521 | Page INFLUENCE OF SAMPLE’S THICKNESS ON Γ-RAY SHIELDING PARAMETERS Kulwinder Singh Mann 1,2 , Manmohan Singh Heer 3 , Asha Rani 4 1 Department of Applied Sciences, I. K. G. Punjab Technical University, Kapurthala, (India) 2 Department of Physics, D.A.V. College, Bathinda-151001, Punjab, (India) 3 Department of Physics, Kanya Maha Vidyalaya, Jalandhar-144001, (India) 4 Department of Physics, Dev Samaj College for Women, Ferozpur-, Punjab, (India) ABSTRACT Optimum-thickness (OT) value for low-Z materials has been estimated in measurement of total mass attenuation coefficient (μ m , cm 2 g -1 ) for γ-rays using narrow-beam transmission geometry at three energies viz. 661.66, 1173.24, 1332.50 keV. OT is the maximum thickness (expressed in mean free path, mfp) of a sample used in μ m measurements with good accuracy. The main objective of this study is to provide the missing information in the literature regarding OT value for low-Z materials. Six samples of commonly used low-Z building materials have been investigated. Monoenergetic γ-rays have been obtained from two standard radioactive sources (Cs 137 and Co 60 ). The μ m values have been measured using narrow-beam transmission geometry for particular sample at 13 different thickness values, starting from 2cm up to 26cm thus OT remains between 0.2-3.5mfp. A self designed and validated user friendly computer program, GRIC2-toolkit has been used for required theoretical computations. It has been concluded that for energy-range 661.66-1332.50 keV, the μ m measurements of low-Z materials with γ-ray transmission-geometry, OT of 0.5mfp is considered as optimum-thickness value. Keywords: Gamma-Ray Shielding Behaviour, Γ-Ray Shielding Parameters, Optimum-Thickness. I INTRODUCTION The experimental measurements of γ-ray shielding parameters (GSP) with good accuracy require perfect narrow-beam transmission-geometry. The total mass attenuation coefficient (μ m , cm 2 g -1 ) is the most important GSP, because many other GSP can be derived from it. μ m is useful in applications such as non-destructive analysis (NDA) of materials and CT-scan [1]. Thus, high accuracy of μ m measurement is essential. However, practical γ-ray transmission-geometry used for μ m measurements usually deviates from the perfect-narrowness. This deviation may be due to intermixing of scattered photons while traversing the γ-ray beam through a material, causing error in measured values of μ m . Variations of the μ m values with absorber (sample) thickness have been pointed in the literature [2-5]. In such variations, Varier et al., have reported the opposite trend than others. But, they have not explained the cause of this type of trend. Other researchers have concluded that for accurate measurements of μ m , the sample’s thickness may be up to 1mfp [2-4]. However, these results are