We elucidated the reason why the average total kinetic energy (TKE) of fission fragments decreases when the excitation energy of the fissioning systems increase as indicated by experimental data for the neutron-induced fission events. To explore this problem, we used a method based on the four-dimensional Langevin equations we have developed. We have calculated the TKE of fission fragments for fissioning systems ${}^{236}\mathrm{U}^{*}$ and ${}^{240}\mathrm{Pu}^{*}$ excited above respective fission barriers, and compared the results with experimental data for n $+$ ${}^{235}\mathrm{U}$ and n $+$ ${}^{239}\mathrm{Pu}$ reactions, respectively. From the Langevin-model analysis, we have found that the shape of the abundant heavy fragments changes from almost spherical for low excitation domain to highly prolate shape for high excitation energy, while that of the light fragments does not change noticeably. The change of the “shape” of the heavy fragments causes an increase of a distance between the charge centers of the nascent fragments just after scission as excitation energy increases. Accordingly, the Coulomb repulsion between the two fragments decreases with an increase of the excitation energy, which causes the decrease of the average TKE. In this manner, we found that the change of the shape of the heavy fragment as a function of the excitation energy is the key issue for the TKE of fission fragments to decrease as the excitation energy of the fissioning nuclei increases. In other words, washing out of the shell effects, which affect the shape of the heavy fragments is the key reason for the decreasing energy dependence of the average TKE of the fission fragments.

Background: An accurate description of the initial and final state interactions in the ${}^{18}\mathrm{O}+{}^{76}\mathrm{Se}$ collision is demanded by the NUMEN project. The study of single and double charge exchange nuclear reactions is the main purpose for NUMEN, since these can be used as tools to provide experimentally driven information about nuclear matrix elements of interest in the context of neutrinoless double-$\beta$ decay. To date, the details of the optical potentials and nuclear response to isospin operators for many of the projectile-target systems proposed for the NUMEN double charge exchange studies are poorly known. The ${}^{18}\mathrm{O}+{}^{76}\mathrm{Se}$ case, here under study, is particularly relevant due to its connection with the ${}^{76}\mathrm{Ge}$ neutrinoless double-$\beta$ decay.

Purpose: In this work the authors want to characterize the initial-state interaction for the ${}^{18}\mathrm{O}+{}^{76}\mathrm{Se}$ reactions at 275 MeV incident energy determining the optical potential and evaluating the effect of couplings with the inelastic scattering on the elastic channel.

Methods: The angular distributions of differential cross section were measured in the angular region between $4^{\circ }$ and ${22}^{\circ }$ in the center-of-mass reference frame. The cross sections were compared with theoretical calculations, that adopt different optical potentials. Coupling effects on the elastic channel were determined into the coupled channels formalism.

Results: The excitation energy spectrum of the colliding nuclei and the cross section angular distributions were measured with satisfactory energy resolution. The elastic scattering cross section is not well reproduced in the full angular range explored when the optical model approach is adopted. A good agreement is found using coupled channel calculations. The initial state interaction for the ${}^{18}\mathrm{O}+{}^{76}\mathrm{Se}$ nuclear reactions at 275 MeV is determined.

Conclusions: Coupled channels effects are crucial to obtain a good description of the measured elastic and inelastic channels cross sections, even at large transferred momenta where the optical model approach fails in reproducing the experimental data. The role of channel coupling could be relevant also in the analysis of other open reaction channels in the same collision and should be accounted for in double charge exchange analyses as well.

The deuterium-tritium (D-T) $\gamma$-to-neutron branching ratio $\left[{}^3\mathrm{H}\right(d,\gamma ){}^5\mathrm{He}/{}^3\mathrm{H}(d,n\left){}^4\mathrm{He}\right]$ has been determined previously under inertial-confinement fusion (ICF) conditions and in beam-target based experiments. In the former case, neutron-induced backgrounds are mitigated compared to the latter due to the short pulse nature of ICF implosions and the use of gas Cherenkov $\gamma$-ray detectors. An added benefit of ICF based measurements is the ability to achieve lower center-of-mass energies as compared to accelerators. Previous ICF based experiments however report a large uncertainty in the D-T $\gamma$-to-neutron branching ratio of $\approx 48\%$, which arises from the necessity of an absolute detector calibration and/or a cross-calibration against the D-${}^3\mathrm{He} \gamma$-to-proton branching ratio. A more precise value for the branching ratio based on data taken at the OMEGA laser facility is reported here, which relies on a cross-calibration against the better known ${}^{12}\mathrm{C}$ neutron inelastic scattering cross section. A D-T branching ratio value of $\left(4.6\pm 0.6\right)\times {10}^{-5}$ is determined by this method.

