The cross section for neutron interactions on argon is an important design and operational parameter for a number of neutrino, dark matter, and neutrinoless double beta decay experiments which use liquid argon as a detection or shielding medium. There is a discrepancy between the evaluated total cross section in the 20 to $70\mathrm{keV}$ neutron kinetic energy region given in the ENDF database and a single measurement conducted by an experiment with a thin target (0.2 atoms/b) optimized for higher cross sections. This gives rise to significant uncertainty in the interaction length of neutrons in liquid argon. This discrepancy is now resolved by new results presented here from the Argon Resonance Transport Interaction Experiment (ARTIE), a thick target experiment (3.3 atoms/b) optimized for the small cross sections in this energy region.

Atomic nuclei can exhibit shape coexistence and multireference physics that enters in their ground states, and accurately capturing the ensuing correlations and entanglement is challenging. We address this problem by applying single-reference coupled-cluster theory based on spherical and deformed reference states and the tailored coupled-cluster method. The latter combines configuration interactions to capture static correlations with coupled-cluster theory for dynamic correlations. We compute the atomic nuclei ${}^{12}\mathrm{C}, {}^{28}\mathrm{Si}$, and ${}^{56}\mathrm{Ni}$ and find that the tailored coupled-cluster method and the single-reference approach based on a deformed Hartree-Fock state yield the most accurate results.

Background: The mercury isotopes around $N=104$ are a well-known example of nuclei exhibiting shape coexistence. Mixing of configurations can be studied by measuring the monopole strength ${\rho }^2\left(E0\right)$, however, currently the experimental information is scarce and lacks precision, especially for the $I^{\pi }\to I^{\pi }$ ($I\ne 0$) transitions.

Purpose: The goals of this study were to increase the precision of the known branching ratios and internal conversion coefficients, to increase the amount of available information regarding excited states in ${}^{182,184,186}\mathrm{Hg}$, and to interpret the results in the framework of shape coexistence using different models.

Method: The low-energy structures in ${}^{182,184,186}\mathrm{Hg}$ were populated in the $\beta$ decay of ${}^{182,184,186}\mathrm{Tl}$, produced at ISOLDE, CERN and purified by laser ionization and mass separation. The $\gamma$-ray and internal conversion electron events were detected by five germanium clover detectors and a segmented silicon detector, respectively, and correlated in time to build decay schemes.

Results: In total, 193, 178, and 156 transitions, including 144, 140, and 108 observed for the first time in a $\beta$-decay experiment, were assigned to ${}^{182,184,186}\mathrm{Hg}$, respectively. Internal conversion coefficients were determined for 23 transitions, out of which 12 had an $E0$ component. Extracted branching ratios allowed the sign of the interference term in ${}^{182}\mathrm{Hg}$ as well as ${\rho }^2(E0;0_2^{+}\to 0_1^{+})$ and $B(E2;0_2^{+}\to 2_1^{+})$ in ${}^{184}\mathrm{Hg}$ to be determined. By means of electron-electron coincidences, the $0_3^{+}$ state was identified in ${}^{184}\mathrm{Hg}$. The experimental results were qualitatively reproduced by five theoretical approaches, the interacting boson model with configuration mixing with two different parametrizations, the general Bohr Hamiltonian, the beyond mean-field model, and the symmetry-conserving configuration-mixing model. However, a quantitative description is lacking.

Conclusions: The presence of shape coexistence in neutron-deficient mercury isotopes was confirmed and evidence for the phenomenon existing at higher energies was found. The new experimental results provide important spectroscopic input for future Coulomb excitation studies.

The level structure of the transitional nucleus ${}^{217}\mathrm{Ra}$ has been extended with the addition of around 20 new transitions. The discrepancies between the placements of several transitions reported in the earlier studies are resolved. The newly established negative-parity sequence at low excitation energies hints at the expected parity-doublet structures in this nucleus. The properties of the observed simplex bands are compared with that of similar bands in neighboring nuclei. Since the presence of parity-doublet structures reflects octupole correlations, theoretical calculations using reflection-asymmetric triaxial particle rotor model (RAT-PRM) have been performed. A comparison of the observed features of the simplex bands with the predictions of the RAT-PRM calculations suggests that ${}^{217}\mathrm{Ra}$ exhibits an intermediate behavior between the extremes of spherical and octupole-deformed nuclei. The termination of the simplex bands at intermediate energies and the structures lying above reflect the dominance of the single-particle excitations at higher excitation energies.

