Evaluating the shielding properties of selected building materials used in Kampala, Uganda for gamma radiation

Abstract

Humans are continuously exposed to ionizing radiation. For high ionizing radiation levels, shielding is needed to mitigate unwanted exposure. In this study, Shielding properties of selected local building materials used in Kampala, Uganda have been evaluated for gamma radiation using Caesium-137 source of characteristic energy 662 KeV and activity of 370 kBq as at 30th October 2019. Properties such as Mass Attenuation Coefficient, Half Value Layer and Kinetic Energy Release per unit Mass were investigated from which the most effective local building material for shielding ionizing radiation amongst the studied local building materials was identified. The materials that were studied such as hardwood, local granite rocks and bricks were shaped into thin square sheets of average thickness 2 mm using an axle blade for black granite rock, a rigid metal frame for bricks samples and an electric saw for hardwood samples. Their densities were calculated and compared with the standard known density value of each sample. A collimated mono energetic gamma ray beam was made to emerge through a narrow slit of approximately 2 mm. Before samples were exposed to this beam, the background radiation was taken to eliminate its contribution to the transmitted beam. The incident and transmitted radiation beam intensities before and after the samples were exposed to the collimated mono energetic gamma ray beam were measured and recorded. Each individual test was run for 60 seconds for which the procedures were repeated three times to obtain consistent results and then the average was calculated. The background radiation was subtracted from the average value to get the radiation beam intensity (I) that was transmitted through the sample. Using Beer Lambert equation, the mass attenuation coefficients were obtained from gradients of the graphs that were plotted of Ln (I) against mass thickness. Mass attenuation coefficient values of the samples were as follows; 0.663 ± 0.0005 cm2/g for lead, 0.097 ± 0.0005 cm2/g for local granite rock, 0.145 ± 0.0005 cm2/g for Mahogany, 0.102 ± 0.0005 cm2/g for Muvule, 0.130 ± 0.0005 cm2/g for Elgon teak, 0.074 ± 0.0005 cm2/g for clay soil and 0.050 ± 0.0005 cm2/g for Red soil. The half value layers were obtained from the attenuation curves that were from plots of I(avg) against sample thickness x(eq). Samples had half value layers that were as follows; 1.05 ± 0.002 mm for lead, 2.71 ± 0.001 mm for local granite rock, 6.59 ± 0.001 mm for Mahogany, 8.91 ± 0.001 mm for Muvule, 9.93 ± 0.001 mm for Elgon Teak, 6.88 ± 0.001 mm for Clay soil and 9.36 ± 0.001 mm for Red soil. The KERMA was obtained as the product of mass attenuation coefficient and characteristic energy of the source. The Kinetic energy released per unit mass (KERMA) values of the samples were as follows; 0.437 (MeV) for lead, 0.064 (MeV) for local granite rock, 0.096 (MeV) for Mahogany, 0.068 (MeV) for Muvule, 0.086 (MeV) for Elgon teak, 0.049 (MeV) for Clay soil and 0.029 (MeV) for Red soil. Samples that displayed favorable radiation shielding properties to gamma rays were those with high atomic number and high density. This made them to have high mass attenuation coefficients, low half value layers and high KERMA values. Of the local building materials that were investigated, Mahogany had the highest mass attenuation coefficient of (0.145 ± 0.0001 cm2/g) after lead, half value layer before lead (6.59 ± 0.001 mm) and highest KERMA value after lead 0.096 (MeV). This made Mahogany an outstanding local building material for shielding gamma rays.