A method for calibrating a first self-powered neutron detector with a second self-powered neutron detector for long term use of the first and second self-powered neutron detectors in a nuclear reactor core, the method comprising: - exposing at least one pair of self-powered neutron detectors to a neutron flux in a nuclear reactor, each of the self-powered neutron detectors in the pair producing a signal proportional to the same neutron flux; - the pair comprising a first self-powered neutron detector and a second self-powered neutron detector (15"); wherein - each self-powered neutron detector (15', 15") in the pair comprises an emitter and a collector; - the emitter of the first self-powered neutron detector in each pair comprising a first emitter material, and the emitter of the second self-powered neutron detector (15") in each pair comprising a second emitter material; wherein - the second emitter material has a neutron absorption cross-section that is greater than the neutron absorption cross-section of the first emitter material; - simultaneously measuring the response of each of the first and second self-powered neutron detectors (15', 15") to the same neutron flux in the nuclear reactor core; and - calibrating the signal produced by the first self-powered neutron detector in the pair in response to the neutron flux with the signal produced by the second self-powered neutron detector (15") in the pair in response to the neutron flux over a period of time sufficient to determine the thermal neutron sensitivity for the first emitter material for a given emitter depletion; characterised by: - monitoring the neutron flux within the nuclear reactor core with the second self-powered neutron detector (15") until the emitter material in the second self-powered neutron detector (15") is depleted, and then monitoring the neutron flux with the calibrated first self-powered neutron detector. - exposing at least one pair of self-powered neutron detectors to a neutron flux in a nuclear reactor, each of the self-powered neutron detectors in the pair producing a signal proportional to the same neutron flux; - the pair comprising a first self-powered neutron detector and a second self-powered neutron detector (15"); wherein - each self-powered neutron detector (15', 15") in the pair comprises an emitter and a collector; - the emitter of the first self-powered neutron detector in each pair comprising a first emitter material, and the emitter of the second self-powered neutron detector (15") in each pair comprising a second emitter material; wherein - the second emitter material has a neutron absorption cross-section that is greater than the neutron absorption cross-section of the first emitter material; - the pair comprising a first self-powered neutron detector and a second self-powered neutron detector (15"); wherein - each self-powered neutron detector (15', 15") in the pair comprises an emitter and a collector; - the emitter of the first self-powered neutron detector in each pair comprising a first emitter material, and the emitter of the second self-powered neutron detector (15") in each pair comprising a second emitter material; wherein - the second emitter material has a neutron absorption cross-section that is greater than the neutron absorption cross-section of the first emitter material; - simultaneously measuring the response of each of the first and second self-powered neutron detectors (15', 15") to the same neutron flux in the nuclear reactor core; and - calibrating the signal produced by the first self-powered neutron detector in the pair in response to the neutron flux with the signal produced by the second self-powered neutron detector (15") in the pair in response to the neutron flux over a period of time sufficient to determine the thermal neutron sensitivity for the first emitter material for a given emitter depletion; characterised by: - monitoring the neutron flux within the nuclear reactor core with the second self-powered neutron detector (15") until the emitter material in the second self-powered neutron detector (15") is depleted, and then monitoring the neutron flux with the calibrated first self-powered neutron detector. - monitoring the neutron flux within the nuclear reactor core with the second self-powered neutron detector (15") until the emitter material in the second self-powered neutron detector (15") is depleted, and then monitoring the neutron flux with the calibrated first self-powered neutron detector.