A sensor for combined measurement of temperature, pressure and neutron fluence, the sensor comprising: a first optical body having at opposing, spaced sides an incident surface and a reflecting surface, and an optical pathway for carrying incident light to the incident surface, the first optical body acting as a Fabry-Pérot etalon, such that a portion of the incident light passes through the incident surface, reflects from the reflecting surface, and then returns along the optical pathway, wherein the first optical body is formed of a material whose optical absorbance increases depending on the neutron fluence experienced by the material, such that a corresponding characteristic attenuation is produced in the reflected light returned along the optical pathway depending on the neutron fluence experienced by the first optical body; a Fabry-Pérot interferometer having a cavity bounded on opposing sides by the reflected surface of the first optical body and a higher finesse mirrored surface of a second optical body, wherein a portion of the incident light passes through the first optical body, enters the cavity, reflects from the higher finesse mirrored surface, and then returns along the optical pathway; wherein varying the temperature of the first optical body leads to changes in the spacing between the incident and reflecting surfaces due to thermal expansion or contraction of the first optical body, such that corresponding temperature-sensitive characteristic changes are produced in the reflected light returned along the optical pathway; and, wherein at least one of the optical bodies is arranged such that it deforms under the influence of external pressure to vary the distance between the mirrored surfaces on the opposing sides of the cavity, leading to corresponding pressure-sensitive characteristic changes in the reflected light returned along the optical pathway, the sensor further comprising a detector arranged to detect the reflected light returned along the optical pathway, and characterised by: a processor unit operatively connected to the detector and configured to determine the neutron fluence experienced by the interferometer from the characteristic attenuation. a first optical body having at opposing, spaced sides an incident surface and a reflecting surface, and an optical pathway for carrying incident light to the incident surface, the first optical body acting as a Fabry-Pérot etalon, such that a portion of the incident light passes through the incident surface, reflects from the reflecting surface, and then returns along the optical pathway, wherein the first optical body is formed of a material whose optical absorbance increases depending on the neutron fluence experienced by the material, such that a corresponding characteristic attenuation is produced in the reflected light returned along the optical pathway depending on the neutron fluence experienced by the first optical body; a Fabry-Pérot interferometer having a cavity bounded on opposing sides by the reflected surface of the first optical body and a higher finesse mirrored surface of a second optical body, wherein a portion of the incident light passes through the first optical body, enters the cavity, reflects from the higher finesse mirrored surface, and then returns along the optical pathway; wherein varying the temperature of the first optical body leads to changes in the spacing between the incident and reflecting surfaces due to thermal expansion or contraction of the first optical body, such that corresponding temperature-sensitive characteristic changes are produced in the reflected light returned along the optical pathway; and, wherein at least one of the optical bodies is arranged such that it deforms under the influence of external pressure to vary the distance between the mirrored surfaces on the opposing sides of the cavity, leading to corresponding pressure-sensitive characteristic changes in the reflected light returned along the optical pathway, the sensor further comprising a detector arranged to detect the reflected light returned along the optical pathway, and characterised by: a processor unit operatively connected to the detector and configured to determine the neutron fluence experienced by the interferometer from the characteristic attenuation.