The understanding of the mechanisms for the production of weakly bound clusters, such as a deuteron $d$, in heavy-ion reactions at mid-rapidity is presently one of the challenging problems which is also known as the “ice in a fire” puzzle. In this study we investigate the dynamical formation of deuterons within the parton-hadron quantum molecular dynamics (PHQMD) microscopic transport approach and advance two microscopic production mechanisms to describe deuterons in heavy-ion collisions from energies available at the GSI Schwerionensynchrotron (SIS) to those at the BNL Relativistic Heavy Ion Collider (RHIC): kinetic production by hadronic reactions and potential production by the attractive potential between nucleons. Differently from other studies, for the “kinetic” deuterons we employ the full isospin decomposition of the various $\pi NN\leftrightarrow \pi d, NNN\leftrightarrow Nd$ channels and take into account the finite-size properties of the deuteron by means of an excluded volume condition in coordinate space and by the projection onto the deuteron wave function in momentum space. We find that considering the quantum nature of the deuteron in coordinate and momentum space reduces substantially the kinetic deuteron production in a dense medium as encountered in heavy-ion collisions. If we add the “potential” deuterons by applying an advanced minimum spanning tree (aMST) procedure, we obtain good agreement with the available experimental data from SIS energies up to the top RHIC energy.

$\mathrm{\Sigma }$ is a new correlation measure, quite recently introduced to heavy-ion physics. This measure, defined in the independent source model as a strongly intensive quantity, is expected to be free of the effects of system volume and volume fluctuations. This paper discusses the forward-backward (FB) correlation quantified with the $\mathrm{\Sigma }$ observable calculated in the framework of the wounded nucleon model (WNM) and wounded quark model (WQM). Findings show that the wounded-constituent approach outperforms the commonly used heavy-ion Monte Carlo generators, such as hijing, ampt, or epos, by accurately describing the experimental data on FB correlations with $\mathrm{\Sigma }$ measured by the ALICE Collaboration in Xe-Xe reactions at $\sqrt{s_{NN}}=5.44$ TeV and in Pb-Pb collisions at $\sqrt{s_{NN}}=2.76$ and 5.02 TeV. This paper demonstrates that $\mathrm{\Sigma }$ can be a unique tool for determining the fragmentation function of a wounded constituent in a symmetric nucleus-nucleus collision. However, in the wounded-constituent framework, it is no longer a strongly intensive quantity.

Particle momentum distributions originating from a quark-gluon plasma as produced in high-energy nuclear collisions can be influenced by thermal fluctuations in fluid dynamic fields. We study this effect by generalizing the commonly used kinetic freeze-out prescription by allowing for small fluctuations around an average in fluid velocity, chemical potentials, and temperature. This leads to the appearance of specific two-body momentum correlations. Combining a blast-wave parametrization of the kinetic freeze-out surface with the thermal correlation functions of an ideal resonance gas, we perform an exploratory study of angular net-charge correlations induced by thermal fluctuations around vanishing chemical potential. We note a diffusion of the near-side peak around $\mathrm{\Delta }y=\mathrm{\Delta }\varphi =0$ induced by variances of different chemical potentials, which could be investigated experimentally.

A qualitative difference in the running sum for the nuclear matrix element of the two-neutrino double-$\beta$ decay of ${}^{136}\mathrm{Xe}$ was found four years ago between quasiparticle random-phase approximation (QRPA) and shell model calculations. The former result has large increase and decrease with respect to the excitation energy of the intermediate state, and the latter one is an almost monotonically and mildly increasing function. My QRPA calculations independently of the above one do not have a remarkable decrease. This discrepancy is a serious problem affecting the reliability of calculations of the neutrinoless double-$\beta$ decay, and the cause was unknown. I perform several relevant test calculations and make an analytical consideration to find the cause, which is found to be in the strength of the attractive interactions. The possible major local decrease in the running sum is also explained analytically. The interactions of my QRPA calculation are appropriate in terms of the strength, thus the almost monotonic behavior is reasonable.

The $\alpha$ decay of ${}^{222}\mathrm{U}$ was reinvestigated using a general least-square superpulse fitting algorithm dedicated to resolving pileup signals in the decay of very short-lived nuclei. The $\alpha$-particle energy of ${}^{222}\mathrm{U}$ was revised to be 9246(8) keV, and the precision was improved significantly compared with previous result. Using the present $\alpha$ energy, the anomaly in the systematics of the $\alpha$-decay reduced width ${\delta }^2$ observed at ${}^{222}\mathrm{U}$ in the $N_p{N}_n$ scheme is solved, all the ${\delta }^2$ values converge into a smooth and narrow band with $N_p{N}_n$ up to $\approx 90$ in the northeast of ${}^{208}\mathrm{Pb}$.

