Experimental and simulated study of detector collimation for a portable 3″ × 3″ NaI(Tl) detector system for in-situ measurements

The effect of detector collimator and scatterer thickness on multiple Compton backscattered gamma photons is studied. Gamma photons from a 137Cs source of 5.8 mCi, is allowed to fall on Carbon, Aluminium, Iron and Copper targets and the scattered photons are detected by a properly shielded 76 mm × 76 mm NaI(Tl) scintillation detector located at 90° to the incident beam. To extract the contribution of multiple scattered photons from the measured spectra, single scattered distribution is remodelled analytically. The thickness at which the multiple scattered photons saturate is determined for different detector collimator apertures and scatterer thicknesses. The variation of saturation thickness, Signal-to-Noise (S/N) ratio, Multiple Scattering Fraction (MSF) for different materials and collimator sizes are studied and compared with the available literature. Monte Carlo simulated calculations using MCNP code supports the present experimental work.