Left: H₂O:CH₃COOCH₃ 100:13 ice-film experiment. Right, top panel: ice column density measurements from IR spectra. The black dashed lines represent the fits to the destruction cross section of water and CH₃COOCH₃. Middle panel: QMS-normalised masses we used to follow water, methanol, and m = 28 (CO, H₂CO, and CO₂) upon irradiation. m = 28 increases atthe beginning because it is mainly composed of fragments from the radiolytic products of the initial ice mixture. Lower panel: CH₃COOCH₃/H₂O abundance ratio deduced from the IR spectra of the ice film as a function of fluence (red dots). Comparison with the QMS-determined abundance ratio of the same desorbed molecules (dark blue line). The blue and cyan star indicates the gas ratio measurement with the QMS during the ice-film deposition.