Examples are given which prove the ICARUS detector quality through relevant physics measurements. We study the μ decay energy spectrum from a sample of stopping μ events acquired during the test run of the ICARUS T600 detector. This detector allows the spatial reconstruction of the events with fine granularity, hence, the precise measurement of the range and dE/dx of the μ with high sampling rate. This information is used to compute the calibration factors needed for the full calorimetric reconstruction of the events. The Michel ρ parameter is then measured by comparison of the experimental and Monte Carlo simulated μ decay spectra, obtaining ρ=0.72±0.06(stat.)±0.08(syst.) . The energy resolution for electrons below ∼50 MeV is finally extracted from the simulated sample, obtaining (Eemeas−EeMC)/EeMC=11%/E[MeV]−−−−−−√⊕2%