Angular dependence of the wall correction factor for air kerma measurements using cylindrical cavity chambers.

The dependence of the results of air kerma measurements on the orientation of the cylindrical cavity chamber used for beam standardizations is reexamined. The wall attenuation correction factors for the (60)Co and (136)Cs air kerma measurements were obtained for various irradiation angles by extrapolation of the dependences of experimental data and by Monte Carlo simulations. Applying the correction based on extrapolation does not provide an angle-invariant air kerma rate, as desired. In contrast, Monte Carlo simulations, which take into account various affecting phenomena, produce a wall correction factor that makes the corrected air kerma independent of the incidence angle.