Monte Carlo study of reference photon fields in the irradiation halls of the Czech Metrology Institute

The MCNPX code was used to obtain characteristics of collimated photon beams from a 60Co source and a radiotherapy 60Co irradiator “Chisobalt 2 B75” was exploited for calibration and verification of dosimeters in the Czech Metrology Institute, Inspectorate for Ionizing Radiation (CMI-IIR; Czech metrology body). First, for each source, a longitudinal profile of air kerma rate on the beam axis and a transversal profile of air kerma rate at a distance of 1 m from the front of the source's collimator were simulated, and the results were compared with the experimental data obtained with ionization chambers. Measured and calculated data were in good agreement. Second, the number of scattered photons in the fluence spectra along and across the beam axes, and their contribution to air kerma were specified. Third, the individual contributions of photons scattered in collimator, air, floor, walls, mobile platform and other parts of the irradiation halls to the total air kerma rate on the beam axis were calculated for the purposes of future improvement of metrological parameters of photon beams in CMI-IIR.