Abstract
A prerequisite for the practical determination of organ doses in high-energy radiotherapy is the measurement of surface doses on the patient. For this task electret ionization chambers (EIC) are being developed. An EIC works like a conventional ionization chamber in the region of saturation, but it possesses an electret as collecting electrode. The principle of EIC is based on the charge decrease and the resulting electret voltage decrease by radiation-induced ionization of the chamber gas. The Response of the EIC has been adapted by varying the electret thickness and the chamber hight. The EIC shows a small dependence on photon energy, ray direction and dose rate. For charging the electrets a corona-charging method has been developed. The time necessary for dosemeter reading is very short, and it is possible to repeat the reading procedure, in contrast to thermoluminescent dosemeters. Dose measurements on different places of the patient at the same time are possible, because EICs need no external voltage supply during irradiation.
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