Angular Momentum Removal by Neutron and $\gamma$-Ray Emissions during Fission Fragment Decays

We investigate the angular momentum removal from fission fragments (FFs) through neutron and $\gamma$-ray emission, finding that about half the neutrons are emitted with angular momenta $\ge 1.5\hslash$ 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 $3.5-5\hslash$, 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 sawtoothlike behavior observed in recent measurements, the average FF spin computed after neutron and statistical $\gamma$ emissions exhibits a shape that resembles a sawtooth. This suggests that the average FF spin measured after statistical emissions is not necessarily connected with the scission mechanism as previously implied.

Figure 1

Distribution of the total angular momentum removed for light (filled circles), heavy (filled triangles) and all (filled squares) FFs after all prompt neutrons have been emitted. The properties of this wide distribution are listed in Table 2.

Figure 2

The average angular momentum of the primary FFs producing select residuals with mass $A_F$ for (a) ${}^{252}\mathrm{Cf}(\mathrm{sf})$ and (b) ${}^{238}\mathrm{U}(n_{1.9\mathrm{MeV}},f)$. We compare the data by Wilson et al. [17] with CGMF simulations. Three sets of results are presented for CGMF: one obtained by averaging the initial spin of all the FFs producing a targeted residual (squares), the second one by applying Eq. (1) (circles), and the third one similar to the second one, with an additional gate to allow only events with $2^{+}\to 0^{+}$ as the last $\gamma$ transition in the cascade. For a more meaningful comparison, we have added $1\hslash$ to the latter two CGMF results, which is the correction applied in the experiment [17] to account for the change in angular momentum during the statistical neutron and $\gamma$ emissions.