Reactor Flux Measurements Using Thermoluminescent Dosimetry

Thermoluminescent dosimetry (TLD) was successfully evaluated as an in-core thermal-neu-tron-flux determinant. The LiF crystals enriched with either ⁶Li or ⁷Li provided two effective neu-tron-gamma discrimination techniques. The first method used both types of crystals. The ⁶LiF dosimeters, which have large thermal-neutron cross sections, detected both neutrons and gamma radiation, while the ⁷LiF dosimeters, possessing negligible thermal-neutron attenuation characteristics, monitored the gamma component only. The dosimeters were inserted into a reactor for a known time interval and read on a commercially available detection system, and the difference in dosimeter exposure yielded a direct measure of neutron flux. The second technique used bare and cadmium-covered ⁷LiF dosimeters. The bare crystals detected reactor gammas, while those encapsulated in cadmium measured reactor gammas plus capture gammas from the Cd(n, γ ) reaction. The difference in exposures provided the capture-gamma contribution, which was proportional to reactor flux. Experiments using a subcritical and a TRIGA reactor revealed exposure rate to neutron flux sensitivities of 1.4 × 10⁻⁷ R/sec per ϕ and 2.6 × 10⁻⁸ R/sec per ϕ for the respective techniques. Accurate flux measurements were obtained over a range spanning 10² to 10¹² n/(cm² sec). At higher fluxes, the dosimeters experienced radiation damage and readings became unreliable. The TLD results were compared against BF₃ detection, foil activation, and fission chambers to derive an empirical exposure rate to the flux conversion factor.