Twenty-one two-proton knockout $(p,3p)$ cross sections were measured from neutron-rich nuclei at $\sim 250\mathrm{MeV}$/nucleon in inverse kinematics. The angular distribution of the three emitted protons was determined for the first time, demonstrating that the $(p,3p)$ kinematics are consistent with two sequential proton-proton collisions within the projectile nucleus. Ratios of $(p,3p)$ over $(p,2p)$ inclusive cross sections follow the trend of other many-nucleon removal reactions, further reinforcing the sequential nature of $(p,3p)$ in neutron-rich nuclei.

The prediction of stellar $(\gamma ,\alpha )$ reaction rates for heavy nuclei is based on the calculation of $(\alpha ,\gamma )$ cross sections at sub-Coulomb energies. These rates are essential for modeling the nucleosynthesis of so-called $p$ nuclei. The standard calculations in the statistical model show a dramatic sensitivity to the chosen $\alpha$-nucleus potential. The present study explains the reason for this dramatic sensitivity which results from the tail of the imaginary $\alpha$-nucleus potential in the underlying optical model calculation of the total reaction cross section. As an alternative to the optical model, a simple barrier transmission model is suggested. It is shown that this simple model in combination with a well-chosen $\alpha$-nucleus potential is able to predict total $\alpha$-induced reaction cross sections for a wide range of heavy target nuclei above $A\gtrsim 150$ with uncertainties below a factor of 2. The new predictions from the simple model do not require any adjustment of parameters to experimental reaction cross sections whereas in previous statistical model calculations all predictions remained very uncertain because the parameters of the $\alpha$-nucleus potential had to be adjusted to experimental data. The new model allows us to predict the reaction rate of the astrophysically important ${}^{176}\mathrm{W}(\alpha ,\gamma ){}^{180}\mathrm{Os}$ reaction with reduced uncertainties, leading to a significantly lower reaction rate at low temperatures. The new approach could also be validated for a broad range of target nuclei from $A\approx 60$ up to $A\gtrsim 200$.

The discovery of presolar grains in primitive meteorites has initiated a new era of research in the study of stellar nucleosynthesis. However, the accurate classification of presolar grains as being of specific stellar origins is particularly challenging. Recently, it has been suggested that sulfur isotopic abundances may hold the key to definitively identifying presolar grains with being of nova origins and, in this regard, the astrophysical ${}^{33}\mathrm{Cl}(p,\gamma ){}^{34}\mathrm{Ar}$ reaction is expected to play a decisive role. As such, we have performed a detailed $\gamma$-ray spectroscopy study of ${}^{34}\mathrm{Ar}$. Excitation energies have been measured with high precision and spin-parity assignments for resonant states, located above the proton threshold in ${}^{34}\mathrm{Ar}$, have been made for the first time. Uncertainties in the ${}^{33}\mathrm{Cl}(p,\gamma )$ reaction have been dramatically reduced and the results indicate that a newly identified $\ell =0$ resonance at $E_r=396.9(13)\mathrm{keV}$ dominates the entire rate for $T=0.25–0.40\mathrm{GK}$. Furthermore, nova hydrodynamic simulations based on the present work indicate an ejected ${}^{32}\mathrm{S}/{}^{33}\mathrm{S}$ abundance ratio distinctive from type-II supernovae and potentially compatible with recent measurements of a presolar grain.

Transverse single-spin asymmetries of very forward neutral pions generated in polarized $p+p$ collisions allow us to understand the production mechanism in terms of perturbative and nonperturbative strong interactions. During 2017, the RHICf Collaboration installed an electromagnetic calorimeter in the zero-degree region of the STAR detector at the Relativistic Heavy Ion Collider (RHIC) and measured neutral pions produced at pseudorapidity larger than 6 in polarized $p+p$ collisions at $\sqrt{s}=510\mathrm{GeV}$. The large nonzero asymmetries increasing both in longitudinal momentum fraction $x_F$ and transverse momentum $p_T$ have been observed at low transverse momentum $p_T<1\mathrm{GeV}/c$ for the first time, at this collision energy. The asymmetries show an approximate $x_F$ scaling in the $p_T$ region where nonperturbative processes are expected to dominate. A non-negligible contribution from soft processes may be necessary to explain the nonzero neutral pion asymmetries.

