Dosimetry commissioning for an industrial cobalt-60 gamma-radiation facility

In order to establish plant operational parameters of the industrial ⁶⁰Co irradiation facility, UTR GAMA-Pi, a commissioning dosimetry procedure was carried out at the National Laboratory for Engineering and Industrial Technology (LNETI), Lisbon, Portugal. Detailed dose mapping, with and without industrial product, was used to measure the distribution of absorbed dose in the radiation field inside the irradiation cell. “Dummy” and “dosimetry” boxes were prepared, each having 200 kg m⁻³ bulk density, using a combination of cardboard sheets and folded and crumpled newspapers. Four dosimeter systems were used for measurements: red and amber Perspex, radiochromic dye films, ethanol-monochlorobenzene solutions and potassium dichromate solutions. All systems were calibrated against the Fricke dosimeter solution. The dosimetry commissioning procedures allowed the establishment of the setting of conveyor dwell times to reach prescribed minimum doses, as well as the computation of a source utilization efficiency of approx. 19.5% and a dose uniformity ratio of approx. 1.25. Using these data, the capacity throughput of the irradiation plant was evaluated to be 4265 m³ yr⁻¹ or 853 tonnes yr⁻¹ for the ⁶⁰Co plaque source with an initial activity of 1.095 × 10¹⁶ Bq (295 kCi) (November 1988), at an operational working time of 7000 h yr⁻¹, and a specified product dwell time setting of 12.6 min, for a minimum absorbed dose of 25 kGy and a product bulk density of 200 kg m⁻³.