ENERGY RESPONSE FACTOR OF BEO DOSEMETER CHIPS: A MONTE CARLO SIMULATION AND GENERAL CAVITY THEORY STUDY

摘要

The objective of this study is to determine the energy response factors for BeO optically simulated dosemeter (OSLD) using general cavity theory and Monte Carlo (MC) simulations. A virtual phantom is constructed in EGSnrc MC program and energy response of BeO OSLDs were simulated at 5 cm depth for x-ray beams ranging from 1.25 to 25 MV and at 2 cm for beams with <250 kV including ISO 4037 narrow beam energies in a virtual water phantom. The energy response factor for a given radiation quality relative to ~(60)Co was determined for BeO and compared to the Al_2O_3:C and LiF:Mg,Ti dosemeters. Burlin cavity theory calculations were done using mean photon energy (MPE) of the beam spectra, while EGSnrc software package was used to carry out MC simulation of full spectra. The cavity theory and MC methods agreed well within the 0.7%. Energy response of x-ray beams at MV range showed a maximum of 1.5% under-response. At energies higher than 150 kV (105 keV MPE) showed no significant difference while a significant under-response were observed at 100 kV (53 keV MPE) and 50 kV (29keV MPE), ～8 and -12%, respectively. BeO, Al_2O_3:C and LiF:Mg,Ti dosemeters exhibited very similar energy response at higher energies mainly in the MeV range. At 50 kV (29keV MPE), however, BeO dosemeter under responded by a factor of 0.878, while Al_2O_3:C and LiF:Mg,Ti dosemeters over responded by a factor of 3.2 and 1.44, respectively. Furthermore, at low energies, BeO energy response showed dependence on photon spectra. For instance, at 100 kV, the difference was ～8, ～6 and 2% for 53, 60 and 83 keV MPE (ISO 4037N-100), respectively. Furthermore, calibration with ~(137)Cs instead of ~(60)Co resulted up to 1.8% differences in energy response. Both energy spectrum and calibration methods make considerable differences in energy response of OSLDs. This study concludes that BeO chips are nearly energy independent at energies higher than 100 keV MPE, while Al_2O_3:C dosemeters show an extremely enhanced energy-response ranging between 1.44 and 3.2 at energies between 170 and 29 keV MPE mainly due to dominance of photoelectric effect.