Photon fluence perturbation correction factors for solid state detectors irradiated in kilovoltage photon beams

Dose perturbation correction factors, gamma(p), for LiF, CaF2 and Li2B4O7 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 gamma(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, (mu(en)/rho)med,det, 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, (kappa(en)/rho)med,det, which is evaluated using the actual photon fluence spectrum in the medium and detector for LiF and CaF2 rather than assuming they are identical. (kappa(en)/rho)med,det 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 (kappa(en)/rho)med,det and (mu(en)/rho)med,det. The value of gamma(p) calculated using (mu(en)/rho)med,det is up to 27% greater than unity for a cavity of CaF2 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 (kappa(en)/rho)med,det and (mu(en)/rho)med,det is small. For instance their difference for LiF is less than 2%. The average mass energy absorption coefficient ratio, (mu(en)(E)/rho)w,LiF, evaluated using the mean or representative energy, E, is up to 8% different from (mu(en)/rho)w,LiF. For calcium fluoride the difference between (mu(en)/rho)w,CaF2 and (mu(en)(E)/rho)w,CaF2 is up to 42% in the energy range studied.