Characterizing a sample by material basis decomposition

A method is provided for characterizing a sample, by estimating a plurality of characteristic thicknesses, each being associated with a calibration material. The method includes acquiring an energy spectrum transmitted through the sample, located in an X and/or gamma spectral band; for each spectrum of a plurality of calibration spectra, calculating a likelihood from said calibration spectrum, and from the spectrum transmitted through the sample, each calibration spectrum corresponding to the energy spectrum transmitted through a stack of gauge blocks, each formed of a known thickness of a calibration material; and estimating the characteristic thicknesses associated with the sample according to the criterion of maximum likelihood.