The origin of octupole deformation for even-even nuclei near the doubly closed shell configurations are investigated by means of the semiclassical periodic orbit theory. In order to focus on the change of shell structure due to deformation, a simple infinite-well potential model is employed with octupole shape parametrized by merging a sphere and a paraboloid. Attention is paid to the contributions of the degenerate families of periodic orbits (POs) confined in the spherical portion of the potential, that are expected to partially preserve the spherical shell effect up to considerably large value of the octupole parameter. The contribution of those POs to the semiclassical trace formula plays an important role in bringing about shell energy gain due to octupole deformation in the system with a few particles added to spherical closed-shell configurations.

${}^{225}\mathrm{Pa}$ and ${}^{221}\mathrm{Ac}$ were produced at the IGISOL facility through proton-induced fusion-evaporation reactions and have been studied using $\alpha$-particle spectroscopy, as well as $\alpha \text{−}\gamma$ and $\alpha$-electron coincidence spectroscopy. The level scheme of ${}^{221}\mathrm{Ac}$, daughter of ${}^{225}\mathrm{Pa}$, and of ${}^{217}\mathrm{Fr}$, daughter of ${}^{221}\mathrm{Ac}$ were reconstructed. An interpretation of ${}^{221}\mathrm{Ac}$ levels as $K=5/2^{\pm }$ and $K=3/2^{\pm }$ parity-doublet bands is proposed. Such bands appear in reflection-asymmetric models and would be an indication of a static reflection asymmetric shape for ${}^{221}\mathrm{Ac}$.

The structure of the energy states and its decay properties in odd-odd ${}^{92–98}\mathrm{Y}$ isotopes have been investigated via the framework of the interacting boson fermion-fermion model (IBFFM-1). To complete the system of the model, the nuclear structure of the even-even ${}^{90–96}\mathrm{Sr}$ isotopes has been described from the IBM-1 as a core part. The nuclear structure of odd-$Z {}^{91–97}\mathrm{Y}$ and odd-$N {}^{91–97}\mathrm{Sr}$ isotopes is studied in the IBFM-1. The single-, quasi-particle energies and the occupation of valence protons (neutrons) $1f_{7/2},1f_{5/2},2p_{3/2},2p_{1/2}$ and $1g_{9/2}(2d_{5/2},2d_{3/2},1g_{7/2},3s_{1/2}$ and $1h_{11/2})$ single-particle orbitals have been calculated. Angular momentum assignments for some uncertain experimental levels made based on the model calculations. The model calculations gave consistent and detailed description of the nuclear structure of even-even, odd-$A$, and odd-odd Sr and Y of this mass region nuclei.

Neutrinoless double-$\beta$ decay, if it is finally observed, has the potential to reveal important features about the neutrino, such as its mass and character as a Majorana or Dirac particle. In the framework of the interacting boson model I have included new terms in the transition operator which allow one to calculate the nuclear matrix elements involved in the half-life of this extremely rare nuclear process. These terms are the next-to-leading order terms corresponding to the $L=0$ case in the multipolar expansion used for the transition operator. The nuclear matrix elements calculated including these terms are analyzed and compared with those of previous calculations.

One-neutron knockout reactions have been widely used to extract information about the single-particle structure of nuclei from the valley of stability to the drip lines. The interpretation of knockout data relies on reaction models, where the uncertainties are typically not accounted for. In this work, we quantify uncertainties of optical potentials used in these reaction models and propagate them, for the first time, to knockout observables using a Bayesian analysis. We study two reactions in the present paper, the first of which involves a loosely bound halo projectile, ${}^{11}\mathrm{Be}$, and the second a tightly bound projectile, ${}^{12}\mathrm{C}$. We first quantify the parametric uncertainties associated with phenomenological optical potentials. Complementing this approach, we also quantify the model uncertainties associated with the chiral forces that can be used to construct microscopic optical potentials. For the phenomenological study, we investigate the impact of the imaginary terms of the optical potential on the breakup and stripping components of the knockout cross sections as well as the impact of the angular range. For the ${}^{11}\mathrm{Be}$ case, the theoretical uncertainty from the phenomenological method is on the order of the experimental uncertainty of the knockout observables; however, for the ${}^{12}\mathrm{C}$ case, the theoretical uncertainty is significantly larger. The widths of the uncertainty bands for the knockout observables obtained for the microscopic study and the phenomenological approach are of similar orders of magnitude. Based on this work we conclude that structure information inferred from the ratio of the knockout cross sections will carry a theoretical uncertainty of at least $20\%$ for halo nuclei and at least $40\%$ for tightly bound nuclei.

