We have performed a direct measurement of the ${}^{19}\mathrm{Ne}(p,\gamma ){}^{20}\mathrm{Na}$ reaction in inverse kinematics using a beam of radioactive ${}^{19}\mathrm{Ne}$. The key astrophysical resonance in the ${}^{19}\mathrm{Ne}+p$ system has been definitely measured for the first time at $E_{\mathrm{c}.\mathrm{m}.}={456}_{-2}^{+5}\mathrm{keV}$ with an associated strength of ${17}_{-5}^{+7}\mathrm{meV}$. The present results are in agreement with resonance strength upper limits set by previous direct measurements, as well as resonance energies inferred from precision (${}^3\mathrm{He}$, $t$) charge exchange reactions. However, both the energy and strength of the 456 keV resonance disagree with a recent indirect study of the ${}^{19}\mathrm{Ne}(d,n){}^{20}\mathrm{Na}$ reaction. In particular, the new ${}^{19}\mathrm{Ne}(p,\gamma ){}^{20}\mathrm{Na}$ reaction rate is found to be factors of $\sim 8$ and $\sim 5$ lower than the most recent evaluation over the temperature range of oxygen-neon novae and astrophysical x-ray bursts, respectively. Nevertheless, we find that the ${}^{19}\mathrm{Ne}(p,\gamma ){}^{20}\mathrm{Na}$ reaction is likely to proceed fast enough to significantly reduce the flux of ${}^{19}\mathrm{F}$ in nova ejecta and does not create a bottleneck in the breakout from the hot CNO cycles into the $rp$ process.

We study the sequential breakup of $E/A=24.0\mathrm{MeV}$ ${}^7\mathrm{Li}$ projectiles excited through inelastic interactions with C, Be, and Al target nuclei. For peripheral events that do not excite the target, we find very large spin alignment of the excited ${}^7\mathrm{Li}$ projectiles longitudinal to the beam axis. This spin alignment is independent of the target used, and we propose a simple alignment mechanism that arises from an angular-momentum-excitation-energy mismatch. This mechanism is independent of the potential used for scattering and should be present in many scattering experiments.

Statistical reaction theories such as the Hauser-Feshbach theory assume that branching ratios follow Bohr’s compound nucleus hypothesis by factorizing into independent probabilities for different channels. Corrections to the factorization hypothesis are known in both nuclear theory and quantum transport theory, particularly an enhanced memory of the entrance channel. We apply the Gaussian orthogonal ensemble to study a complementary suppression of exit channel branching ratios. The combined effect of the width fluctuation and the limitation on the transmission coefficient can provide a lower bound on the number of exit channels. The bound is demonstrated for the branching ratio in neutron-induced reactions on a ${}^{235}\mathrm{U}$ target.

Nuclear clustering describes the appearance of structures resembling smaller nuclei such as alpha particles (${}^4\text{He}$ nuclei) within the interior of a larger nucleus. In this Letter, we present lattice Monte Carlo calculations based on chiral effective field theory for the ground states of helium, beryllium, carbon, and oxygen isotopes. By computing model-independent measures that probe three- and four-nucleon correlations at short distances, we determine the shape of the alpha clusters and the entanglement of nucleons comprising each alpha cluster with the outside medium. We also introduce a new computational approach called the pinhole algorithm, which solves a long-standing deficiency of auxiliary-field Monte Carlo simulations in computing density correlations relative to the center of mass. We use the pinhole algorithm to determine the proton and neutron density distributions and the geometry of cluster correlations in ${}^{12}\mathrm{C}$, ${}^{14}\mathrm{C}$, and ${}^{16}\mathrm{C}$. The structural similarities among the carbon isotopes suggest that ${}^{14}\mathrm{C}$ and ${}^{16}\mathrm{C}$ have excitations analogous to the well-known Hoyle state resonance in ${}^{12}\mathrm{C}$.

A nonlocal dispersive optical-model analysis has been carried out for neutrons and protons in ${}^{48}\mathrm{Ca}$. Elastic-scattering angular distributions, total and reaction cross sections, single-particle energies, the neutron and proton numbers, and the charge distribution have been fitted to extract the neutron and proton self-energies both above and below the Fermi energy. From the single-particle propagator resulting from these self-energies, we have determined the charge and neutron matter distributions in ${}^{48}\mathrm{Ca}$. A best fit neutron skin of $0.249\pm 0.023\mathrm{fm}$ is deduced, but values up to 0.33 fm are still consistent. The energy dependence of the total neutron cross sections is shown to have a strong sensitivity to the skin thickness.

Few-body nuclear physics often relies upon phenomenological models, with new efforts at the ab initio theory reported recently; both need high-quality benchmark data, particularly at low center-of-mass energies. We use high-energy-density plasmas to measure the proton spectra from ${}^3\mathrm{He}+\mathrm{T}$ and ${}^3\mathrm{He}+{}^3\mathrm{He}$ fusion. The data disagree with $R$-matrix predictions constrained by neutron spectra from $\mathrm{T}+\mathrm{T}$ fusion. We present a new analysis of the ${}^3\mathrm{He}+{}^3\mathrm{He}$ proton spectrum; these benchmarked spectral shapes should be used for interpreting low-resolution data, such as solar fusion cross-section measurements.

