Method for calibrating a spectrometry detector, the spectrometry detector being configured to: • detect interactions of an ionising radiation in a detection material during an acquisition duration; • acquire an amplitude spectrum ( S A) corresponding to a histogram of amplitudes of pulses detected during the interactions; the spectrometry detector operating according to an unstable mode during an instability time period following application of voltage to the detection material, then tending progressively towards a stable operating mode; the method including the following step: a) exposing the detector to a calibration radiation and acquisition of a calibration amplitude spectrum ( S A,calib), the detector being assumed to be operating according to the unstable mode, the calibration spectrum extending over a plurality of channels, one amplifier value corresponding to each channel; the method being characterized in that it furthermore comprises the following steps: b) taking into account a reference spectrum ( S A,ref, S E,ref) representing a spectrum measured by the detector when it is exposed to the calibration radiation, the detector being assumed to be operating according to the stable mode; c) determining a calibration function ( f), intended to be applied to the value of the channels of the calibration spectrum ( S A,calib), on the basis of a comparison between: - the reference spectrum ( S A,ref, S E,ref) taken into account during step b); - readjusted calibration spectrum ( S A,calib, S E,calib), obtained by applying the calibration function ( f) to the value of the channels of the calibration amplitude spectrum, optionally weighted by a strictly positive weighting term ( c); the calibration function minimising a deviation, or a mean square deviation, between the reference spectrum and the readjusted calibration spectrum. • detect interactions of an ionising radiation in a detection material during an acquisition duration; • acquire an amplitude spectrum ( S A) corresponding to a histogram of amplitudes of pulses detected during the interactions; a) exposing the detector to a calibration radiation and acquisition of a calibration amplitude spectrum ( S A,calib), the detector being assumed to be operating according to the unstable mode, the calibration spectrum extending over a plurality of channels, one amplifier value corresponding to each channel; b) taking into account a reference spectrum ( S A,ref, S E,ref) representing a spectrum measured by the detector when it is exposed to the calibration radiation, the detector being assumed to be operating according to the stable mode; c) determining a calibration function ( f), intended to be applied to the value of the channels of the calibration spectrum ( S A,calib), on the basis of a comparison between: - the reference spectrum ( S A,ref, S E,ref) taken into account during step b); - readjusted calibration spectrum ( S A,calib, S E,calib), obtained by applying the calibration function ( f) to the value of the channels of the calibration amplitude spectrum, optionally weighted by a strictly positive weighting term ( c); the calibration function minimising a deviation, or a mean square deviation, between the reference spectrum and the readjusted calibration spectrum. - the reference spectrum ( S A,ref, S E,ref) taken into account during step b); - readjusted calibration spectrum ( S A,calib, S E,calib), obtained by applying the calibration function ( f) to the value of the channels of the calibration amplitude spectrum, optionally weighted by a strictly positive weighting term ( c);