Background: The existence of fusion hindrance in the light heavy-ion systems of astrophysical interest is not well established, so investigating slightly heavier cases may allow a reliable extrapolation towards the lighter ones. The recent observation of a very high hindrance threshold in ${}^{12}\mathrm{C}+{}^{24}\mathrm{Mg}$ (with a positive $Q$ value for fusion) at ${\sigma }_{\mathrm{fus}}\simeq 0.75$ mb, misses a valid interpretation within current theoretical models.

Purpose: Our aim has been to search evidence for fusion hindrances in the nearby system ${}^{12}\mathrm{C}+{}^{26}\mathrm{Mg}$ also having $Q_{\mathrm{fus}}>0$, and to obtain information on the underlying physics from a comparison of the two cases and from coupled-channels calculations.

Methods: The experiment was performed in inverse kinematics using the ${}^{26}\mathrm{Mg}$ beam from the XTU Tandem accelerator of Laboratori Nazionali di Legnaro (LNL). The targets were thin ${}^{12}\mathrm{C}$ evaporations isotopically enriched to $99.9\%$. The fusion-evaporation residues were detected at small angles by a $E-\mathrm{\Delta }E$-ToF detector telescope following an electrostatic beam deflector.

Results: The fusion excitation function of ${}^{12}\mathrm{C}+{}^{26}\mathrm{Mg}$ has been measured down to $\approx 5\mu \mathrm{b}$. The astrophysical $S$ factor shows a maximum at an energy where the cross section is $\approx 0.03$ mb, significantly lower than for ${}^{12}\mathrm{C}+{}^{24}\mathrm{Mg}$. This difference is confirmed by the comparison of the two $S$ factors. coupled channel calculations give a good account of the data, but they overpredict the cross sections below $\approx 0.03$ mb. The logarithmic slopes of the two excitation functions are superimposable to a large extent, with visible oscillations, more noticeable for ${}^{12}\mathrm{C}+{}^{24}\mathrm{Mg}$.

Conclusions: The hindrance phenomenon is clearly observed in ${}^{12}\mathrm{C}+{}^{26}\mathrm{Mg}$. The difference between the corresponding threshold energies for ${}^{12}\mathrm{C}+{}^{24,26}\mathrm{Mg}$ might (only qualitatively) be attributed to the $\alpha$-like structure of ${}^{24}\mathrm{Mg}$. In the Jiang's phenomenological systematics, the different behaviors of ${}^{12}\mathrm{C}+{}^{24,26}\mathrm{Mg}$ make the situation more complex, and call into question the extrapolation procedure toward the lighter systems of astrophysical interest.

We utilize the Boltzmann transport model to investigate the sequential suppression pattern of the $\mathrm{{\rm Y}}(1S,2S,3S)$ states in both small ($p$-Pb) and large (Pb-Pb) collision systems at a center-of-mass energy of $\sqrt{s_{NN}}=5.02$ TeV. The cold nuclear matter effects occur prior to the formation of bottomonium, which is equivalent for the various bottomonium states $\mathrm{{\rm Y}}(1S,2S,3S)$. The sequential suppression pattern of the bottomonium states is regarded as a manifestation of the hot medium effects, where the bottomonium states experience different levels of color screening and parton inelastic scatterings due to their distinct geometrical sizes and binding energies. Excited states of bottomonium are more susceptible to dissociation due to their smaller binding energies. By incorporating both cold and hot medium effects, the transport model provides a consistent explanation for the experimental observations of bottomonium in both small and large collision systems.

The decay rate of ${}^7\mathrm{Be}$ electron capture in ${\mathrm{C}}_{70}$ and Be metal was measured employing a reference method. The half-life ($T_{1/2}$) of ${}^7\mathrm{Be}$ endohedral ${\mathrm{C}}_{70}$ (${}^7\mathrm{Be}$@${\mathrm{C}}_{70}$) was found to be $T_{1/2}=52.49\pm 0.04$ d at room temperature ($T=293$ K) and $T_{1/2}=52.42\pm 0.04$ d at liquid helium temperature ($T=5$ K). Furthermore, the $T_{1/2}$ of ${}^7\mathrm{Be}$ in Be metal was $T_{1/2}=53.25\pm 0.04$ d at room temperature ($T=293$ K) and $T_{1/2}=53.39\pm 0.03$ d at liquid helium temperature ($T=5$ K). These values for ${}^7\mathrm{Be}$@${\mathrm{C}}_{70}$ at $T=5$ K are approximately $1.6\%(1.8\%$) smaller than those for ${}^7\mathrm{Be}$ in Be metal at $T=293$ K ($T=5$ K), indicating the difference in the electron wave functions for ${}^7\mathrm{Be}$ inside ${\mathrm{C}}_{70}$ and ${}^7\mathrm{Be}$ in ${\mathrm{C}}_{60}$ and Be metal. The average charge transfer from the $L\left(2s\right)$ electrons of the ${}^7\mathrm{Be}$ atom influences such variations in the decay constant ($\lambda =ln2/T_{1/2}$) in the environment. The experimental and theoretical investigations revealed that the change in the EC-decay rate of ${}^7\mathrm{Be}$ could be largely related to the potential configurations and the environment inside ${\mathrm{C}}_{60}$ and/or ${\mathrm{C}}_{70}$ cages. The motion of ${}^7\mathrm{Be}$ inside cages was found to be restricted according to temperature.

