Initial recombination of ions in ionization chambers filled with hydrocarbon gases

Ion recombination in ionization chambers is usually considered as a side effect, which influences the accuracy of the determination of the absorbed dose. This is generally true for volume recombination, but not always for initial recombination, i.e. the recombination which occurs between the ions created in the same track of a single ionizing particle or resulting from a single ionization event. Contrary to volume recombination, the initial one does not depend on dose rate but it depends on ion density within the tracks. Therefore, the measurable parameters associated with initial recombination carry information about the local density of ions created in the tracks of the ionizing particles in small volumes of the traversed medium (with a mass of the order of one femtogram) and can be correlated with the linear energy transfer (LET) or with other parameters describing radiation quality. The phenomenon of initial recombination and its relationship with radiation quality has been used in the so-called recombination chambers, where the conditions for the initial recombination are created purposely. The chambers are mostly tissue equivalent, filled with hydrocarbon gases at elevated pressure, in order to keep the gas density in the chamber volume above approximately 2 kg/m. The chamber volume, the polarizing voltage and the distance between electrodes in the chamber are optimized in such a way that the ions are Initial recombination of ions in ionization chambers filled with hydrocarbon gases Michał A. Gryziński, Natalia Golnik, Mieczysław Zielczyński