1. An identification method of an isotope present in a medium to be characterized by a measurement system for measuring energy-resolved ionising radiations, the identification method comprising: a) measuring at least one reference spectrum for at least two reference isotopes; b) normalizing the reference spectra; c) delimiting for each reference isotope measurement windows in the reference spectrum of the isotope, width of the measurement windows being dependent on the reference spectrum of the reference isotope; d) measuring a measured spectrum on the medium to be characterized; e) normalizing the measured spectrum; f) for each reference isotope, delimiting in the measured spectrum measurement windows identical to those delimited for the reference isotope and calculating for each of the measurement windows a deviation value which is representative of the deviation between the measured spectrum and that of the reference isotope in the measurement window; g) for each reference isotope, determining from the deviation values calculated for the reference isotope an identity coefficient; h) identifying the isotope from the determined identity coefficients.