Excitation of the anomalously low lying nuclear isomer ${}^{229m}\mathrm{Th}(3/2^{+},8.28\pm 0.17\mathrm{eV})$ in the process of inelastic electron scattering is studied theoretically in the framework of the perturbation theory for the quantum electrodynamics. The calculated cross sections of ${}^{229m}\mathrm{Th}$ by the extremely low energy electrons in the range 9–12 eV for the Th atom and ${\mathrm{Th}}^{1+,4+}$ ions lie in the range ${10}^{-25}–{10}^{-26}{\mathrm{cm}}^2$. Being so large, the cross section opens up new possibilities for the effective nonresonant excitation of ${}^{229m}\mathrm{Th}$ in experiments with an electron beam or electron (electric) current. This can be crucial, since the energy of the isomeric state is currently known with an accuracy insufficient for the resonant excitation by photons. In addition, the cross section of the time reversed process is also large, and as a consequence, the probability of the nonradiative ${}^{229m}\mathrm{Th}$ decay via the conduction electrons in metal is $\approx {10}^6{\mathrm{s}}^{-1}$, that is, close to the internal conversion probability in the Th atom.

We report on a comparison between the theoretically predicted and experimentally measured spectra of the first-forbidden nonunique $\beta$-decay transition ${}^{137}\mathrm{Xe}(7/2^{-})\to {}^{137}\mathrm{Cs}(7/2^{+})$. The experimental data were acquired by the EXO-200 experiment during a deployment of an AmBe neutron source. The ultralow background environment of EXO-200, together with dedicated source deployment and analysis procedures, allowed for collection of a pure sample of the decays, with an estimated signal to background ratio of more than 99 to 1 in the energy range from 1075 to 4175 keV. In addition to providing a rare and accurate measurement of the first-forbidden nonunique $\beta$-decay shape, this work constitutes a novel test of the calculated electron spectral shapes in the context of the reactor antineutrino anomaly and spectral bump.

Working with Hamiltonians from chiral effective field theory, we develop a novel framework for describing arbitrary deformed medium-mass nuclei by combining the in-medium similarity renormalization group with the generator coordinate method. The approach leverages the ability of the first method to capture dynamic correlations and the second to include collective correlations without violating symmetries. We use our scheme to compute the matrix element that governs the neutrinoless double beta decay of ${}^{48}\mathrm{Ca}$ to ${}^{48}\mathrm{Ti}$, and find it to have the value 0.61, near or below the predictions of most phenomenological methods. The result opens the door to ab initio calculations of the matrix elements for the decay of heavier nuclei such as ${}^{76}\mathrm{Ge}$, ${}^{130}\mathrm{Te}$, and ${}^{136}\mathrm{Xe}$.

The ground-state-to-ground-state $\beta$-decay $Q$ value of ${}^{135}\mathrm{Cs}(7/2^{+})\to {}^{135}\mathrm{Ba}(3/2^{+})$ has been directly measured for the first time. The measurement was done utilizing both the phase-imaging ion-cyclotron resonance technique and the time-of-flight ion-cyclotron resonance technique at the JYFLTRAP Penning-trap setup and yielded a mass difference of 268.66(30) keV between ${}^{135}\mathrm{Cs}(7/2^{+})$ and ${}^{135}\mathrm{Ba}(3/2^{+})$. With this very small uncertainty, this measurement is a factor of 3 more precise than the currently adopted $Q$ value in the Atomic Mass Evaluation 2016. The measurement confirms that the first-forbidden unique ${\beta }^{-}$-decay transition ${}^{135}\mathrm{Cs}(7/2^{+})\to {}^{135}\mathrm{Ba}(11/2^{-})$ is a candidate for antineutrino mass measurements with an ultralow $Q$ value of 0.44(31) keV. This $Q$ value is almost an order of magnitude smaller than those of nuclides presently used in running or planned direct (anti)neutrino mass experiment.

We report the measurement of reaction cross sections (${\sigma }_R^{\mathrm{ex}}$) of ${}^{27,29}\mathrm{F}$ with a carbon target at RIKEN. The unexpectedly large ${\sigma }_R^{\mathrm{ex}}$ and derived matter radius identify ${}^{29}\mathrm{F}$ as the heaviest two-neutron Borromean halo to date. The halo is attributed to neutrons occupying the $2p_{3/2}$ orbital, thereby vanishing the shell closure associated with the neutron number $N=20$. The results are explained by state-of-the-art shell model calculations. Coupled-cluster computations based on effective field theories of the strong nuclear force describe the matter radius of ${}^{27}\mathrm{F}$ but are challenged for ${}^{29}\mathrm{F}$.

