Study of the gamma radiation fields of irradiators in a calibration laboratory using Monte Carlo codes

The Laborato<acute accent>rio de Calibra & ccedil;ao de Dos & iacute;metros (LCD), a dosimeter calibration laboratory of the Centro de Desenvolvimento da Tecnologia Nuclear (CDTN), acquired a new gamma irradiator, the Hopewell Designs model G10, with a source with a nominal activity of 1.85 TBq. To employ the new irradiator in the LCD, it is necessary to characterize its gamma radiation field. This work aims to study the gamma radiation field around the new LCD/CDTN irradiator, using Monte Carlo computational codes MCNP and PHITS. The laboratory and the new (Hopewell G10) and old (STS OB85) irradiators were modeled. The air kerma and ambient dose equivalent rates were determined for various irradiation scenarios, from a simple free point-like source in air, to a more complex model, considering both irradiators in the irradiation rooms of the laboratory. Radiation levels in areas adjacent to those rooms were also assessed, for radioprotection purposes. Air kerma rate, Gamma Ka delta , and ambient dose equivalent rate, Gamma H*(10) delta , constants were also calculated and compared with values available in the literature. Experimental measurements were made with calibrated ionization chambers to validate the results obtained through the simulations. The less complex simulations were useful to validate the methodology and to assess the influence of scattered radiation. Comparisons were made between the results obtained using the PHITS and MCNP codes. Simulation results obtained in various points inside the irradiation rooms agreed well with the experimental measurements (within 1.84 % when using the MCNP code). Simulations carried out in areas adjacent to the irradiation rooms presented higher discrepancies compared to measured values. In those cases, variance reduction techniques must be used to improve the agreement with the experimental measurements. However, the simulated models can be used in dosimetric studies performed inside the irradiation rooms.