The $0^{+}$ ground state of the ${}^{10}\mathrm{He}$ nucleus produced in the ${}^3\mathrm{H}({}^8\mathrm{He},p){}^{10}\mathrm{He}$ reaction was found at about $2.1\pm 0.2\mathrm{MeV}$ ($\Gamma \sim 2\mathrm{MeV}$) above the three-body ${}^8\mathrm{He}+n+n$ breakup threshold. Angular correlations observed for ${}^{10}\mathrm{He}$ decay products show prominent interference patterns allowing us to draw conclusions about the structure of low-energy excited states. We interpret the observed correlations as a coherent superposition of a broad $1^{-}$ state having a maximum at energy 4–6 MeV and a $2^{+}$ state above 6 MeV, setting both on top of the $0^{+}$ state “tail.” This anomalous level ordering indicates that the breakdown of the $N=8$ shell known in ${}^{12}\mathrm{Be}$ thus extends also to the ${}^{10}\mathrm{He}$ system.

The current evaluation of the triple-$\alpha$ reaction rate assumes that the $\alpha$ decay of the 7.65 MeV, $0^{+}$ state in ${}^{12}\mathrm{C}$, commonly known as the Hoyle state, proceeds sequentially via the ground state of ${}^8\mathrm{Be}$. This assumption is challenged by the recent identification of two direct $\alpha$-decay branches with a combined branching ratio of $17(5)\%$. If correct, this would imply a corresponding reduction in the triple-$\alpha$ reaction rate with important astrophysical consequences. We have used the ${}^{11}\mathrm{B}({}^3\mathrm{He},d)$ reaction to populate the Hoyle state and measured the decay to three $\alpha$ particles in complete kinematics. We find no evidence for direct $\alpha$-decay branches, and hence our data do not support a revision of the triple-$\alpha$ reaction rate. We obtain an upper limit of $5\times {10}^{-3}$ on the direct $\alpha$ decay of the Hoyle state at $95\%$ C.L., which is 1 order of magnitude better than a previous upper limit.

The best examples of halo nuclei, exotic systems with a diffuse nuclear cloud surrounding a tightly bound core, are found in the light, neutron-rich region, where the halo neutrons experience only weak binding and a weak, or no, potential barrier. Modern direct-reaction measurement techniques provide powerful probes of the structure of exotic nuclei. Despite more than four decades of these studies on the benchmark one-neutron halo nucleus ${}^{11}\mathrm{Be}$, the spectroscopic factors for the two bound states remain poorly constrained. In the present work, the ${}^{10}\mathrm{Be}(d,\text{}p)$ reaction has been used in inverse kinematics at four beam energies to study the structure of ${}^{11}\mathrm{Be}$. The spectroscopic factors extracted using the adiabatic model were found to be consistent across the four measurements and were largely insensitive to the optical potential used. The extracted spectroscopic factor for a neutron in an $n\ell j=2s_{1/2}$ state coupled to the ground state of ${}^{10}\mathrm{Be}$ is 0.71(5). For the first excited state at 0.32 MeV, a spectroscopic factor of 0.62(4) is found for the halo neutron in a $1p_{1/2}$ state.

We report on the first experimental study of quadrupole collectivity in the very neutron-rich nuclei ${}^{47,48}\mathrm{Ar}$ using intermediate-energy Coulomb excitation. These nuclei are located along the path from doubly magic Ca to collective S and Si isotopes, a critical region of shell evolution and structural change. The deduced $B(E2)$ transition strengths are confronted with large-scale shell-model calculations in the sdpf shell using the state-of-the-art SDPF-Uand EPQQM effective interactions. The comparison between experiment and theory indicates that a shell-model description of Ar isotopes around $N=28$ remains a challenge.