Using the extended optical model, a simultaneous analysis is carried out of elastic scattering and fusion reactions induced by the neutron halo ${}^6\mathrm{He}$ projectile on a ${}^{64}\mathrm{Zn}$ target. Dynamic polarization potentials $U_F$, $U_D$ are introduced which can be associated to fusion and direct couplings, respectively. Appropriate fitting parameters leading to dispersive potentials are found which provide a good simultaneous description of the data. An unexpected local maximum is uncovered in the energy dependence of the total reaction cross sections, indicating a transition in the involved direct reaction processes, and a possible underlying mechanism is discussed. $U_F$ is found to follow the normal threshold anomaly but with an unusually large threshold energy. $U_D$, on the other hand, exhibits a breakup threshold anomaly, but with a strikingly high value of the peak energy, which can be associated to the above-mentioned transition. The results also are discussed within the context of possible halo effects.

Background: We constructed the Kyushu chiral $g$ matrix and confirmed its reliability at $30\lesssim E_{\mathrm{in}}\lesssim 100$ MeV and $250\lesssim E_{\mathrm{in}}\lesssim 400$ MeV for ${}^{12}\mathrm{C}$ scattering. Reaction cross-section data of ${}^4\mathrm{He}$ scattering are available for some nuclides including ${}^{208}\mathrm{Pb}$. The PREX II collaboration reported a thick neutron skin for ${}^{208}\mathrm{Pb}$.

Purpose: Our purpose is to deduce neutron skin thicknesses of ${}^{208}\mathrm{Pb}$ and some other nuclides from reaction cross sections calculated in terms of the double folding model with the $g$ matrix.

Methods: We fold the $g$ matrix and densities given by mean-field calculations. To remedy the weaker constraint of the neutron sector, we renormalize densities so as to reproduce the observed cross sections.

Results: We found that a $3.4\%$ renormalization is necessary for ${}^{208}\mathrm{Pb}$. The neutron density obtained from renormalization results in $R_{\mathrm{skin}}=0.416\pm 0.146$ fm by confronting the precision proton radius.

Conclusions: Our result is consistent with PREX II and therefore supports larger slope parameter $L$. Results for ${}^{40}\mathrm{Ca}$ and ${}^{124}\mathrm{Sn}$ are also consistent with $R_{\mathrm{skin}}$ deduced from other experiments. For ${}^{116,120}\mathrm{Sn}$ the present method gives thicker skins.

Angular correlations of identified particles measured in ultrarelativistic proton-proton (pp) and heavy-ion collisions exhibit a number of features which depend on the collision system and particle type under consideration. Those features are produced by mechanisms, such as (mini)jets, elliptic flow, resonance decays, and conservation laws. In addition, of particular importance are those related to the quantum statistics (QS) and final-state interactions (FSIs). In this paper we show how to unfold the QS and FSI contributions in angular correlation functions by employing a Monte Carlo approach and using momentum correlations (femtoscopy), focusing on pp reactions. We validate the proposed method with pythia 8 Monte Carlo simulations of pp collisions at $\sqrt{s}=13\mathrm{TeV}$ coupled to calculations of QS and FSI effects with the Lednický and Lyuboshitz formalism and provide predictions for the unfolded effects. In particular, we show how those effects modify the shape of the angular correlation function with emphasis on pions and protons. Most importantly, specific structures observed in the near-side region for both baryon-baryon and baryon-antibaryon pairs, originating from the strong interaction, are unveiled with the proposed method.

