Effective point of measurement of cylindrical ionization chambers for heavy charged particles

Cylindrical ionization chambers are used for the determination of absorbed dose in beams of heavy charred particles, where the effective point of meas urement, P-eff (the point in depth to which the measured dose refers), is a priori not known. A measurement of P-eff for a Farmer-type chamber in a ca rbon ion beam is presented. It is based on a comparison of relative depth d ose curves measured with a cylindrical chamber and a plane-parallel Markus chamber. Both measurements were compared against another high-precision rel ative depth dose measurement using large-area plane-parallel chambers. For P-eff, a value of 72 +/- 7% of the inner radius of the chamber is obtained. The relative depth dose curve for the cylindrical chamber is calculated us ing an averaging of the depth dose values over the curved inner surface of the active volume while taking account of the different depths of points on the inner surface. Within the measurement uncertainty of 0.2 mm the measur ements agree well with the calculated Bragg curve for the Farmer chamber. T he result for P-eff is in correspondence with the value suggested in a new code of practice by the IAEA for protons and ions, and somewhat less than t hat suggested by Palmans for protons. The measurements show that cylindrica l chambers are in general well suited for depth dose measurements in fields of heavy charged particles if the correct P-eff is used.