Łukasz W. Iskra

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 \(A=70\), \(T=1\) isobaric multiplet exhibits an anomalous Coulomb energy difference (CED) behavior, with the \(^{70}\)Br/\(^{70}\)Se pair showing a decrease in CED with increasing spin, contrary to the typical trend observed in other \(pf\)-shell nuclei. To investigate this phenomenon, a dedicated experiment was carried out at the Heavy Ion Laboratory of the University of Warsaw using an 88 MeV \(^{32}\)S beam impinging on a \(^{40}\)Ca target. Excited states in \(A \approx 70\) neutron-deficient nuclides were populated in fusion–evaporation reactions. Gamma rays were measured with the EAGLE spectrometer, whereas the reaction channel identification was provided by the combined set of neutron — NEDA and charged particle — DIAMANT detectors. Preliminary data analysis shows the satisfactory performance of the setup, which will allow for in-depth studies of the structure of the nuclei of interest. Published by the Jagiellonian University 2026 authors

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.

The lifetime of the $11/2_1^{+}$ state in the ${}^{131}\mathrm{Sb}$ nucleus was measured at the LOHENGRIN spectrometer of the Institut Laue-Langevin via neutron-induced fission of ${}^{235}\mathrm{U}$ using $\gamma$-ray fast-timing techniques. The obtained value of $T_{1/2}=3$(2) ps, at the edge of the sensitivity of the experimental method, is the first result for the $11/2_1^{+}$ state half-life in neutron-rich Sb isotopes. The corresponding quadrupole reduced transition probability to the ground state is $B\left(E2\right)=1.4_{-0.6}^{+1.5}$W.u., indicating a noncollective nature of this state. Realistic shell-model calculations performed in a large valence space reproduce well the experimental value and point to a dominant $2^{+}\left({}^{130}\mathrm{Sn}\right)\otimes \pi {\mathrm{g}}_{7/2}$ configuration for the $11/2_1^{+}$ state, as expected in a weak-coupling scenario. At the same time, the sum of the quadrupole strength of the multiplet states is predicted to exceed the one of the ${}^{130}\mathrm{Sn}$ core as a consequence of the equal contribution of the proton and the proton-neutron quadrupole matrix elements, pointing to possible development of collectivity already in the close neighborhood of ${}^{132}\mathrm{Sn}$.

We present recent results on the structure of the one-valence-particle ${}^{41}\mathrm{Ca}$ and ${}^{49}\mathrm{Ca}$ nuclei and the one-valence-hole ${}^{47}\mathrm{Ca}$ nucleus. The isotopes of interest were populated via the cold-neutron-capture reactions ${}^{40}\mathrm{Ca}(n,\gamma ), {}^{48}\mathrm{Ca}(n,\gamma )$, and ${}^{46}\mathrm{Ca}(n,\gamma )$, respectively. The experiments were performed at the Institut Laue-Langevin, within the EXILL campaign, which employed a large array of high-purity germanium (HPGe) detectors. The $\gamma$ decay and level schemes of these nuclei were investigated by $\gamma$-ray coincidence relationships, leading to the identification of 41, 10, and 6 new transitions in ${}^{41}\mathrm{Ca}, {}^{47}\mathrm{Ca}$, and ${}^{49}\mathrm{Ca}$, respectively. Branching ratios and intensities were extracted for the $\gamma$ decay from each state, and $\gamma$-ray angular correlations were performed to establish a number of transition multipolarities and mixing ratios, thus helping in the spin assignment of the states. The experimental findings are discussed along with microscopic, self-consistent beyond-mean-field calculations performed with the hybrid configuration mixing model, based on a Skyrme SkX Hamiltonian. The latter suggests that a fraction of the low-spin states of the ${}^{41}\mathrm{Ca}, {}^{49}\mathrm{Ca}$, and ${}^{47}\mathrm{Ca}$ nuclei is characterized by the coexistence of either 2p-1h (two-particle–one-hole) and 1p-2h excitations, or couplings between single-particle or single-hole degrees of freedom and collective vibrations (phonons) of the doubly-magic “core.”

Medium and high spin states of the ${}^{96}\mathrm{Y}$ nucleus, located in the shape-coexistence region near $Z=40$ and $N=60$, were populated in thermal-neutron-induced fission of ${}^{233}\mathrm{U}$ and ${}^{235}\mathrm{U}$ targets, diluted in a scintillator. $\gamma$ rays were measured with the FIssion Product Prompt $\gamma$-ray Spectrometer (FIPPS) high-purity germanium (HPGe) detector array, using double and triple $\gamma$-ray coincidence techniques and taking advantage of the efficient fission tag provided by the scintillating target material. A complex level scheme, extending up to 5.2 MeV and including excitations above the $8^{+}\beta$-decaying isomer, was investigated, and firm spin and parity assignments were given to a number of states, on the basis of angular correlation analysis and considerations on the $\gamma$-decay patterns. While the structures built on the $0^{-}$ ground state and the $8^{+}$ isomer show irregular patterns typical for spherical shapes, the ($6^{+}$) isomeric state at 1655 keV [with half-life of 181(9) ns], and the rotational band built on it [with spin-parity values between ($6^{+}$) and ($9^{+}$)], can be explained by Hartree-Fock-Bogoliubov calculations, if an oblate deformation is assumed. This is the first observation of a deformed structure in an $N=57$ isotone, lying three neutrons away from the $N=60$ line. An important finding is also the 115-keV transition which connects the ($6^{+}$) 181(9)-ns isomer to the $\beta$-decaying $8^{+}$ spherical isomer, allowing us to firmly place the latter at 1541 keV excitation energy. This may be relevant for calculations of electron and antineutrino spectra from fission of actinides, for which ${}^{96}\mathrm{Y}$ is a prominent product.