To investigate the interaction between the nucleon $N$ and nucleon resonance $N\left(1535\right)1/2^{-}$, the $\eta d$ threshold structure connected to the isoscalar $S$-wave $N\text{−}N\left(1535\right)1/2^{-}$ system has been experimentally studied in the $\gamma d\to {\pi }^0\eta d$ reaction at incident photon energies ranging from the reaction threshold to 1.15 GeV. A strong enhancement is observed near the $\eta d$ threshold over the three-body phase-space contribution in the $\eta d$ invariant-mass distribution. An analysis incorporating the known isovector resonance ${\mathcal{D}}_{12}$ with a spin parity of $2^{+}$ in the ${\pi }^{0}d$ channel reveals the existence of a narrow isoscalar resonance-like structure with $1^{-}$ in the $\eta d$ system. Using a Flatté parametrization, the mass is found to be $2.{427}_{-0.006}^{+0.013}$ GeV, close to the $\eta d$ threshold, and the width is $(0.{029}_{-0.029}^{+0.006}\mathrm{GeV})+(0.{00}_{-0.00}^{+0.41})p_{\eta }c$, where $p_{\eta }$ denotes the $\eta$ momentum in the rest frame of the $\eta d$ system. The observed structure would be attributed to a predicted isoscalar $1^{-}\eta NN$ bound state from $\eta NN$ and $\pi NN$ coupled-channel calculation, or an $\eta d$ virtual state owing to strong $\eta d$ attraction.

The nucleon-nucleon scattering problem is usually analyzed in terms of partial waves and the corresponding coupled-channel phase shifts and mixing angles, but the available experiments induce correlations among the corresponding channels with different quantum numbers. Based on the Granada 2013 database we analyze the meaning and impact of those correlations, taking into account both the purely statistical ones reflecting the primary experimental data uncertainties as well as the systematic ones exhibiting the ambiguities in the form of the potential representing the unknown nuclear force for distances below 3 fm. We find that the combined uncertainties not only display a dominance of systematic over statistical effects, but also show that these correlations are almost compatible with zero. These findings support the frequent practice of determining potentials from separated channel-by-channel direct fits to phase shifts with the combined systematic and statistical effects without the full-fledged partial wave analysis inferred from experimental data but only with much larger uncertainties.

Background: The evolution of nuclear shell structure far from stability can be explored by identifying and measuring the properties of isomers. Neutron-rich nuclei between the $Z=28$ and the $Z=50$ closed shells have been the subject of recent studies which have identified a number of $0.1–10\mu \mathrm{s}$ isomers and measured detailed spectroscopic properties.

Purpose: The purpose of this analysis was to identify and measure the properties of short-lived isomeric states populated following $\beta$ decay in $Z\approx 30,N\approx 50$ nuclei near the doubly magic nucleus ${}^{78}\mathrm{Ni}$.

Methods: Radioactive ions produced by beam fragmentation at the National Superconducting Cyclotron Laboratory were implanted into a ${\mathrm{CeBr}}_3$ scintillator coupled to a pixelated photomultiplier tube. Ancillary arrays of HPGe clover and ${\mathrm{LaBr}}_3$ detectors were positioned around the implantation detector to measure $\beta$-delayed $\gamma$ rays.

Results: The previously observed 2634-keV level in ${}^{76}\mathrm{Zn}$, populated following the $\beta$ decay of ${}^{76}\mathrm{Cu}$, was identified as isomeric with a half-life of 25.4(4) ns. A combination of timing and $\gamma$-ray spectroscopy was used to confirm this assignment. Shell-model calculations were performed and indicate that this state may be a negative-parity state formed by the occupation of the $\nu 0g_{9/2}$ orbital.

Conclusions: A new isomeric state in ${}^{76}\mathrm{Zn}$ has been identified, and its half-life was measured. Ambiguity about the structure of this state could be resolved with further experiments.

