REPLACEMENT CORRECTIONS OF A FARMER-TYPE IONIZATION-CHAMBER FOR THE CALIBRATION OF CS-137 AND IR-192 SOURCES IN A SOLID PHANTOM

Calibrating Cs-137 and Ir-192 brachytherapy sources in a solid phantom has the advantage over calibration in air that the positioning of the sources is very accurate and straightforward. In order to determine t he air kerma rate at the point of measurement it is, however, necessar y to take the replacement of the phantom material by the ionization ch amber into account. The replacement correction factor p(r) of a Farmer -type ionization chamber has been determined for a few types of Cs-137 and Ir-192 sources at a source to chamber distance of 5 cm. For spher ical Cs-137 sources the replacement correction was determined by means of measurements with chambers with decreasing diameter and length. Ad ditional measurements were performed for some other source configurati ons in order to determine p(r) for Cs-137 micro-seed trains. For an Ir -192-HDR source p(r) was determined for a source chamber configuration equal to that for spherical Cs-137 sources by comparing measurements in-phantom with measurements free in-air. Finally, measurements were p erformed with source configurations that yielded p(r) values for Ir-19 2 Seed trains and Ir-192 wires. The resulting replacement correction f actors are all within +/-2% of unity. It can be concluded that, althou gh the dose nonuniformity over the chamber volume caused by each sourc e is rather substantial, the replacement correction that has to be app lied is rather small.