The invention relates to a dosimeter of the optical fibre calorimeter type employed to measure the integrally absorbed dose in irradiation with accelerated particle beams. According to the invention, the dosimeter is based on the use of an intrinsic sensor consisting of an optical fiber (3) wherein a Bragg diffraction network (4) is mounted to measure the temperature variation occurred as a consequence of the energy transfer from an accelerated particle beam (11) to a graphite calorimetric body (15) inside which there is mounted the portion of the optical fibre (3) wherein the Bragg diffraction network (4) is mounted, the body (15) being insulated in relation with the environment by a thermally insulating enclosure (14) of polystyrene foam, where the temperature variation is highlighted as a change of the nominal wavelength of the Bragg diffraction network (4), which is determined by means of an optical spectroscopy equipment (6) to which the optical fibre (3) is coupled.