A sensor is provided that includes a first optical body having at opposing, spaced sides an incident surface and a reflecting surface. The sensor includes an optical pathway for carrying incident light to the incident surface. The first optical body acts 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. 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. The sensor may measure pressure in combination with the measurement of temperature and neutron fluence, and a further sensor is provided for combined measurement of pressure and neutron fluence.