title: 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 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.
mesh_terms: D004601:Elementary Particles; D005720:Gamma Rays; D008961:Models, Structural; D011829:Radiation Dosage; D011874:Radiometry / Q000295:instrumentation*; D012542:Scattering, Radiation; D014965:X-Rays