The atomic numbers and the masses of fragments formed in quasifission reactions are simultaneously measured at scission in ${}^{48}\mathrm{Ti}+{}^{238}\mathrm{U}$ reactions at a laboratory energy of 286 MeV. The atomic numbers are determined from measured characteristic fluorescence x rays, whereas the masses are obtained from the emission angles and times of flight of the two emerging fragments. For the first time, thanks to this full identification of the quasifission fragments on a broad angular range, the important role of the proton shell closure at $Z=82$ is evidenced by the associated maximum production yield, a maximum predicted by time-dependent Hartree-Fock calculations. This new experimental approach gives now access to precise studies of the time dependence of the $N/Z$ (neutron over proton ratios of the fragments) evolution in quasifission reactions.

Fission-fragment mass distributions were measured for ${}^{237–240}\mathrm{U}$, ${}^{239–242}\mathrm{Np}$, and ${}^{241–244}\mathrm{Pu}$ populated in the excitation-energy range from 10 to 60 MeV by multinucleon transfer channels in the reaction ${}^{18}\mathrm{O}+{}^{238}\mathrm{U}$ at the Japan Atomic Energy Agency tandem facility. Among them, the data for ${}^{240}\mathrm{U}$ and ${}^{240,241,242}\mathrm{Np}$ were observed for the first time. It was found that the mass distributions for all the studied nuclides maintain a double-humped shape up to the highest measured energy in contrast to expectations of predominantly symmetric fission due to the washing out of nuclear shell effects. From a comparison with the dynamical calculation based on the fluctuation-dissipation model, this behavior of the mass distributions was unambiguously attributed to the effect of multichance fission.

We investigate the influence of a strong laser electromagnetic field on the $\alpha$-decay rate by using the Hennenberger frame of reference. We introduce an adimensional parameter $D=S_0/R_0$, where $R_0$ is the geometrical nuclear radius and $S_0\sim \sqrt{I}/{\omega }^2$ is a length parameter depending on the laser intensity $I$ and frequency $\omega$. We show that the barrier penetrability has a strong increase for intensities corresponding to $D>D_{\text{crit}}=1$, due to the fact that the resulting Coulomb potential becomes strongly anisotropic even for spherical nuclei. As a consequence, the contribution of the monopole term increases the barrier penetrability by 2 orders of magnitude, while the total contribution has an effect of 6 orders of magnitude at $D\sim 3D_{\text{crit}}$. In the case of deformed nuclei, the electromagnetic field increases the penetrability by an additional order of magnitude for a quadrupole deformation ${\beta }_2\sim 0.3$. The influence of the electromagnetic field can be expressed in terms of a shifted Geiger-Nuttal law by a term depending on $S_0$ and deformation.

The nuclear matrix elements for the spin operator and the momentum quadrupole operator are important for the interpretation of precision atomic physics experiments that search for violations of local Lorentz and $CPT$ symmetry and for new spin-dependent forces. We use the configuration-interaction nuclear shell model and self-consistent mean-field theory to calculate the momentum matrix elements for ${}^{21}\mathrm{Ne}$, ${}^{23}\mathrm{Na}$, ${}^{133}\mathrm{Cs}$, ${}^{173}\mathrm{Yb}$, and ${}^{201}\mathrm{Hg}$. We show that these momentum matrix are strongly suppressed by the many-body correlations, in contrast to the well-known enhancement of the spatial quadrupole nuclear matrix elements.

In this Letter, the observation of two previously unknown isotopes is presented for the first time: ${}^{72}\mathrm{Rb}$ with 14 observed events and ${}^{77}\mathrm{Zr}$ with one observed event. From the nonobservation of the less proton-rich nucleus ${}^{73}\mathrm{Rb}$, we derive an upper limit for the ground-state half-life of 81 ns, consistent with the previous upper limit of 30 ns. For ${}^{72}\mathrm{Rb}$, we have measured a half-life of 103(22) ns. This observation of a relatively long-lived odd-odd nucleus, ${}^{72}\mathrm{Rb}$, with a less exotic odd-even neighbor, ${}^{73}\mathrm{Rb}$, being unbound shows the diffuseness of the proton drip line and the possibility of sandbanks to exist beyond it. The ${}^{72}\mathrm{Rb}$ half-life is consistent with a $5^{+}\to 5/2^{-}$ proton decay with an energy of 800–900 keV, in agreement with the atomic mass evaluation proton-separation energy as well as results from the finite-range droplet model and shell model calculations using the GXPF1A interaction. However, we cannot explicitly exclude the possibility of a proton transition between $9^{+}({}^{72}\mathrm{Rb})\to 9/2^{+}({}^{71}\mathrm{Kr})$ isomeric states with a broken mirror symmetry. These results imply that ${}^{72}\mathrm{Kr}$ is a strong waiting point in x-ray burst $rp$-process scenarios.

