Thermal Spectrum Measurements in Slightly Enriched Uranium, Light-Water-Moderated Lattices, by the Lutetium Activation Method

Thermal spectrum measurements in 1% enriched uranium, 0.387-in. diam rod lattices, light water moderated, have been performed by the lutetium activation method in order to test the usefulness of this technique in not-well-thermalized lattices. Dysprosium-164 and ¹⁷⁵Lu were used as reference thermal-neutron detectors. The experimental results are presented as the average values of the normalized (lattice/Maxwellian) activation ratios of ¹⁷⁶Lu/Dy and ¹⁷⁶Lu/ ¹⁷⁵Lu with the pertinent cadmium-ratios, in the fuel and in the moderator over the H₂O/U range from 1:1 to 4:1. The agreement with the results of a calculation performed using the THERMOS code is satisfactory. It is concluded that the ¹⁷⁶Lu/Dy set appears to be adequate for the thermal spectrum analysis of light-water-moderated lattices, while the ¹⁷⁵Lu/Dy set can provide rather accurate integral data for the epithermal component of the neutron spectrum.