Clustering in low density nuclear matter has been investigated using the NIMROD multidetector at Texas A&M University. Thermal coalescence modes were employed to extract densities, $\rho$, and temperatures, $T$, for evolving systems formed in collisions of $47A$ MeV ${}^{40}\mathrm{Ar}+{}^{112}\mathrm{Sn}$, ${}^{124}\mathrm{Sn}$ and ${}^{64}\mathrm{Zn}+{}^{112}\mathrm{Sn}$, ${}^{124}\mathrm{Sn}$. The yields of $d$, $t$, ${}^3\mathrm{He}$, and ${}^4\mathrm{He}$ have been determined at $\rho =0.002$ to 0.03 $\mathrm{nucleons}/{\mathrm{fm}}^3$ and $T=5$ to 11 MeV. The experimentally derived equilibrium constants for $\alpha$ particle production are compared with those predicted by a number of astrophysical equations of state. The data provide important new constraints on the model calculations.

A new experimental technique is presented using proton-$\gamma \mathrm{\text{−}}\gamma$ correlations from ${}^{94}\mathrm{Mo}(d,p){}^{95}\mathrm{Mo}$ reactions which allows for the model-independent extraction of the photon strength function at various excitation energies using primary $\gamma$-ray decay from the quasicontinuum to individual low-lying levels. Detected particle energies provide the entrance excitation energies into the residual nucleus while $\gamma$-ray transitions from low-lying levels specify the discrete states being fed. Results strongly support the existence of the previously reported low-energy enhancement in the photon strength function.

A measurement of the final state distribution of the ${}^8\mathrm{B}$ $\beta$ decay, obtained by implanting a ${}^8\mathrm{B}$ beam in a double-sided silicon strip detector, is reported here. The present spectrum is consistent with a recent independent precise measurement performed by our collaboration at the IGISOL facility, Jyväskylä [O. S. Kirsebom et al., Phys. Rev. C 83, 065802 (2011)]. It shows discrepancies with previously measured spectra, leading to differences in the derived neutrino spectrum. Thanks to a low detection threshold, the neutrino spectrum is for the first time directly extracted from the measured final state distribution, thus avoiding the uncertainties related to the extrapolation of $R$-matrix fits. Combined with the IGISOL data, this leads to an improvement of the overall errors and the extension of the neutrino spectrum at high energy. The new unperturbed neutrino spectrum represents a benchmark for future measurements of the solar neutrino flux as a function of energy.

We report on the spectroscopic quadrupole moment measurement of the $7/2_1^{-}$ isomeric state in ${}_{16}{}^{43}\mathrm{S}_{27}$ [$E^{*}=320.5(5)\mathrm{keV}$, $T_{1/2}=415(3)\mathrm{ns}$], using the time dependent perturbed angular distribution technique at the RIKEN RIBF facility. Our value, $\mid Q_s\mid =23(3)e{\mathrm{fm}}^2$, is larger than that expected for a single-particle state. Shell model calculations using the modern SDPF-U interaction for this mass region reproduce remarkably well the measured $\mid Q_s\mid$, and show that non-negligible correlations drive the isomeric state away from a purely spherical shape.

We evaluate quasielastic double-differential antineutrino cross sections obtained in a phenomenological model based on the superscaling behavior of electron scattering data and estimate the contribution of the vector meson-exchange currents in the two-particle–two-hole sector. We show that the impact of meson-exchange currents for charge-changing antineutrino reactions is much larger than in the neutrino case.

Evidence for the ground state of the neutron-unbound nucleus ${}^{26}\mathrm{O}$ was observed for the first time in the single proton-knockout reaction from a $82\mathrm{MeV}/\mathrm{u}$ ${}^{27}\mathrm{F}$ beam. Neutrons were measured in coincidence with ${}^{24}\mathrm{O}$ fragments. ${}^{26}\mathrm{O}$ was determined to be unbound by ${150}_{-150}^{+50}\mathrm{keV}$ from the observation of low-energy neutrons. This result agrees with recent shell-model calculations based on microscopic two- and three-nucleon forces.

