Potential of radioluminescent silica-based optical fibers for 14 MeV neutron beam monitoring

The present paper proposes a solution for 14 MeV neutron beam monitoring employing Ce-doped optical fibers, based on the radiation-induced luminescence (RIL) phenomenon. Context is given on the possible use of these fiber-based monitors in fusion experiments, exploiting the advantages of optical fibers and of the proposed setup to overcome the limitations given by the harsh environments of fusion-related facilities. Here, the advantages of optical fibers (size, weight, electromagnetic immunity, among others) are complemented by the possibility of a very simple single-ended setup, which can be operated remotely, avoiding the exposure of the detectors to radiation. Two 2-cm long Ce-optical fibers were tested, one pristine and one that was X-ray pre-irradiated at 250 kGy(SiO₂). The investigated optical fibers have been characterized at the Frascati Neutron Generator (FNG) of ENEA (Italy) under 14 MeV neutrons and a flux varying between 4 × 10⁷ and 4.5 × 10⁸ n cm⁻² s⁻¹. Two runs have been performed, a first one to calibrate the RIL response to the neutron flux, via a linear fit between the facility monitors and the optical fiber response. A second run was then performed to validate the calculated sensitivity. The positive effect of pre-irradiation is demonstrated by a clear improvement in RIL response. The final results show a maximum deviation in the fluence measurements from the alpha counter of the facility of 6 % for the pristine fiber and 4 % for the pre-irradiated one, both values are lower than the 7 % deviation between the alpha counter and the fission chamber of the FNG facility.