In nuclear fission, the primary fragments deexcite through the emission of neutrons and $\gamma$ rays. This prompt emission conveys information about the initial angular momentum and excitation energy of the fragments at scission and is therefore important for improved understanding and modeling of the fission process. A long-standing question is how much the angular momentum $J$ of a fragment is changed after scission due to the evaporation of neutrons. Though few experimental data are available, the consensus has been that the neutrons carry only a small amount of $J$ on average (around $0.5\hslash$ per neutron), while the majority (typically $6\hslash –7\hslash$) is removed via $\gamma$-ray emission. This consensus was recently challenged [I. Stetcu et al., Phys. Rev. Lett. 127, 222502 (2021)], instead suggesting values $>1.2\hslash$ per neutron. Since the question of how much neutron emission impacts $J$ is important for understanding fission fragment deexcitation, we seek here to put experimental constraints on this value. An experiment was carried out with the nu-Ball2 Ge detector array to perform $\gamma$-ray spectroscopy of fission fragments produced in the ${}^{238}\mathrm{U}(n,\mathrm{f})$ reaction. A spectroscopic method [Y. Abdelrahman et al., Phys. Lett. B 199, 504 (1987)] was employed to determine average angular momenta after neutron emission for even-even fission fragments. By simultaneously selecting $\gamma$ rays from both a particular fission fragment and its partner fragment, the total number of neutrons emitted could be deduced in each event. The average fragment $J_{\text{e}}$ is extracted from measurements of the yrast feeding pattern of the selected fragment. Here we investigate the dependence of this observable on the number of neutrons emitted by studying 53 different fragment pairs. The uncertainty weighted dependences of the average change in entry-line spin with the total number of neutrons emitted are $(-0.03\pm 0.03)\hslash$ and $(-0.21\pm 0.04)\hslash$ for the light and heavy fragments, respectively. These results are interpreted in combination with other recent works in the literature.

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• Fission
• A ≥ 220
• Angular momentum