An apparatus comprising a calibration source and a gamma-ray spectrometer arranged to receive gamma-rays from the calibration source, the gamma-ray spectrometer comprising: a scintillator material optically coupled to two or more photomultipliers, the two or more photomultipliers being arranged to detect photons generated in the scintillator material associated with gamma-ray interactions between the scintillator material and gamma-rays received from the calibration source, wherein the two or more photomultipliers are operable to output respective detection signals associated with the gamma-ray interactions; the calibration source comprising: a radioactive material having a decay transition associated with emission of a radiation particle and a gamma-ray, a detector arranged to receive radiation particles emitted from the radioactive material, and a gating circuit coupled to the detector and arranged to output a gating signal in response to a detection of a radiation particle by the detector: the apparatus being characterized by further comprising: a switch coupled to receive the respective detection signals from the two or more photomultipliers and operable to select detection signals from one of the two or more photomultipliers; and a stabilization circuit (48, 50) coupled to the switch and the gating circuit, wherein the stabilization circuit is operable to receive the selected detection signal of the respective photomultiplier and to stabilize the gain of the photomultiplier that output the selected detection signals based on the detection signals measured in association with gating signals from the gating circuit, wherein the stabilization circuit is operable to generate a gain-change signal for the photomultiplier associated with the selected detection signals based on a comparison of a median channel of an observed energy spectrum calculated from the selected detection signals and a median channel of a reference energy spectrum of the calibration source. a scintillator material optically coupled to two or more photomultipliers, the two or more photomultipliers being arranged to detect photons generated in the scintillator material associated with gamma-ray interactions between the scintillator material and gamma-rays received from the calibration source, wherein the two or more photomultipliers are operable to output respective detection signals associated with the gamma-ray interactions; the calibration source comprising: a radioactive material having a decay transition associated with emission of a radiation particle and a gamma-ray, a detector arranged to receive radiation particles emitted from the radioactive material, and a gating circuit coupled to the detector and arranged to output a gating signal in response to a detection of a radiation particle by the detector: the apparatus being characterized by further comprising: a switch coupled to receive the respective detection signals from the two or more photomultipliers and operable to select detection signals from one of the two or more photomultipliers; and a stabilization circuit (48, 50) coupled to the switch and the gating circuit, wherein the stabilization circuit is operable to receive the selected detection signal of the respective photomultiplier and to stabilize the gain of the photomultiplier that output the selected detection signals based on the detection signals measured in association with gating signals from the gating circuit, wherein the stabilization circuit is operable to generate a gain-change signal for the photomultiplier associated with the selected detection signals based on a comparison of a median channel of an observed energy spectrum calculated from the selected detection signals and a median channel of a reference energy spectrum of the calibration source. a switch coupled to receive the respective detection signals from the two or more photomultipliers and operable to select detection signals from one of the two or more photomultipliers; and a stabilization circuit (48, 50) coupled to the switch and the gating circuit, wherein the stabilization circuit is operable to receive the selected detection signal of the respective photomultiplier and to stabilize the gain of the photomultiplier that output the selected detection signals based on the detection signals measured in association with gating signals from the gating circuit, wherein the stabilization circuit is operable to generate a gain-change signal for the photomultiplier associated with the selected detection signals based on a comparison of a median channel of an observed energy spectrum calculated from the selected detection signals and a median channel of a reference energy spectrum of the calibration source.