Effects of Shell Model-Constrained Level Densities on Reaction Cross-Sections

The reaction cross sections of radioactive nuclei are notoriously difficult to measure, especially for nuclei that lie far away from the line of stability. At present, various theoretical models exist for extrapolating nuclear properties (masses, radii, even cross sections) based on available experimental data. These extrapolations become more uncertain as one moves further from available experimental data and the uncertainties, in turn, propagate to the reaction cross sections of interest. We can utilize microscopic nuclear theory models that correlate observables across multiple nuclei to further constrain these uncertainties. We developed a process to generate new level density constraints based on the nuclear shell model and studied how these constraints affect the (n, γ) and (n, 2n) reaction cross sections. We constrained the level densities of six magnesium isotopes using shell model calculations and then found the covariance between the level densities of different isotopes and used that to further constrain the inputs to Hauser-Feshbach calculations. We finally calculated new cross sections using the constrained files and calculated their respective uncertainties. We found that the cross section uncertainty is smaller under the shell model constraints. Our results demonstrate that correlated shell model predictions for the nuclear level density can be used to constrain capture and activation reaction cross sections for nuclei away from stability, where experimental data is scarce.