Odd-mass gadolinium isotopes around $N=90$ were populated by the ($p$,$d$) reaction, utilizing 25-MeV protons, resulting in population of low-spin quasineutron states at energies near and below the Fermi surface. Systematics of the single quasineutron levels populated are presented. A large excitation energy gap is observed between levels originating from the $2d_{3/2}$, $1h_{11/2}$, and $3s_{1/2}$ spherical parents (above the $N=64$ gap), and the $2d_{5/2}$ (below the gap), indicating that the spherical shell model level spacing is maintained at least to moderate deformations.

The ${}^8$He($d,p$) reaction was studied in inverse kinematics at 15.4$A$ MeV using the MUST2 Si-CsI array in order to shed light on the level structure of ${}^9$He. The well known ${}^{16}$O($d,p$)${}^{17}$O reaction, performed here in reverse kinematics, was used as a test to validate the experimental methods. The ${}^9$He missing mass spectrum was deduced from the kinetic energies and emission angles of the recoiling protons. Several structures were observed above the neutron-emission threshold and the angular distributions were used to deduce the multipolarity of the transitions. This work confirms that the ground state of ${}^9$He is located very close to the neutron threshold of ${}^8$He and supports the occurrence of parity inversion in ${}^9$He.

The ${}^{87}$Se nucleus has been studied via prompt $\gamma$-ray spectroscopy using the Eurogam2 Ge array to measure $\gamma$ rays following fission of ${}^{248}$Cm. Excited levels in ${}^{87}$Se have been observed for the first time. The yrast excitation scheme in this nucleus is similar to the excitations schemes of its $N=53$ neighbors and fits the energy systematics, indicating $j-1$ anomaly below $Z=38$. Large-scale shell-model calculations reproduce in detail yrast excitations in ${}^{87}$Se and other $N=53$ isotones. The $j-1$ anomaly is explained as due to the enhancement of collectivity towards the proton midshell. The coexistence of collective and single-particle excitations is predicted in ${}^{87}$Se.

A superdeformed (SD) band has been identified in a non-$\alpha$-conjugate nucleus ${}^{35}\text{Cl}$. It crosses the negative-parity ground band above $11/2^{-}$ and becomes the yrast at $15/2^{-}$. Lifetimes of all relevant states have been measured to follow the evolution of collectivity. Enhanced $B\left(E2\right)$, $B\left(E1\right)$ values as well as energetics provide evidence for superdeformation and existence of parity doublet cluster structure in an odd-$A$ nucleus in the $A\simeq 40$ region. Large-scale shell-model calculations assign ${\left(sd\right)}^{16}{\left(pf\right)}^3$ as the origin of these states. Calculated spectroscopic factors correlate the SD states in ${}^{35}\text{Cl}$ to those in ${}^{36}\text{Ar}$.

The spherical quasiparticle random-phase approximation method is used for calculations of the $\beta$-decay properties of neutron-rich nuclei in the region near the neutron magic numbers $N=82$ and $N=126$ which are important for determination of the $r$-process path. Our calculations differ from previous works by the use of realistic forces for the proton-neutron interaction. Both allowed and first-forbidden $\beta$ decays are included. Detailed comparisons with the experimental measurements and the previous shell-model calculations are performed. The results for half-lives and $\beta$-delayed neutron emission probabilities will serve as input for $r$-process nucleosynthesis simulations.

New polarization observables with which we can study the two-nucleon weak interactions at low energies are considered. In the breakup of the deuteron by photons, polarization of outgoing neutrons can depend on the parity-violating component of two-nucleon interactions. We express the parity-violating polarization in general forms, and perform numerical calculations with a pionless effective field theory. The theory has three unknown parity-violating low energy constants, and new polarization observables are expressed in linear combinations of them. We discuss how the unknown constants may be determined and their implication to the understanding of the hadronic weak interactions.

Background: In presolar low-density graphite grains, an extraordinarily large ${}^{22}$Ne/${}^{20}$Ne ratio or even nearly pure ${}^{22}$Ne is found, pointing to the condensation of radioactive ${}^{22}$Na in grains. Supernovae and neon-rich novae are the main events that produce ${}^{22}$Na via the explosive hydrogen burning process. The ${}^{22}$Na($p$, $\gamma$)${}^{23}$Mg reaction is one of the key reactions that influences the ${}^{22}$Na abundance in ejecta.

Purpose:The present work aims to explore the proton resonant states in ${}^{23}$Mg relevant to the astrophysical ${}^{22}$Na($p$, $\gamma$)${}^{23}$Mg reaction. The determined ${}^{23}$Mg resonant parameters can be used to evaluate the ${}^{22}$Na($p$, $\gamma$)${}^{23}$Mg reaction rate.

