The water-to-air stopping power ratio is a quantity used in the measurement of absolute dose to water with ionization chambers. In previous works, this ratio has been calculated using experimental data, tables from the literature and different Monte Carlo codes. In the currently available dosimetry protocols, this quantity is known for carbon ions with an accuracy of 2%. Using the possibility of the Monte Carlo code FLUKA to track secondary electrons, the effect of delta ray production on the stopping power ratio calculation is studied. The results of this study, along with the latest experimental data available, allow us to propose a new expression for the water-to-air stopping power ratio of carbon ions which reduces the uncertainty to below 0.8 %, which results in a reduction of the overall uncertainty in the ionization chamber measurements of absorbed dose to water.

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