| dc.description |
Dose perturbation correction factors, (p ), for LiF, CaF<sub>2</sub> and Li<sub>2</sub> B<sub>4</sub> O<sub>7</sub> solid state detectors have been determined using the EGS4 Monte Carlo code. Each detector was simulated in the form of a disc of diameter 3.61 mm and thickness 1 mm irradiated in a clinical kilovoltage photon beam at a depth of 1 cm in a water phantom. The perturbation correction factor (p ) is defined as the deviation of the absorbed dose ratio from the average mass energy absorption coefficient ratio of water to the detector material, ( <sub>en</sub> / )<sub>med,det</sub> , which is evaluated assuming that the photon fluence spectrum in the medium and in the detector material are identical. We define another mass energy absorption coefficient ratio, ( <sub>en</sub> / )<sub>med,det</sub> , which is evaluated using the actual photon fluence spectrum in the medium and detector for LiF and CaF<sub>2</sub> rather than assuming they are identical. ( <sub>en</sub> / )<sub>med,det</sub> predicts the average absorbed dose ratio of the medium to the detector material within 0.3%. When the difference in atomic number between the cavity and the phantom material is large then their photon fluence spectra will differ substantially resulting in a difference between ( <sub>en</sub> / )<sub>med,det</sub> and ( <sub>en</sub> / )<sub>med,det</sub> . The value of (p ) calculated using ( <sub>en</sub> / )<sub>med,det</sub> is up to 27% greater than unity for a cavity of CaF<sub>2</sub> in 50 kV x-rays. When the atomic number of the medium and detector are similar, their photon fluence spectra are similar, and the difference between ( <sub>en</sub> / )<sub>med,det</sub> and ( <sub>en</sub> / )<sub>med,det</sub> is small. For instance their difference for LiF is less than 2%. The average mass energy absorption coefficient ratio, ( <sub>en</sub> ( )/ )<sub>w ,LiF</sub> , evaluated using the mean or representative energy, , is up to 8% different from ( <sub>en</sub> / )<sub>w ,LiF</sub> . For calcium fluoride the difference between ( <sub>en</sub> / )<sub>w ,CaF<sub>2</sub> </sub> and ( <sub>en</sub> ( )/ )<sub>w ,CaF<sub>2</sub> </sub> is up to 42% in the energy range studied. |
