A method for characterizing a sample, by estimating a plurality of characteristic thicknesses, each being associated with a calibration material, including acquiring an energy spectrum (Sech) transmitted through this sample, located in an X and/or gamma spectral band; for each spectrum of a plurality of calibration spectra (sbase(Lk; Lt)) calculating a likelihood from said calibration spectrum (Sbase(Lk; Lt)), and from the spectrum transmitted through the sample (Sech), each calibration spectrum (Sbase(Lk; Lt)) corresponding to the energy spectrum transmitted through a stack of gauge blocks, each formed of a known thickness of a calibration material; estimating the characteristic thicknesses (L1, L2) associated with the sample according to the criterion of maximum likelihood.