Chemical binding effects on resonance line shape and neutron capture rates

Calculated capture rates and Doppler coefficients in uranium compounds do not agree with experimental results. The difference can be due to: (1) errors in the resonance parameters and/or that (2) the Maxwell gas treatment, which considers nuclei free when in fact they are bound in a crystalline structure, is inadequate. Models are introduced which account for chemical binding in the resonance treatment and to determine if such models are capable of removing the discrepancy. Both harmonic and damped vibrational models of chemical binding were considered. In the harmonic model the vibrational amplitude of an atom in a crystal lattice is independent of time. In the damped vibrational model this amplitude is allowed to decay in time. It was found that the line shape obtained with the harmonic approximation relative to the free-gas, Maxwell model, is shorter at the peak and broader in the wings. This leads to less resonance self shielding and yields a slight increase in the discrepancy between theory and experiments (1-3%). The line shape obtained with the damped vibrational model is taller at the peak and narrower in the wings than that obtained with the Maxwell model. This leads to more resonance self shielding and yields a slight decrease in the discrepancy between theory and experiment (1-3%). This decrease, however, is not enough to remove all the discrepancy (approximately 10%). Further evaluation of the magnitude of the damping coefficient is required. (author)