The gamma strength function and level density of $1^{-}$ states in ${}^{96}\mathrm{Mo}$ have been extracted from a high-resolution study of the ($\overrightarrow{p}$, ${\overrightarrow{p}}^{\prime }$) reaction at 295 MeV and extreme forward angles. By comparison with compound nucleus $\gamma$ decay experiments, this allows a test of the generalized Brink-Axel hypothesis in the energy region of the pygmy dipole resonance. The Brink-Axel hypothesis is commonly assumed in astrophysical reaction network calculations and states that the gamma strength function in nuclei is independent of the structure of the initial and final state. The present results validate the Brink-Axel hypothesis for ${}^{96}\mathrm{Mo}$ and provide independent confirmation of the methods used to separate gamma strength function and level density in $\gamma$ decay experiments.

The apparent splitting between orbitals that are spin-orbit partners can be substantially influenced by the effects of weak binding. In particular, such effects can account for the observed decrease in separation of the neutron $1p_{3/2}$ and $1p_{1/2}$ orbitals between the ${}^{41}\mathrm{Ca}$ and ${}^{35}\mathrm{Si}$ isotopes. This behavior has been the subject of recent experimental and theoretical works and cited as evidence for a proton “bubble” in ${}^{34}\mathrm{Si}$ causing an explicit weakening of the spin-orbit interaction. The results reported here suggest that the change in the separation between the $1p_{3/2}$ and $1p_{1/2}$ partners occurs dominantly because of the behavior of the energies of these $1p$ neutron states near zero binding.

We report the measurement of the beam-vector and tensor asymmetries $A_{ed}^V$ and $A_d^T$ in quasielastic $(\overrightarrow{e},e^{\prime }p)$ electrodisintegration of the deuteron at the MIT-Bates Linear Accelerator Center up to missing momentum of $500\mathrm{MeV}/c$. Data were collected simultaneously over a momentum transfer range $0.1<Q^2<0.5(\mathrm{GeV}/c{)}^2$ with the Bates Large Acceptance Spectrometer Toroid using an internal deuterium gas target polarized sequentially in both vector and tensor states. The data are compared with calculations. The beam-vector asymmetry $A_{ed}^V$ is found to be directly sensitive to the $D$-wave component of the deuteron and has a zero crossing at a missing momentum of about $320\mathrm{MeV}/c$, as predicted. The tensor asymmetry $A_d^T$ at large missing momentum is found to be dominated by the influence of the tensor force in the neutron-proton final-state interaction. The new data provide a strong constraint on theoretical models.

The charge and magnetic form factors, $F_C$ and $F_M$, respectively, of ${}^3\mathrm{He}$ are extracted in the kinematic range $25{\mathrm{fm}}^{-2}\le Q^2\le 61{\mathrm{fm}}^{-2}$ from elastic electron scattering by detecting ${}^3\mathrm{He}$ recoil nuclei and scattered electrons in coincidence with the two High Resolution Spectrometers of the Hall A Facility at Jefferson Lab. The measurements find evidence for the existence of a second diffraction minimum for the magnetic form factor at $Q^2=49.3{\mathrm{fm}}^{-2}$ and for the charge form factor at $Q^2=62.0{\mathrm{fm}}^{-2}$. Both minima are predicted to exist in the $Q^2$ range accessible by this Jefferson Lab experiment. The data are in qualitative agreement with theoretical calculations based on realistic interactions and accurate methods to solve the three-body nuclear problem.

The differential production cross sections of $B^{\pm }$ mesons are measured via the exclusive decay channels $B^{\pm }\to J/\psi K^{\pm }\to {\mu }^{+}{\mu }^{-}{K}^{\pm }$ as a function of transverse momentum in $p$$p$ and Pb-Pb collisions at a center-of-mass energy $\sqrt{s_{NN}}=5.02\mathrm{TeV}$ per nucleon pair with the CMS detector at the LHC. The $pp(\mathrm{Pb-Pb})$ data set used for this analysis corresponds to an integrated luminosity of $28.0{\mathrm{pb}}^{-1}$ ($351\mu {\mathrm{b}}^{-1}$). The measurement is performed in the $B^{\pm }$ meson transverse momentum range of 7 to $50\mathrm{GeV}/c$, in the rapidity interval $|y|<2.4$. In this kinematic range, a strong suppression of the production cross section by about a factor of 2 is observed in the Pb-Pb system in comparison to the expectation from $pp$ reference data. These results are found to be roughly compatible with theoretical calculations incorporating beauty quark diffusion and energy loss in a quark-gluon plasma.

Direct laser excitation of the lowest known nuclear excited state in ${}^{229}\mathrm{Th}$ has been a long-standing objective. It is generally assumed that reaching this goal would require a considerably reduced uncertainty of the isomer’s excitation energy compared to the presently adopted value of $(7.8\pm 0.5)\mathrm{eV}$. Here we present a direct laser excitation scheme for ${}^{229m}\mathrm{Th}$, which circumvents this requirement. The proposed excitation scheme makes use of already existing laser technology and therefore paves the way for nuclear laser spectroscopy. In this concept, the recently experimentally observed internal-conversion decay channel of the isomeric state is used for probing the isomeric population. A signal-to-background ratio of better than $10^4$ and a total measurement time of less than three days for laser scanning appear to be achievable.