Upper panel: optical depth spectrum evolution upon 46 MeV Ni swift ion irradiation at 13.6 K of a thick water ice film crystallised at 150 K. The crystalline ice spectrum and the spectrum at the highest fluence are labelled; the intermediate spectra correspond to the fluence evolution given in the lower panel. Lower panel: integrated optical depth I_OD over the (ν₁ =) 3000 to (ν₂ =) 3250 cm^-1 water-ice stretching-mode profile interval, where the amorphous-to-crystalline contrast is higher, as a function of fluence (filled circles). A model fit (from Eq. (5)) is overplotted (full lines). The fit obtained without any water ice sputtering contribution is also shown (dashed lines) for comparison.