GEANT4 simulation of the angular dependence of TLD-based monitor response

In this work, the response of thermoluminescent (TL) monitors to X-ray beams impinging on them at different angles was investigated and compared with results of simulations performed with the GEANT4 radiation transport toolkit. Each monitor used contains four TL detectors (TLD): two CaF2 pellets and two TLD-100 (one of each type within lead filter and the other without filter). Monitors were irradiated free-in-air with X-ray beams of the narrow and wide spectrum with effective energy of 61 and 130 keV and angles of incidence of 0°, 30°, 45°, and 60°. Curves of TL response relative to air kerma as a function of photon effective energy for each detector, with and without filter, are used to correct the energetic dependence of TL response. Such curves were also obtained from the data of radiation energy stored in the TLDs provided by the simulations. The attenuation increases with the increase of the incidence angle, since the thickness of lead filter traversed by the beam also enlarges. As the monitor calibration is usually performed with the beams impinging the monitor at 0°, changes in the attenuation become a source of error in the energy determination and consequently in the value of dose equivalent obtained with this monitor. The changes in attenuation observed in experiments were corroborated by the Monte Carlo simulations.