We study three-charged-particle low-energy elastic collision and particle-exchange reactions with special attention to the systems with Coulomb interaction and an additional nuclear interaction employing a close-coupling expansion scheme to a set of coupled two-component few-body equations. First we apply our formulation to compute low-energy elastic scattering phase shifts for the $d+{\left(t{\mu }^{-}\right)}_{1s}$ collision, which is of significant interest for the muon-catalyzed-fusion D-T cycle. Next, we study the particle-exchange reaction $d+\left(p{X}^{-}\right)\to p+\left(d{X}^{-}\right)$ with the long-lived elementary heavy lepton stau $X^{-}$ which can play a critical role in the understanding of the big-bang nucleosynthesis and the nature of dark matter. We also study the total cross sections and rates for two-particle-exchange reactions involving antiprotons ($\overline{p}$), deuterons $\left(d\right)$, and tritons $\left(t\right)$, e.g., $\overline{p}+{\left(d{\mu }^{-}\right)}_{1s}\to {\left(\overline{p}d\right)}_{1s}+{\mu }^{-}$ and $\overline{p}+{\left(t{\mu }^{-}\right)}_{1s}\to {\left(\overline{p}t\right)}_{1s}+{\mu }^{-}$, where ${\mu }^{-}$ is a muon. The effect of the final state short-range strong ($\overline{p}d$) and ($\overline{p}t$) nuclear interactions is significant in these reactions, which increases the reaction rates by a factor of $\approx 3$.

The low- and medium-spin level structure of ${}^{77}\mathrm{As}$ nucleus has been investigated using the ${}^{76}\mathrm{Ge}(\alpha ,p2n)$ fusion evaporation reaction and the standard in-beam $\gamma$ ray spectroscopic technique. The deexciting $\gamma$ rays were detected with the Indian National Gamma Array spectrometer comprising Clover and LEPS detectors. The previously known $9/2^{+}$ yrast positive-parity band, based on the $\pi \left(g_{9/2}\right)$ configuration, has been extended to the excitation energy of $E_x\approx 7.5$ MeV with $J^{\pi }=(33/2^{+})$. An extension of the high-lying negative-parity band, based on the $\pi \left[\left(g_{9/2}\right)\right]\otimes \nu \left[\left(g_{9/2}\right)\left(fp\right)\right]$ configuration, has been made up to $E_x\approx 5.1$ MeV, $J^{\pi }=(25/2^{-})$. The yrast positive-parity $\alpha =-1/2$ signature partner band has newly been identified and the occurrence of large signature splitting associated with the partner bands have been observed. A quasi-$\gamma$ vibrational band structure built on the nonyrast $13/2^{+}$ state has also newly been established. The experimental findings have been interpreted on the basis of the predicted results from the total Routhian surface, particle plus triaxial-rotor model, and shell-model calculations. The band crossing phenomena correlated to the $\nu \left(g_{9/2}\right)$ alignment and the perseverance of triaxial shape up to the highest observed excitation regime of the $9/2^{+}$ yrast positive-parity band have been discussed. The evidence for the possible onset of the stapler-like mechanism prevailing in the high-lying negative-parity band structure are presented.

The analysis of self-consistency and proton-neutron interaction effects in the buildup of differential charge radii were carried out in covariant density functional theoretical calculations without pairing interaction on the example of selected configurations in the Pb isotopes. The proton-neutron interaction of neutron(s) added to the neutron $N=126$ core and the protons forming the $Z=82$ proton core is responsible for a major contribution to the buildup of differential charge radii. It depends on the products of proton and neutron wave functions and thus on their nodal structure. This interaction leads to a redistribution of single-particle density of occupied proton states which in turn modifies the charge radii. The microscopic origin of this redistribution and its consequences for differential charge radii were investigated for the first time. Self-consistency effects affecting the shape of proton potential, total proton densities and the energies of the single-particle proton states provide only minor contribution to differential charge radii.