Evaluation of the biological shields of the secondary standard dosimetry laboratory of Ghana using MCNP5

Abstract

The primary objective with radiation sources and facilities is the protection of both radiation workers and the general public. The biological shields of the Secondary Standard Dosimetry Laboratory of the Radiation Protection Institute (RPI) Ghana had been evaluated for a collimated isotropic cesium-137 source for calibration purpose using MCNP5 code. The dose rate at supervised areas ranged from 0.57 to 8.35 μSv/h and 0.26 to 10.22 μSv/h at control areas when the source was panoramic. When the source was collimated, the dose rate ranged from 0.05 to 0.30 μSv/h at supervised areas and 0.23 to 8.88 μSv/h at control areas for 22.2 GBq of the cesium-137 source. The scatter contribution from the surfaces of the walls and roofs were also accounted for. The scatter radiation in the room decreased to 400 μSv/h when the source was first collimated and to 3.5 μSv/h when the source was further collimated. These results agreed quite well with experimental measurement. To effectively protect the staff, a narrow beam of 1.2 cm diameter which was defined at 1.0 m by the total surface of the ISO slab phantom was recommended to reduce the dose rate to less than 1.5 μSv/h outside the calibration bunker even when the current activity is doubled. It was concluded that the 4.7 cm diameter of the existing narrow beam should be decreased to 1.2 cm by further collimation of the beam. © Maxwell Scientific Organization, 2012.