The nuclear charge radius of ${}^{12}\mathrm{Be}$ was precisely determined using the technique of collinear laser spectroscopy on the $2s_{1/2}\to 2p_{1/2,3/2}$ transition in the ${\mathrm{Be}}^{+}$ ion. The mean square charge radius increases from ${}^{10}\mathrm{Be}$ to ${}^{12}\mathrm{Be}$ by $\delta ⟨r_c^2{⟩}^{10,12}=0.69(5){\mathrm{fm}}^2$ compared to $\delta ⟨r_c^2{⟩}^{10,11}=0.49(5){\mathrm{fm}}^2$ for the one-neutron halo isotope ${}^{11}\mathrm{Be}$. Calculations in the fermionic molecular dynamics approach show a strong sensitivity of the charge radius to the structure of ${}^{12}\mathrm{Be}$. The experimental charge radius is consistent with a breakdown of the $N=8$ shell closure.

We obtained the total radiation widths of $s$-wave resonances through an $\mathcal{R}$-matrix analysis of ${}^{147}\mathrm{Sm}(n,\gamma )$ cross sections. Distributions of these widths differ markedly for resonances below and above $E_n=300\mathrm{eV}$, which is in stark contrast to long-established theory. We show that this change, as well as a similar change in the neutron-width distribution reported previously, is reflected in abrupt increases in both the average ${}^{147}\mathrm{Sm}(n,\gamma )$ cross section and fluctuations about the average near 300 eV. Such effects could have important consequences for applications such as nuclear astrophysics and nuclear criticality safety.

$\gamma$ softness in atomic nuclei is investigated in the framework of energy density functionals. By mapping constrained microscopic energy surfaces for a set of representative nonaxial medium-heavy and heavy nuclei to a Hamiltonian of the proton-neutron interacting boson model (IBM-2) containing up to three-body interactions, low-lying collective spectra and transition rates are calculated. Observables are analyzed that distinguish between the two limiting geometrical pictures of nonaxial nuclei: the rigid-triaxial rotor and the $\gamma$-unstable rotor. It is shown that neither of these pictures is realized in actual nuclei, and that a microscopic description leads to results that are almost exactly in between the two geometrical limits. This finding points to the optimal choice of the IBM Hamiltonian for $\gamma$-soft nuclei.

Studies of ${}^6\mathrm{He}$ beta decay along with tritium can play an important role in testing ab initio nuclear wave-function calculations and may allow for fixing low-energy constants in effective-field theories. Here, we present an improved determination of the ${}^6\mathrm{He}$ half-life to a relative precision of $3\times {10}^{-4}$. Our value of $806.89\pm {0.11}_{\mathrm{stat}}{\genfrac{}{}{0ex}{}{+0.23}{-0.19}}_{\mathrm{syst}}\mathrm{ms}$ resolves a major discrepancy between previous measurements. Calculating the statistical rate function we determined the $ft$ value to be ${803.04}_{-0.23}^{+0.26}\mathrm{s}$. The extracted Gamow-Teller matrix element agrees within a few percent with ab initio calculations.

We demonstrate the QCD factorization for inclusive hadron production in $p\mathrm{\text{−}}A$ collisions in the saturation formalism at one-loop order, with explicit calculation of both real and virtual gluon radiation diagrams. In particular, we find that the cross section can be written into a factorization form in the coordinate space at the next-to-leading order, while the naive form of the convolution in the transverse momentum space does not hold. The collinear divergences associated with the incoming parton distribution of the nucleon and the outgoing fragmentation function of the final-state hadron, as well as the rapidity divergence with small-$x$ dipole gluon distribution of the nucleus are factorized into the splittings of the associated parton distribution and fragmentation functions and the energy evolution of the dipole gluon distribution function. The hard coefficient function is evaluated at one-loop order, and contains no divergence.

