The effective resonance absorption integral of 238U in heavy-atom mixtures

Abstract

The resonance absorption of neutrons by 238U in admixtures containing elements of medium or high atomic weight is considered. The contributions of the eighteen resolved resonances are calculated for three different dilutions corresponding to potential scattering cross-sections per 238U atom of 10 barns (pure 238U metal), 100 barns and 1000 barns. A good approximation is obtained to the exact Hurwitz solution for a non-hydrogeneous moderator, from which the effective resonance integrals are calculated. The effect of Doppler broadening at T = 300°K is included. The region of unresolved resonances is taken from 420 eV to 1 MeV and the resonance integrals are evaluated for this region, taking into account levels associated with l > 0 neutrons. Important differences are found from the predictions of a simple ‘narrow resonance’ approximation.