Method:A low-energy ${}^{22}$Na radioactive ion beam is produced via the ${}^1$H(${}^{22}$Ne, ${}^{22}$Na)$n$ reaction, and used to measure the experimental excitation function of the ${}^{22}$Na + $p$ resonant scattering with a conventional thick-target inverse kinematic method. $R$-matrix analysis is applied to deduce the ${}^{23}$Mg resonance parameters from the experimental excitation function.

Results: Three proton resonance states in ${}^{23}$Mg are observed. Spins/parities and the proton partial widths are determined. The deduced excitation energies agree with the compiled values.

Conclusions: The new spin and parity assignments allow us to perform a shell-model calculation of the $\gamma$ widths of the ${}^{23}$Mg resonant states for the evaluation of the ${}^{22}$Na($p$, $\gamma$)${}^{23}$Mg astrophysical reaction rate. The two $s$-wave resonant states established in this work at 8.793 and 8.916 MeV in ${}^{23}$Mg, respectively, increase the total reaction rate by about 5% at a temperature greater than 2 GK.

The study of neutron cross sections for elements used as efficient “absorbers” of ultracold neutrons (UCN) is crucial for many precision experiments in nuclear and particle physics, cosmology and gravity. In this context, “absorption” includes both the capture and upscattering of neutrons to the energies above the UCN energy region. The available data, especially for hydrogen, do not agree between themselves or with the theory. In this report we describe measurements performed at the Los Alamos National Laboratory UCN facility of the UCN upscattering cross sections for vanadium and for hydrogen in CH${}_2$ using simultaneous measurements of the radiative capture cross sections for these elements. We measured ${\sigma }_{\mathrm{up}}=1972\pm 130$ b for hydrogen in CH${}_2$, which is below theoretical expectations, and ${\sigma }_{\mathrm{up}}=25\pm 9$ b for vanadium, in agreement with the expectation for the neutron heating by thermal excitations in solids.

Decay mode of the $2_1^{+}$ resonant state of ${}^6$He populated by the ${}^6$He breakup reaction by ${}^{12}$C at 240 MeV/nucleon is investigated. The continuum-discretized coupled-channels method is adopted to describe the formation of the $2_1^{+}$ state, whereas its decay is described by the complex-scaled solutions of the Lippmann-Schwinger equation. From analysis of invariant mass spectra with respect to the $\alpha$-$n$ and $n$-$n$ subsystems, coexistence of two decay modes is found. One is the simultaneous decay of two neutrons correlating with each other and the other is the emission of two neutrons to the opposite directions. The latter is found to be free from the final-state interaction and suggests existence of a dineutron in the $2_1^{+}$ state of ${}^6$He.

We present the derivation of a novel third-order hydrodynamic evolution equation for the shear stress tensor from kinetic theory. The Boltzmann equation with a relaxation time approximation for the collision term is solved iteratively using a Chapman-Enskog-like expansion to obtain the nonequilibrium phase-space distribution function. Subsequently, the evolution equation for the shear stress tensor is derived from its kinetic definition up to third order in gradients. We quantify the significance of the new derivation within a one-dimensional scaling expansion and demonstrate that the results obtained using the third-order viscous equations derived here provides a very good approximation to the exact solution of the Boltzmann equation in a relaxation time approximation. We also show that the time evolution of pressure anisotropy obtained using our equations is in better agreement with transport results than that obtained with an existing third-order calculation based on the second law of thermodynamics.

The radial basis function (RBF) approach is applied in predicting nuclear masses for eight widely used nuclear mass models, ranging from macroscopic-microscopic to microscopic types. A significantly improved accuracy in computing nuclear masses is obtained, and the corresponding rms deviations with respect to the known masses are reduced by up to $78\%$. Moreover, strong correlations are found between a target nucleus and the reference nuclei within about three units in distance; these play critical roles in improving nuclear mass predictions. Based on the latest Weizsäcker-Skyrme mass model, the RBF approach can achieve an accuracy comparable with the extrapolation method used in atomic mass evaluation. In addition, the necessity of new high-precision experimental data to improve the mass predictions with the RBF approach is emphasized as well.

The structure of the neutron-rich nucleus ${}^{58}$Ti was investigated via proton inelastic scattering in inverse kinematics at a mean energy of 42.0 MeV/nucleon. By measuring the deexcitation $\gamma$ rays, three transitions with the energies of 1046(11) keV, 1376(18) keV, and 1835(27) keV were identified. The angle-integrated cross section for the 1046-keV excitation, which corresponds to the decay from the first 2${}^{+}$ state, was determined to be 13(7) mb. The deformation length ${\delta }_{p,p^{\prime }}$ was extracted from the cross section to be ${0.83}_{-0.30}^{+0.22}$ fm. The energy of the first 2${}^{+}$ state and the ${\delta }_{p,p^{\prime }}$ value are comparable to the ones of ${}^{56}$Ti, which indicates that the collectivity of the Ti isotopes does not increase significantly with neutron number until $N=36$. This fact indicates that ${}^{58}$Ti is outside of the region of the deformation known in the neutron-rich nuclei around $N=40$.

