Liquid-filled ionization chamber temperature dependence

Temperature and pressure corrections of the read-out signal of ionization chambers have a crucial importance in order to perform high-precision absolute dose measurements. In the present work the temperature and pressure dependences of a sealed liquid isooctane filled ionization chamber (previously developed by the authors) for radiotherapy applications have been studied. We have analyzed the thermal response of the liquid ionization chamber in a similar to 20 degrees C interval around room temperature. The temperature dependence of the signal can be considered linear, with a slope that depends on the chamber collection electric field. For example, a relative signal slope of 0.27 x 10(-2) K-1 for an operation electric field of 1.67 x 106 V m(-1) has been measured in our detector. On the other hand, ambient pressure dependence has been found negligible, as expected for liquid-filled chambers. The thermal dependence of the liquid ionization chamber signal can be parametrized within the Onsager theory on initial recombination. Considering that changes with temperature of the detector response are due to variations in the free ion yield, a parametrization of this dependence has been obtained. There is a good agreement between the experimental data and the theoretical model from the Onsager framework. (c) 2006 Elsevier B.V. All rights reserved.