Shielding design for high-intensity Co-60 and Ir-192 gamma sources used in industrial radiography based on PHITS Monte Carlo simulations

Protection in industrial radiography facilities has been a worldwide concern for decades and the most appropriate protection for the public and operators in small-scale facilities is the engineering protection including shielding first, time and distance. Monte Carlo methods were used for thickness determination in shielding design for gamma imaging using gamma sources in fixed facilities. Computation was done to develop the concrete design that can slow down the effective dose rate of the gamma rays from 100 Ci (or less) of Co-60 and 50 Ci (or less) of Ir-192 less than the value of 2.5 μSv/h (the limit for Public area) as required by the ALARA principle and regulation of different countries. The minimum concrete wall thickness necessary to achieve the ALARA principle in the previous conditions was found to be 120 cm for Co-60 source and 70 cm for the Ir-192 source. From the optimized design using a single corner maze, it was found that the central source position gives rise to high exposure and should not be used as a source position. Only the left and right positions of the source are preferred depending on the facility’s dimensions. For Co-60 with a maze corridor less than 7 m, the right position of the source is the most appropriate while the left position is preferred for larger dimensions. For Ir-192, the right position if preferred if the corridor length is less than 5 m and the left otherwise. Double-corner maze design was found to be the most appropriate shielding design for gamma radiation with the left and right position of the sources the best for both sources used. The double-corner maze required only 4 m maze length to achieve 2.5 µSv/h or less ALARA principle while single corner design requires 12 m or 14 m for left and right positions, respectively.