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Its uncertainty alone produces millimetre-scale uncertainty in where a proton beam stops in water.","posed_since":"","precise":"Determine the mean excitation energy $I$ of liquid water, which enters the Bethe stopping-power formula, to an uncertainty below 1 eV; the ICRU 90 recommendation is $I = 78 \\pm 2\\,\\mathrm{eV}$, and published experimental and dielectric-response values spread over several eV around it. An answer is a value with uncertainty consistent between direct range or stopping-power measurements and optical-data calculations.","problem_ref":null,"references":"","settled_by":"Precision range measurements of monoenergetic protons and ions in liquid water at 1 to 250 MeV combined with an evaluation of the optical oscillator strength of liquid water.","status_note":"","title":"Mean excitation energy of liquid 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