Left panel: CO column density evolution at 9K as a function of at 33 MeV ⁵⁸ Ni ⁹⁺ ion fluence for CO ice films deposited with different initial thickness (different colours). The radiolytically produced CO₂ is shown. Middle panel: experimentally measured differential evolution −dN∕dF as a function of column density. The fit of Eq. (1) to the data for the thickest film is shown (long black dashed line), as well as a fit not taking into account the finite depth of sputtering (straight dashed line). Right panel: sputtering yield evolution and fitted contribution as a function of column density.