Saturation Thickness Determination and Effective Atomic Number of Materials for 662 keV γ-Photons Using Multiple Compton Scattering Technique

Multiple Compton scattering of γ-photons is a powerful tool used for precisely evaluating the electronic momentum distribution (EMD) in Compton profile studies, X-ray spectroscopy, non-destructive testing and evaluation (NDTE), medical imaging, etc. The present work aims to study the behaviour of the intensity distribution of multiple scattered photons (MSP) originating from interactions of 662 keV γ-rays from elemental targets of carbon, aluminium, zinc, tin, and lead as well as binary alloys of brass, bronze, and soldering material, and further parameterize their Z-dependence. For this purpose, a well-collimated beam of γ-rays from a 137Cs radioactive source (strength 0.22 TBq) is allowed to impinge on targets (rectangular or cylindrical in shape) of different materials with varying thickness. The detection of scattered γ-photons is carried out with a properly shielded NaI(Tl) scintillation detector (having the diameter as well as thickness of 51 mm) positioned at 90° scattering angle relative to the primary incident γ-beam. It is discerned that the MSP initially gets enhanced linearly with target thickness and then gets saturated beyond a specific thickness. This specific thickness is known as saturation thickness and is a parameter for radiation shielding and determination of effective atomic number (Zeff) of mixtures. Using this method, the unknown thickness of a target can also be determined from the linear portion of the calibration line drawn between the intensity of the scattered photons and the target thickness. Furthermore, the effective atomic number (Zeff) of binary alloys is determined from the calibration curve of saturation thickness as a function of the Z-number of elemental targets. The experimental results agree well with Monte Carlo simulated data (1981). Two important parameters, multiple scattered fraction (MSF) and signal to noise ratio (SNR), are determined from the experimental data which are of great importance in Compton scatter imaging.