Applying a relatively simple particle-rotor model to odd-odd nuclei, the possible presence of multi-chiral-pair bands is looked for, where chiral-pair bands are defined not only by near degeneracy of the levels of two bands but also by similar expectation values of squared components of three angular momenta that define chirality. In the angular-momentum region where two pairs of chiral-pair bands are obtained, the possible interband $M1/E2$ decay from the second-lowest chiral-pair bands to the lowest chiral-pair bands is studied, with the intention of finding how to experimentally identify the multi-chiral-pair bands. It is found that up until almost the bandhead the intraband $M1/E2$ decay within the second chiral-pair bands is preferred rather than the interband $M1/E2$ decay to the lowest chiral-pair bands, though the decay possibility depends on the ratio of actual decay energies. It is also found that chiral-pair bands in our model and definition are hardly obtained for $\gamma$ values outside the range ${25}^{\circ }<\gamma <{35}^{\circ }$, although either a near-degeneracy or a constant energy difference of several hundred keV between the two levels for a given angular momentum $I$ in “a pair bands” is sometimes obtained in some limited region of $I$. In the present model calculations the energy difference between chiral-pair bands is always one or two orders of magnitude smaller than a few hundred keV, and no chiral-pair bands are obtained, which have an almost constant energy difference of the order of a few hundred keV in a reasonable range of $I$.

The abnormally large collectivity of neutron-rich magnesium isotopes in the “island of inversion” has not been well understood. It has been commented that the unexpectedly large deformations observed in the magnesium isotopes are attributed to the neutron $f_{7/2}$ intruder orbits involved remarkably even in the ground states, which points to nuclear force directly. Recently, a new isospin-dependent Nilsson potential was suggested to improve the calculations of the ground states of magnesium isotopes. With the improved Nilsson potential, in the present work we investigate the collectivity of excited states by using the projected shell model. To avoid the collapse of the BCS pairing, which occurs in weak pairing, we improved pairing calculations by using the Lipkin-Nogami approach. The collectivity and the erosion of the $N=20$ shell in neutron-rich magnesium isotopes are discussed by calculating spectra and electric quadrupole transitions. The gyromagnetic factors are calculated and compared with existing data to get insight into the configurations of collective states at different spins. A two-quasineutron $K^{\pi }=6^{+}$ state in ${}^{38}$Mg is predicted to be a possible isomer with an excitation energy lower than the $I^{\pi }=6^{+}$ member of the ground-state band.

We perform a study of ${}^{16}\mathrm{O}+{}^{16}\mathrm{O}$ fusion at above and below the interaction barrier energies using three-dimensional time-dependent Hartree-Fock (TDHF) calculations at above-barrier energies and density-constrained TDHF calculations for the entire energy range. We discuss the variations of the experimental data at above the barrier energies. Calculations reasonably reproduce the observed energy-dependent broad oscillations in the fusion excitation functions. These oscillations result from overcoming $L$-dependent fusion barriers. The role of the coupling to low-lying octupole states is also discussed.

Production cross sections of Rf isotopes in the ${}^{248}$Cm$+$${}^{18}$O reaction were measured at the beam energy range of 88.2 to 101.3 MeV by use of a gas-filled recoil ion separator. The excitation functions of ${}^{260}$Rf, ${}^{261}$Rf${}^a$, and ${}^{262}$Rf were obtained together with those of spontaneously fissioning nuclides which have few-second half-lives and have been assigned to ${}^{261}$Rf${}^b$ and a longer-lived state of ${}^{262}$Rf. The excitation function of few-second spontaneously fissioning nuclide exhibited the maximum cross section at the ${}^{18}$O beam energy of 94.8 MeV. The shape of the excitation function was almost the same as that of ${}^{261}$Rf${}^a$, whereas it was quite different from those of ${}^{260}$Rf and ${}^{262}$Rf. A few-second spontaneously fissioning nuclide previously reported as ${}^{261}$Rf${}^b$ and ${}^{262}$Rf observed in ${}^{248}$Cm$+$${}^{18}$O reaction was identified as ${}^{261}$Rf${}^b$.

Background: The discrepancy between proton electromagnetic form factors extracted using unpolarized and polarized scattering data is believed to be a consequence of two-photon exchange (TPE) effects. However, the calculations of TPE corrections have significant model dependence, and there is limited direct experimental evidence for such corrections.

Purpose: The TPE contributions depend on the sign of the lepton charge in $e^{\pm }p$ scattering, but the luminosities of secondary positron beams limited past measurement at large scattering angles, where the TPE effects are believe to be most significant. We present the results of a new experimental technique for making direct $e^{\pm }p$ comparisons, which has the potential to make precise measurements over a broad range in $Q^2$ and scattering angles.

