Angular momentum removal by neutron and $\gamma$-ray emissions during fission fragment decays

TL;DR

This study examines how neutron and gamma-ray emissions during fission fragment decays significantly reduce angular momentum, challenging previous assumptions and revealing complex distributions that obscure initial spin correlations.

Contribution

The paper introduces a model that quantifies angular momentum removal during fission, reproduces experimental spin data, and questions the link between measured spins and the scission mechanism.

Findings

Neutrons often carry angular momentum ≥ 1.5ħ.

Angular momentum change per event is 3.5–5ħ with large variability.

Average fragment spin after emissions resembles a saw-tooth pattern.

Abstract

We investigate the angular momentum removal from fission fragments (FFs) through neutron and $\gamma$-ray emission, where we find that about half the neutrons are emitted with angular momenta $\ge 1.5\hbar$ and that the change in angular momentum after the emission of neutrons and statistical $\gamma$ rays is significant, contradicting usual assumptions. Per fission event, in our simulations, the neutron and statistical $\gamma$-ray emissions change the spin of the fragment by 3.5 -- 5~$\hbar$, with a large standard deviation comparable to the average value. Such wide angular momentum removal distributions can hide any underlying correlations in the fission fragment initial spin values. Within our model, we reproduce data on spin measurements from discrete transitions after neutron emissions, especially in the case of light FFs. The agreement further improves for the heavy fragments if one removes from the analysis the events that would produce isomeric states. Finally, we show that while in our model the initial FF spins do not follow a saw-tooth like behavior observed in recent measurements, the average FF spin computed after neutron and statistical $\gamma$ emissions exhibits a shape that resembles a saw tooth. This suggests that the average FF spin measured after statistical emissions is not necessarily connected with the scission mechanism as previously implied.