Effect of finite size of ion chambers used for neutron dosimetry

The effective point of measurement for spherical ionisation chambers was determined free-in-air for d + T neutrons and inside a human water phantom for d + T neutrons and 60Co gamma rays. Measurements were performed with spherical chambers of different diameters and a disc-type chamber. For spherical ion chambers free-in-air, the geometrical centre of the chamber was found to be the point of measurement for sufficiently great distances from the neutron source. In-phantom for spherical ion chambers, displacement correction factors of 1 - (0.25 +/- 0.06) x 10(-2) r for d + T neutrons and of 1 - (0.37 +/- 0.04) x 10(-2) r for 60Co gamma rays (r being the cavity radius in mm) were observed. The differences in displacement for d + T neutrons and 60Co gamma rays can be attributed to differences in attenuation and scatter characteristics of these radiation qualities. The replacement of phantom material by the cavity of an ion chamber was simulated by introducing Styrofoam spheres of comparable dimensions into the phantom. The dose distributions measured over the cavities could explain the different displacement corrections for 60Co gamma rays and 15 MeV neutrons.