Despite over eighty years of study since nuclear fission was discovered, important details of this nuclear process remain poorly understood. Among them is the mechanism by which fission fragments acquire substantial spins, or angular momenta, despite originating from a heavy nucleus that may start with no spin. Theorists and experimentalists have recently put forth renewed effort toward revealing this spin generation mechanism. This work aims to measure correlations between the spin generated in the fission fragments and the energy available to them. Practically, we measure correlations between fission gamma-ray emission and properties of the fragments that emit them. We first measure the gamma-ray multiplicity and spectrum from 239Pu(n,f) as a function of incident neutron energy using the Chi-Nu liquid scintillator array at the Los Alamos Neutron Science Center, finding qualitative evidence for positive spin-energy correlations. We then develop and characterize a specialized fission fragment detector that measures the fragments’ kinetic energies and masses at Argonne National Laboratory. Loaded with a 252Cf(sf) source, we place that fragment detector inside Gammasphere, a high-resolution and high-granularity gamma-ray spectrometer, to measure correlations between the kinetic energies of specific fragments and the gamma-ray spectra they emit. We leverage our knowledge of a fragment's nuclear level scheme to reconstruct its spin distribution as a function of fragment total kinetic energy. Focusing on 144Ba, we find its average spin to be insensitive to energy—-inconsistent with predictions from solely statistical excitation of rotational modes in the fragments. We discuss how these new results fit into the existing body of correlated fission measurements that are relevant to spin generation, and provide our perspective on what our observations mean for spin generation in fission.
