Monte Carlo Study of a 137Cs calibration field of the China institute of atomic energy

The MCNP code was used to study the characteristics of gamma radiation field with collimated beam geometry. A close-to-reality simulation model of the facility was used for calculation air-kerma along the whole range of source-detector-distance (SDD) along the central beam and air-kerma off-axis beam profiles at two different source-detector-distance (SDD). The simulation results were tested by the measured results which were acquired in the Radiation Metrology Center of CIAE. Other characteristics such as the individual contributions of photons scattered in collimator, 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 purpose of future improvement of metrological parameters in CIAE. Finally, factors which influence the simulation results were investigated, including e.g., detector volume effects or source density effects.