This study was conducted to find the best combined ratio of galena, hematite and limonite in concrete shielding structure against neutron and gamma rays emitted from 18 MeV Linac source in radiation therapy centers. The MCNPX code was used to calculate the linear attenuation coefficient of different concretes for the 18 MeV Linac source. The concrete samples were then produced in a laboratory, and their compressive strength was measured. Then, by comparing the quality factor diagrams that simultaneously express the power of neutron and gamma attenuation and compressive strength, the optimal point for the percentage of different minerals in concrete was identified. The results showed that the used minerals have a high gamma absorption capacity due to their high atomic number and density. It also showed that the neutron attenuation power increases with the increase in the concentration of galena, limonite, hematite in concrete and reaches to its maximum value. And if the concentration of minerals rises above a certain value, the neutron attenuation power of concrete decreases, due to the decrease in the concentration of lighter elements such as hydrogen in the concrete structure, as well as the increase in the probability of secondary neutrons production. The highest value of QF was obtained for samples containing 40% galena, 50% hematite and 40% limonite, and the highest value of QF/ρ was obtained for samples G30, G40, H50, L30 and L40. This type of concretes can be used to create an effective concrete shield for radiotherapy centers.