Methods: We use the Jefferson Laboratory electron beam and the Hall B photon tagger to generate a clean but untagged photon beam. The photon beam impinges on a converter foil to generate a mixed beam of electrons, positrons, and photons. A chicane is used to separate and recombine the electron and positron beams while the photon beam is stopped by a photon blocker. This provides a combined electron and positron beam, with energies from 0.5 to 3.2 GeV, which impinges on a liquid hydrogen target. The large acceptance CLAS detector is used to identify and reconstruct elastic scattering events, determining both the initial lepton energy and the sign of the scattered lepton.

Results: The data were collected in two days with a primary electron beam energy of only 3.3 GeV, limiting the data from this run to smaller values of $Q^2$ and scattering angle. Nonetheless, this measurement yields a data sample for $e^{\pm }p$ with statistics comparable to those of the best previous measurements. We have shown that we can cleanly identify elastic scattering events and correct for the difference in acceptance for electron and positron scattering. Because we ran with only one polarity for the chicane, we are unable to study the difference between the incoming electron and positron beams. This systematic effect leads to the largest uncertainty in the final ratio of positron to electron scattering: $R=1.027\pm 0.005\pm 0.05$ for $\langle Q^2\rangle =0.206$ GeV${}^2$ and $0.830⩽\epsilon ⩽0.943$.

Conclusions: We have demonstrated that the tertiary $e^{\pm }$ beam generated using this technique provides the opportunity for dramatically improved comparisons of $e^{\pm }p$ scattering, covering a significant range in both $Q^2$ and scattering angle. Combining data with different chicane polarities will allow for detailed studies of the difference between the incoming $e^{+}$ and $e^{-}$ beams.

The breakup cross section (BUX) of ${}^{22}$C by ${}^{12}$C at 250 MeV/nucleon is evaluated by the continuum-discretized coupled-channels method incorporating the cluster-orbital shell model (COSM) wave functions. Contributions of the low-lying $0_2^{+}$ and $2_1^{+}$ resonances predicted by COSM to the BUX are investigated. The $2_1^{+}$ resonance gives a narrow peak in the BUX, as in usual resonant reactions, whereas the $0_2^{+}$ resonant cross section has a peculiar shape due to the coupling to the nonresonant continuum, i.e., the background-phase effect (BPE). By changing the scattering angle of ${}^{22}$C after the breakup, a variety of shapes of the $0_2^{+}$ resonant cross sections are obtained. The mechanism of the appearance of the sizable BPE in the breakup of ${}^{22}$C is discussed.

We examine the interplay between the percolation and the deconfinement phase transitions of Yang-Mills matter at finite temperature, quark chemical potential ${\mu }_Q$, and number of colors $N_c$. We find that, whereas the critical $N_c$ for percolation goes down with density, the critical $N_c$ for confinement generally goes up. Because of this, Yang-Mills matter falls into two qualitatively different regimes: the “low-$N_c$ limit,” where percolation does not occur because matter deconfines before it percolates, and the “high-$N_c$ limit,” where there are three distinct phases—confined, deconfined, and confined but percolating matter—characterizing Yang-Mills matter at finite temperature and density. The latter can be thought of as the recently conjectured “quarkyonic phase.” We attempt an estimate of the critical $N_c$ to see if the percolating phase can occur in our world. We find that, while percolation will not occur at normal nuclear density as in the large-$N_c$ limit, a sliver of the phase diagram in $N_c$, energy density and baryonic density where percolation occurs while confinement persists is possible. We conclude by speculating on the phenomenological properties of such percolating “quarkyonic” matter and suggesting avenues by which to study it quantitatively and to look for it in experiment.

Background: The inner crust of a neutron star is believed to consist of Coulomb-frustrated complex structures known as “nuclear pasta” that display interesting and unique low-energy dynamics.

Purpose: To elucidate the structure and composition of the neutron-star crust as a function of temperature, density, and proton fraction.

Methods: A new lattice-gas model, the charged Ising model (CIM), is introduced to simulate the behavior of neutron-star matter. Preliminary Monte Carlo simulations on ${30}^3$ lattices are performed for a variety of temperatures, densities, and proton fractions.

Results: Results are obtained for the heat capacity, pair-correlation function, and static structure factor for a variety of conditions appropriate to the inner stellar crust.

Conclusions: Although relatively simple, the CIM captures the essence of Coulomb frustration that is required to simulate the subtle dynamics of the inner stellar crust. Moreover, the computationally demanding long-range Coulomb interactions have been precomputed at the appropriate lattice sites prior to the start of the simulation, resulting in enormous computational gains. This work demonstrates the feasibility of future CIM simulations involving a large number of particles as a function of density, temperature, and proton fraction.