Bogdan Fornal

In nuclear fission, the primary fragments deexcite through the emission of neutrons and $\ensuremath{\gamma}$ 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 $0.5\ensuremath{\hbar}$ per neutron), while the majority (typically $6\ensuremath{\hbar}--7\ensuremath{\hbar}$) is removed via $\ensuremath{\gamma}$-ray emission. This consensus was recently challenged [I. Stetcu et al., Phys. Rev. Lett. 127, 222502 (2021)], instead suggesting values $>1.2\ensuremath{\hbar}$ 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 $\ensuremath{\gamma}$-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 $\ensuremath{\gamma}$ 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 $(\ensuremath{-}0.03\ifmmode\pm\else\textpm\fi{}0.03)\ensuremath{\hbar}$ and $(\ensuremath{-}0.21\ifmmode\pm\else\textpm\fi{}0.04)\ensuremath{\hbar}$ for the light and heavy fragments, respectively. These results are interpreted in combination with other recent works in the literature.

The \(A\sim 40\) mass region is ideal to study the evolution of nuclear structure and to probe different theoretical approaches. In particular, in these nuclei, phenomena like shape coexistence are expected to appear. The objective of this work is to study the evolution of such phenomena across the Ca isotopic chain by performing complete low-spin spectroscopy of even–even \(^{42,44}\)Ca and odd–even \(^{43,45}\)Ca isotopes and, together with the already published results on \(^{41,47,49}\)Ca, track the evolution of nuclear structure along \(Z=20\). All four isotopes relevant to this work were populated by (\(n_\mathrm {th}\),\(\gamma \)) neutron-capture reactions exploiting thermal neutrons from the ILL (Grenoble) nuclear reactor. In this article, we present preliminary results on \(^{42}\)Ca and \(^{44}\)Ca. In our experiments, we used the FIPPS spectrometer to detect \(\gamma \)-rays, and both double-\(\gamma \)- and triple-\(\gamma \)-coincidence techniques were used to reconstruct the decay schemes. The result is 10 new levels and 109 new transitions for \(^{42}\)Ca, and 27 new levels and 280 new \(\gamma \) rays in \(^{44}\)Ca. Preliminary angular correlation studies of \(^{42}\)Ca excited states will also be presented, and the population of \(0^+\) excited states, possibly associated with shape coexistence, will be discussed. Published by the Jagiellonian University 2026 authors

Lifetime measurements of the ($3^{-}$), ($4^{-}$), and ($6^{-}$) intraband states in the neutron-rich, odd-odd ${}^{96}\mathrm{Rb}$ nucleus were performed at the LOHENGRIN spectrometer of Institut Laue-Langevin, using thermal-neutron-induced fission of ${}^{235}\mathrm{U}$ and fast-timing techniques with ${\mathrm{LaBr}}_3:\mathrm{Ce}$ scintillator detectors. The nanosecond isomeric nature of the ($3^{-}$) bandhead was established as well as the ${\beta }_2=0.39\left(3\right)$ deformation parameter of the band, pointing to a robust deformation in ${}^{96}\mathrm{Rb}$. Moreover, a hindered $B\left(E2\right)$ value of $3.9_{-13}^{+19}\times {10}^{-2}$ W.u. was found for the $\gamma$ decay of the deformed ($4^{-}$) state to the spherical $2^{-}$ ground state. A retardation was also found for the ($3^{-}$) $\to 2^{-}$ transition, possibly due to the shape change and giving strong support to a shape coexistence scenario in this nucleus, at the borders of the island of deformation at $N=60$. Analogies with the structure of the ${}^{98}\mathrm{Y}$ isotone are discussed.