We report $J^{\pi }=0^{+}$ ground-state energies and point-proton radii of ${}^4\mathrm{He}$, ${}^8\mathrm{Be}$, ${}^{12}\mathrm{C}$, ${}^{16}\mathrm{O}$, and ${}^{20}\mathrm{Ne}$ nuclei calculated by the ab initio no-core Monte Carlo shell model with the JISP16 and Daejeon16 nonlocal $NN$ interactions. Ground-state energies are obtained in the basis spaces up to seven oscillator shells $(N_{\mathrm{shell}}=7)$ with several oscillator energies $\left(\hslash \omega \right)$ around the optimal oscillator energy for the convergence of ground-state energies. These energy eigenvalues are extrapolated to obtain estimates of converged ground-state energies in each basis space using energy variances of computed energy eigenvalues. We further extrapolate these energy-variance-extrapolated energies obtained in the finite basis spaces to infinite basis-space results with an empirical exponential form. This form features a dependence on the basis-space size but is independent of the value of $\hslash \omega$ used for the harmonic-oscillator basis functions. Point-proton radii for these states of atomic nuclei are also calculated following techniques employed for the energies. From these results, it is found that the Daejeon16 $NN$ interaction provides good agreement with experimental data up to approximately ${}^{16}\mathrm{O}$, while the JISP16 $NN$ interaction provides good agreement with experimental data up to approximately ${}^{12}\mathrm{C}$. Beyond these nuclei, the interactions produce overbinding accompanied by radii that are too small. These findings suggest and encourage further revisions of nonlocal $NN$ interactions towards the investigation of nuclear structure in heavier-mass regions.

We studied $\alpha$ cluster states in ${}^{26}\mathrm{Mg}$ via the ${}^{22}\mathrm{Ne}({}^6\mathrm{Li},d\gamma ){}^{26}\mathrm{Mg}$ reaction in inverse kinematics at an energy of 7 MeV/nucleon. States between $E_x$ = 4–14 MeV in ${}^{26}\mathrm{Mg}$ were populated and relative $\alpha$ spectroscopic factors were determined. Some of these states correspond to resonances in the Gamow window of the ${}^{22}\mathrm{Ne}(\alpha ,\mathrm{n})$${}^{25}\mathrm{Mg}$ reaction, which is one of the main neutron sources in the astrophysical $s$-process. Using our new ${}^{22}\mathrm{Ne}(\alpha ,\mathrm{n})$${}^{25}\mathrm{Mg}$ and ${}^{22}\mathrm{Ne}(\alpha ,\gamma ){}^{26}\mathrm{Mg}$ reaction rates, we performed new $s$-process calculations for massive stars and asymptotic giant branch stars and compared the resulting abundances with the abundances obtained using other ${}^{22}\mathrm{Ne}+\alpha$ rates from the literature. We observe an impact on the $s$-process abundances up to a factor of three for intermediate-mass AGB stars and up to a factor of ten for massive stars. Additionally, states in ${}^{25}\mathrm{Mg}$ at $E_x <$ 7.5 MeV are identified via the ${}^{22}\mathrm{Ne}({}^6\mathrm{Li},t){}^{25}\mathrm{Mg}$ reaction for the first time. We present the $({}^6\mathrm{Li}, t)$ spectroscopic factors of these states and note similarities to the $(d,p)$ reaction in terms of reaction selectivity.

We systematically study the hyperon global polarization's sensitivity to a collision system's initial longitudinal flow velocity in hydrodynamic simulations. By explicitly imposing local energy-momentum conservation when mapping the initial collision geometry to macroscopic hydrodynamic fields, we study the evolution of the system's orbital angular momentum (OAM) and fluid vorticity. We find that a simultaneous description of the $\mathrm{\Lambda }$ hyperon's global polarization and the slope of the pion's directed flow can strongly constrain the size of longitudinal flow at the beginning of hydrodynamic evolution. We extract the size of the initial longitudinal flow and the fraction of orbital angular momentum in the produced quark-gluon plasma fluid as a function of collision energy with the STAR measurements in the Beam Energy Scan program at the BNL Relativistic Heavy-Ion Collider. We find that there is about 100–200 $\hslash$ OAM that remains in the mid-rapidity fluid at the beginning of hydrodynamic evolution. We further examine the effects of different hydrodynamic gradients on the spin polarizations of $\mathrm{\Lambda }$ and $\overline{\mathrm{\Lambda }}$. The gradients of ${\mu }_B/T$ can change the ordering between $\mathrm{\Lambda }$'s and $\overline{\mathrm{\Lambda }}$'s polarizations.

The Bose-Einstein condensation and the liquid-gas first-order phase transition are studied in the interacting pion matter. Two phenomenological models are used: The mean-field model and the hybrid model. Free model parameters are fixed by fitting the lattice QCD data on the pion Bose condensate density at zero temperature. In spite of some minor differences, the two models demonstrate an identical qualitative and very close quantitative behavior for the thermodynamic functions and electric charge fluctuations. A peculiar property of the considered models is an intersection of the Bose-Einstein condensation line and the line of the first-order phase transition at the critical endpoint.

