Resonance Self-Shielding Corrections for Detector Foils in Fast Neutron Spectra

Activation measurements in fast neutron spectra, using detector foils in which the neutron mean-free-path in a resonance is small compared to the foil thickness, are revised by detailed space and energy self-shielding corrections. The experimental data are then comparable with the reaction rate density calculations. The self-shielding factors of individual Doppler-broadened resonances have been calculated with the integral transport theory. A multiple collision technique was required to treat resonance scattering inside the foil because of the small mean-free-path. The foil correction factor at each experimental position is obtained by combining the self-shielding factors of individual resonances with the calculated multigroup reaction rates at the foil locations. This gives a space-dependent foil correction factor for regions with spatially varying spectra. Applications of the foil correction technique to the integral reaction rate measurements of ¹⁹⁷Au, ⁵⁵Mn, ¹⁸⁶W, and ²³²Th in a fast reactor blanket mockup are presented.