Determining charge collection efficiency in parallel-plate liquid ionization chambers.

An accurate computation of the collection efficiency due to recombination has a capital importance in order to perform high-precision dose measurements with ionization chambers. The two-voltage method, developed to compute charge collection efficiency in gas chambers, cannot be directly applied to liquid-filled ionization chambers (LICs) due to the ionized charge strong dependence on the collection electric field, which is caused by initial recombination. It is shown that in order to apply the two-voltage method to parallel-plate LICs it is necessary to introduce explicitly the slope of the ionization charge yield with respect to the polarization voltage applied in the chamber. A three-voltage method is also proposed to avoid the introduction of the slope parameter. That method allows decoupling initial and general recombination. It leads to a nonlinear system of equations which can be solved using numerical methods.