Speaker
Description
Fission fragment properties are set, to a large extent, during the scission process. It is generally assumed that the fully accelerated fission fragments can be treated as compound nuclei, and their de-excitation towards a long-lived state via prompt neutron and gamma emissions can be treated within a statistical model like the Hauser-Feshbach approach to nuclear reactions. Accurate modeling of prompt fission neutrons, and to a lesser extent prompt fission gamma-rays, is important, as measurements cannot detect fission fragments before neutron emission, and have to correct for the emitted particles to extract information about the scission process. One often makes the assumption that because most of the prompt fission neutrons are emitted with energies of about 1 MeV in the center-of-mass frame of the fragment, they do not carry much angular momentum when emitted from fission fragments. Using our state-of-the-art code CGMF, which treats the fission fragments as compound nuclei, we show that a significant number of neutrons are emitted with angular momentum of 3/2 and higher. As a consequence of quantum effects, the p-wave strength function peaks in the 90 – 100 mass region (the Ramsauer effect), emphasizing the importance of higher partial waves for the light fission fragments even at low neutron energies. When all of the contributions to the angular momentum removal from neutrons and statistical gamma rays (emitted before electromagnetic transitions between discrete levels) are counted, we find that on average the statistical decays remove about 4-5 units of angular momentum, with comparable standard deviation. This finding can be at odds with some other statistical models and assumptions made in the analysis of experimental data, and blurs the connection with the scission process, making difficult a direct connection between observables before and after neutron emission. We show that we can reproduce the shape of the average fission fragment spins as a function of their mass, as recently measured by Wilson et al., with somewhat different assumptions regarding the initial spin of the fission fragment, as illustrated in Fig. 1. We also show the impact of the known nuclear structure information, in particular isomeric states, illustrated in Fig. 1 by the difference between the CGMF results marked by green circles and purple pentagons. Finally, because different models disagree on details of the angular momenta of neutrons and statistical gamma rays, one should be aware of the possible model dependence when interpretation for some measurements is given.
LA-UR-21-28347