Evaluation of Neutron Spectral Effects in Systems with Thermal Resonance Absorbers by Applying a Correlation Method to Measured Spectral Indexes

Spectral indexes have been measured in homogeneous systems (solutions of europium and boron). The measured values have been compared with the results of spectra calculations and, independently, have been utilized for a correlation method devised to deduce the reaction rates which are undetectable directly. The reliability of the THERMOS code for spectra calculations in such systems has been shown. A satisfactory test of the correlation method has been performed. The adequacy of the cross-section sets available in the literature has been shown for the detectors ¹⁹⁷Au, ⁶³Cu, ⁵⁵Mn, ¹⁷⁶Lu, ²³⁹Pu, and ²³⁵U. The sets available for ¹⁵¹Eu and ¹⁷⁵Lu are not satisfactory: new evaluations have been carried out by properly utilizing measurements reported in the literature and/or new measurements performed for this purpose in a known spectrum. When using the new sets, the agreement between calculated and experimental spectral indexes in the absorber solutions is significantly improved. When reliable cross sections are available for the detectors, the correlation method can be considered a powerful tool for the evaluation of an unknown reaction rate: for instance the ²³⁹Pu absorption rate in the solutions is evaluated by means of the measured activation rates without introducing uncertainties due to the correlation procedure itself.