Uncertainty quantification of optical models in fission fragment angular momentum removal*

Understanding the post-scission state of fission fragments, and their subsequent energy and angular momentum removal by emitted prompt neutrons and gammas, is an important open problem. Recent measurements have assumed that prompt neutrons are predominantly emitted as s-waves, which is contradictory to phenomenological optical model predictions. Despite this contradiction, Monte Carlo Hauser fesbach (MCHF) fission event generators like CGMF are still generally predictive of fission observables, such as prompt neutron and gamma multiplicities and energy distributions. Are the optical models, level densities, and models for post-scission fragment energy and angular momentum valid individually, or do canceling errors lead to the predictive powers of MCHF? Workhorse phenomenological optical models, Koning Delaroche and Chapel Hill 89, have been re-fit to their corpora of experimental data using Markov Chain Monte Carlo. We propagate the resulting posterior distributions, representing model parameter uncertainty, through CGMF, an MCHF fission event generator, to determine the uncertainty in angular momentum and energy removed from the fragments by neutrons. These models are trained on experimental scattering data on β-stable targets, but using them for fission requires extrapolation to neutron-rich regions, where their reliability is unknown. The Whitehead-Lim-Holt optical model is derived ab-initio from chiral effective field theory, and should not suffer from extrapolation error. We add this model to CGMF, and propagate its theoretical uncertainties as well, comparing its predictions to the phenomenological models.