We develop the equation of state of dense neutron star matter with nucleons, nucleons and hyperons, nucleons and delta, as well as including strange interactions through strange meson-hyperon couplings to assess the effect of both the soft and stiff symmetry energy, determined in the recent PREX-II experiment, on the properties of neutron stars. The macroscopic properties of neutron stars such as the mass, radius and tidal deformability are a direct consequence of the underlying equation of state of dense neutron star matter. Given the recent advances in constraining the above macroscopic observations from gravitational wave analysis, NICER and radio observations, we study here the effect of a variation in the symmetry energy on such observational properties of neutron stars with different possible compositions in a relativistic quark model. We observe that the stiff symmetry energy satisfies the current constraints on maximum mass and radius, but creates tension with tidal deformability values.

Following the many successful implementations of effective universal configuration-interaction Hamiltonians, we endeavored to produce a universal $fp$ shell interaction tailored for the calcium isotopes, which we call UFP-CA. Starting from a state-of-the-art in-medium similarity renormalization group (IMSRG) interaction, linear combinations of Hamiltonian parameters that define the natural basis of the parameter space are constrained by the latest experimental data for the neutron-rich calcium isotopes. We show that this data-driven method for improving the Hamiltonian provides an excellent description of the known binding energies and spectra for the calcium isotopes within the $fp$ model space. This together with comparisons to results from energy-density functional models leads us to conclude that ${}^{60}\mathrm{Ca}$ is doubly magic at a similar level to ${}^{68}\mathrm{Ni}$. Several predictions are presented for unobserved low-lying excited states in ${}^{55-59}\mathrm{Ca}$ that will be accessible to future experiments.

The nuclear landscape is investigated within the triaxial relativistic Hartree-Bogoliubov theory with the PC-PK1 density functional, and the beyond-mean-field dynamical correlation energies are taken into account by a microscopically mapped five-dimensional collective Hamiltonian without additional free parameters. The effects of triaxial deformation and dynamical correlations on the nuclear landscape are analyzed. The present results provide a better description of the experimental binding energies of even-even nuclei, in particular for medium and heavy mass regions, in comparison with previous global calculations with the state-of-the-art covariant density functionals DD-PC1 and TMA. The inclusion of the dynamical correlation energies plays an important role in the PC-PK1 results. It is emphasized that the nuclear landscape is considerably extended by the PC-PK1 functional in comparison with the previous results obtained with the covariant density functionals DD-PC1 and TMA, which may be due to the different isovector properties in the density functionals.

Background: The density profile of exotic nuclei can be a rich source of information on the nuclear surface. In particular, the nuclear surface diffuseness is correlated with the occupation probability of nucleons in distinct nuclear orbits, especially those with low angular momenta.

Purpose: The aim of this paper is to investigate the relationship between the nuclear surface diffuseness and spectroscopic information of neutron-rich Ne and Mg isotopes both at the cusp and inside the island of inversion.

Method: We use the microscopic antisymmetrized molecular dynamics model to calculate the densities and other spectroscopic information related to Ne and Mg isotopes. A two-parameter Fermi density distribution is then used to define the diffuseness parameter and the matter radius. These quantities are extracted by minimizing the difference between these two densities. To relate them with observables, the two densities are given as inputs to a Glauber model calculation of nucleon-nucleus elastic scattering differential cross section, with the demand that they reproduce the first peak position and its magnitude.

Results: A marked increase in the occupation of neutrons in the $pf$ orbit is noted in Ne and Mg isotopes from $N=19$ onwards. We observed that the nuclear surface diffuseness is strongly correlated with the nuclear deformation, in the island of inversion, and gradually increases with the occupation of neutrons in the $1p_{3/2}$ orbit. This result is also confirmed by a single-particle estimate of the valence neutron density distribution, with ${}^{29}\mathrm{Ne}$ as a test case. An exception is noted for ${}^{35–37}\mathrm{Mg}$, where the filling up of the holes in the $sd$ shell partially compensates the increase in diffuseness due to filling up of the $1p_{3/2}$ orbit.

