o处的反向散射光子。实验上观察到，反向散射光子的强度最初随目标厚度的增加而增加，然后在一定厚度以上达到饱和。通过知道元素目标的已知原子序数的饱和厚度值，Z_EFF复合材料已被确定。将实验测得的Z _eff值与通过直接法，幂律法和Aut​​oZeff代码预测的理论值进行了比较。