A new approach for the experimental determination of the spectral indices 28ρ and 25δ

A new experimental approach for the experimental determination of the spectral indices (28)rho and (25)delta is proposed in this work. The proposed method, which is based on a gamma scanning technique of the fuel rod, eliminates the need of correction factors for the cadmium and foil perturbations, and also allows the bare and cadmium covered positions of the fuel rods to be irradiated simultaneously. The results are obtained with very high degree of accuracy. The theoretical analyses reveal that for the spectral index (25)delta the results are in a very good agreement for several libraries. Considering the case of 28 rho the nuclear data libraries ENDF/B-VI.8, JENDL3.3 and JEF3.0 slightly overestimated this parameter. In contrast, the newly released nuclear data for U-238 (ORNL2) and preliminary ENDFB-VII data for U-235 overestimated heavily the spectral index (28)rho.