Conclusion: Information on nuclear density profile of neutron-rich medium mass nuclei can be reliably extracted by studying the first diffraction peak of the nucleon-nucleus elastic scattering differential cross section. The enormous surface diffuseness of Ne and Mg isotopes, in the island of inversion, could be attributed to the increasing neutron occupation of the $1p_{3/2}$ orbit.

We trace a systematic and consistent method to precisely numerate the magnitude range for various structural and isospin compositional properties of the neutron star. Incompressibility, symmetry energy, slope parameter, and curvature of a neutron star are investigated using the relativistic energy density functional within the framework of coherent density fluctuation model. The analytical expression for the energy density functional of the neutron star matter is motivated from the Brückner functional and acquired by the polynomial fitting of the saturation curves for three different relativistic mean-field parameter sets (NL3, G3, and IU-FSU). The modified functional is folded with the neutron star's density-dependent weight function to calculate the numerical values for incompressibility and symmetry energy using the coherent density fluctuation model. The NL3 parameter set, being the stiffest equation of state, has a higher magnitude of all the properties compared to the other two parameter sets.

In the nucleus ${}^{48}\mathrm{Ti}$, whose structure is essential in evaluating the half-life of the neutrinoless double-$\beta$ decay ($0\nu \beta \beta$) of ${}^{48}\mathrm{Ca}$, the existence of the $\alpha$ cluster structure is shown for the first time. A unified description of scattering and structure is performed for the $\alpha +{}^{44}\mathrm{Ca}$ system. By using a global potential, which reproduces experimental $\alpha +{}^{44}\mathrm{Ca}$ scattering over a wide range of incident energies, $E_{\alpha }=18\text{–}100\mathrm{MeV}$, it is shown that the observed ($\alpha +{}^{44}\mathrm{Ca}$)-fusion excitation function at $E_{\alpha }=9\text{–}18\mathrm{MeV}$ is described well. The bump at $E_{\alpha }=10.2\mathrm{MeV}$ is found to be due to a resonance which is a $7^{-}$ state of the higher nodal band with the ($\alpha +{}^{44}\mathrm{Ca}$)-cluster structure in ${}^{48}\mathrm{Ti}$. The local potential ($\alpha +{}^{44}\mathrm{Ca}$)-cluster model locates the ground band of ${}^{48}\mathrm{Ti}$ in agreement with experiment and reproduces the enhanced $B\left(E2\right)$ values in the ground band well. This shows that collectivity due to $\alpha$ clustering in ${}^{48}\mathrm{Ti}$ should be taken into account in the evaluation of the nuclear matrix element in the $0\nu \beta \beta$ double-$\beta$ decay of ${}^{48}\mathrm{Ca}$.

Background: The $r$ process is thought to be a dominant mechanism for the production of medium mass and heavy nuclei. Despite extensive study, neither the process nor its site, and, consequently, the origin of these elements, is well understood. One of the principal reasons is the lack of adequate knowledge of neutron capture cross sections in neutron-rich nuclei, especially up to a few hundred keV. Existing statistical model calculations differ widely and are unreliable more than a few nucleons beyond stability.

Purpose: To provide a new, more reliable empirical method to estimate neutron capture cross sections.

Method: To use an entirely empirical approach by exploiting a newly discovered correlation between two-neutron separation energies and neutron capture cross sections.

Results: It is shown that there is a compact correlation between neutron capture cross sections and $S_{2n}$ for $S_{2n}$ values from 10 to 16 MeV, encompassing nuclei in five mass regions comprising seven classes of nuclei from $A\approx 110$ through the actinides over Maxwellian energy distributions from $kT=5$ to 100 keV. As a consequence, many unknown cross sections can be predicted by interpolation, with accuracies on the order of $\pm 25\%$. In other cases, the cross sections can still be predicted with greatly improved accuracy compared to current models. Finally, it is shown that, in very neutron-rich nuclei, measurements of $S_{2n}$ can replace more difficult or impossible neutron capture measurements to estimate $r$-process abundances.

Conclusions: A new approach to estimate neutron capture cross section allows much improved predictions of these key ingredients into $r$-process calculations, perhaps providing an enhanced ability to model this process and to better define its site(s).