We present a novel technique for studying the quenching of shell gaps in exotic isotopes. The method is based on extracting Gamow-Teller ($\Delta L=0$, $\Delta S=1$) transition strengths [$B(\mathrm{GT})$] to low-lying states from charge-exchange reactions at intermediate beam energies. These Gamow-Teller strengths are very sensitive to configuration mixing between cross-shell orbitals, and this technique thus provides an important complement to other tools currently used to study cross-shell mixing. This work focuses on the $N=8$ shell gap. We populated the ground and 2.24 MeV $0^{+}$ states in ${}^{12}\mathrm{Be}$ using the ${}^{12}\mathrm{B}(1^{+})$ (${}^7\mathrm{Li}$, ${}^7\mathrm{Be}$) reaction at $80\mathrm{MeV}/u$ in inverse kinematics. Using the ground-state $B(\mathrm{GT})$ value from $\beta$-decay measurements ($0.184\pm 0.007$) as a calibration, the $B(\mathrm{GT})$ for the transition to the second $0^{+}$ state was determined to be $0.214\pm 0.051$. Comparing the extracted Gamow-Teller strengths with shell-model calculations, it was determined that the wave functions of the first and second $0^{+}$ states in ${}^{12}\mathrm{Be}$ are composed of $25\pm 5\%$ and $60\pm 5\%$ $(0s{)}^4(0p{)}^8$ configurations, respectively.

An atomic clock based on x-ray fluorescence yields has been used to estimate the mean characteristic time for fusion followed by fission in reactions ${}^{238}\mathrm{U}+{}^{64}\mathrm{Ni}$ at $6.6\mathrm{MeV}/\mathrm{A}$. Inner shell vacancies are created during the collisions in the electronic structure of the possibly formed $Z=120$ compound nuclei. The filling of these vacancies accompanied by a x-ray emission with energies characteristic of $Z=120$ can take place only if the atomic transitions occur before nuclear fission. Therefore, the x-ray yield characteristic of the united atom with 120 protons is strongly related to the fission time and to the vacancy lifetimes. $K$ x rays from the element with $Z=120$ have been unambiguously identified from a coupled analysis of the involved nuclear reaction mechanisms and of the measured photon spectra. A minimum mean fission time ${\tau }_f=2.5\times {10}^{-18}\mathrm{s}$ has been deduced for $Z=120$ from the measured x-ray multiplicity.

We report the first measurement of the parity-violating asymmetry $A_{\mathrm{PV}}$ in the elastic scattering of polarized electrons from ${}^{208}\mathrm{Pb}$. $A_{\mathrm{PV}}$ is sensitive to the radius of the neutron distribution ($R_n$). The result $A_{\mathrm{PV}}=0.656\pm 0.060(\mathrm{stat})\pm 0.014(\mathrm{syst})\mathrm{ppm}$ corresponds to a difference between the radii of the neutron and proton distributions $R_n-R_p={0.33}_{-0.18}^{+0.16}\mathrm{fm}$ and provides the first electroweak observation of the neutron skin which is expected in a heavy, neutron-rich nucleus.

The inelastic response of a bosonic trimer is explored in the confines of the Borromean region. To this end we model the interaction between the external field and the bosonic system as a photoabsorptionlike process and study the response of the trimer in the quadrupole approximation. We utilize the hyperspherical-harmonics expansion to solve the Schrödinger equation and the Lorentz integral transform method to calculate the reaction. It is found that the magnitude of the response function and corresponding sum rules increase exponentially when approaching the 3-body threshold. It is also found that this increase is governed by unnatural exponents. The connection between our results and radio-frequency experiments in ultracold atom systems is made.

We report on the first observation of dineutron emission in the decay of ${}^{16}\mathrm{Be}$. A single-proton knockout reaction from a $53\mathrm{MeV}/\mathrm{u}$ ${}^{17}\mathrm{B}$ beam was used to populate the ground state of ${}^{16}\mathrm{Be}$. ${}^{16}\mathrm{Be}$ is bound with respect to the emission of one neutron and unbound to two-neutron emission. The dineutron character of the decay is evidenced by a small emission angle between the two neutrons. The two-neutron separation energy of ${}^{16}\mathrm{Be}$ was measured to be 1.35(10) MeV, in good agreement with shell model calculations, using standard interactions for this mass region.