Effective measuring point of ion chambers for photon dosimetry in phantoms

The effective point of measurement for spherical ionization chambers has been determined for 60Co and 137Cs gamma 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 gamma rays and 1-(0.22 +/- 0.05) r. 10(-2) for 137 Cs gamma 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.