Effective measuring point of ion chambers for photon dosimetry in phantoms

Abstract The effective point of measurement for spherical ionization chambers has been determined for 60Co and 137Cs γ rays and for 300 kV X rays inside a water phantom. For reasons of comparison, measurements were also performed with a thimble-type Baldwin-Farmer ion chamber. For spherical ion chambers the displacement correction factors were 1 – (0.37±0.04) r.10−2 for 60Co γ rays and 1 – (0.22±0.05) r.10−2 for 137Cs γ rays, where r is expressed in mm, whereas no displacement was observed for X rays. The differences in displacement obtained for the various types of radiation can be attributed to differences in attenuation and scatter characteristics of the various radiation qualities.