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Diamond based thermal neutron flux monitors have been fabricated using single crystal diamond films, grown by chemical vapour deposition. A 3 μm thick 6LiF layer was thermally evaporated on the detector surface as a converting material for thermal neutron monitoring via the 6Li(n, α) T nuclear reaction. The detectors were tested in a fission nuclear reactor. One of them was positioned 80 cm above the core mid-plane, where the neutron flux is 2.2 × 109 neutrons/cm2 s at 1 MW resulting in a device count rate of about 150000 cps. Good stability and reproducibility of the device output were proved over the whole reactor power range (up to 1 MW). During the irradiation, several pulse height spectra were recorded, in which both products of the 6Li(n, α) T reaction, e.g. 2.73 MeV tritium and the 2.06 MeV α, were clearly identified, thus excluding a degradation of the detector response. A comparison with a reference fission chamber monitor pointed out a limitation of the adopted readout electronics at high count rates, due to multiple pile-up processes. However, once this effect is properly accounted for, a good linearity of the diamond flux monitor response is observed as a function of the fission chamber one, as well as an excellent agreement between the temporal behaviour of the two detector response. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)