Here we present new information on the shape evolution of the very neutron-rich ${}^{92,94}\mathrm{Se}$ nuclei from an isomer-decay spectroscopy experiment at the Radioactive Isotope Beam Factory at RIKEN. High-resolution germanium detectors were used to identify delayed $\gamma$ rays emitted following the decay of their isomers. New transitions are reported extending the previously known level schemes. The isomeric levels are interpreted as originating from high-$K$ quasineutron states with an oblate deformation of $\beta \sim 0.25$, with the high-$K$ state in ${}^{94}\mathrm{Se}$ being metastable and $K$ hindered. Following this, ${}^{94}\mathrm{Se}$ is the lowest-mass neutron-rich nucleus known to date with such a substantial $K$ hindrance. Furthermore, it is the first observation of an oblate $K$ isomer in a deformed nucleus. This opens up the possibility for a new region of $K$ isomers at low $Z$ and at oblate deformation, involving the same neutron orbitals as the prolate orbitals within the classic $Z\sim 72$ deformed hafnium region. From an interpretation of the level scheme guided by theoretical calculations, an oblate deformation is also suggested for the ${}^{94}{\mathrm{Se}}_{60}$ ground-state band.

The beta decay of tritium in the form of molecular ${\mathrm{T}}_2$ is the basis of sensitive experiments to measure neutrino mass. The final-state electronic, vibrational, and rotational excitations modify the beta spectrum significantly and are obtained from theory. We report measurements of the branching ratios to specific ionization states for the isotopolog HT. Two earlier, concordant measurements gave branching ratios of HT to the bound ${\mathrm{HHe}}^{+}$ ion of 89.5% and 93.2%, in sharp disagreement with the theoretical prediction of 55%–57%, raising concerns about the theory’s reliability in neutrino mass experiments. Our result, 56.5(6)%, is compatible with the theoretical expectation and disagrees strongly with the previous measurements.

The possibility to control the $\alpha$ decay channel of atomic nuclei with electromagnetic fields of extreme intensities envisaged for the near future at multipetawatt and exawatt laser facilities is investigated theoretically. Using both analytic arguments based on the Wentzel-Kramers-Brillouin approximation and numerical calculations for the imaginary time method applied in the framework of the $\alpha$ decay precluster model, we show that no experimentally detectable modification of the $\alpha$ decay rate can be observed with super-intense lasers at any so-far-available wavelength. Comparing our predictions with those reported in several recent publications, where a considerable or even giant laser-induced enhancement of the decay rate has been claimed, we identify there the misuse of a standard approximation.

Understanding the dynamics of equilibration processes in quantum systems as well as their interplay with dissipation and fluctuation is a major challenge in quantum many-body theory. The timescales of such processes are investigated in collisions of atomic nuclei using fully microscopic approaches. Results from time-dependent Hartree-Fock and time-dependent random-phase approximation calculations are compared for 13 systems over a broad range of energies. The timescale for full mass equilibration ($\sim 2\times {10}^{-20}\mathrm{s}$) is found to be much larger than timescales for neutron-to-proton equilibration, kinetic energy, and angular momentum dissipations which are on the order of ${10}^{-21}\mathrm{s}$. Fluctuations of mass numbers in the fragments and correlations between their neutron and proton numbers build up within only a few ${10}^{-21}\mathrm{s}$. This indicates that dissipation is basically not impacted by mass equilibration, but is mostly driven by the exchange of nucleons between the fragments.

The structure of a neutron-rich ${}^{25}\mathrm{F}$ nucleus is investigated by a quasifree ($p,2p$) knockout reaction at $270A\mathrm{MeV}$ in inverse kinematics. The sum of spectroscopic factors of $\pi 0{\mathrm{d}}_{5/2}$ orbital is found to be $1.0\pm 0.3$. However, the spectroscopic factor with residual ${}^{24}\mathrm{O}$ nucleus being in the ground state is found to be only $0.36\pm 0.13$, while those in the excited state is $0.65\pm 0.25$. The result shows that the ${}^{24}\mathrm{O}$ core of ${}^{25}\mathrm{F}$ nucleus significantly differs from a free ${}^{24}\mathrm{O}$ nucleus, and the core consists of $\sim 35\%$ ${{}^{24}\mathrm{O}}_{\mathrm{g}.\mathrm{s}.}$. and $\sim 65\%$ excited ${}^{24}\mathrm{O}$. The result may infer that the addition of the $0d_{5/2}$ proton considerably changes neutron structure in ${}^{25}\mathrm{F}$ from that in ${}^{24}\mathrm{O}$, which could be a possible mechanism responsible for the oxygen dripline anomaly.

Taking benefit of the $R3B/\mathrm{SOFIA}$ setup to measure the mass and the nuclear charge of both fission fragments in coincidence with the total prompt-neutron multiplicity, the scission configurations are inferred along the thorium chain, from the asymmetric fission in the heavier isotopes to the symmetric fission in the neutron-deficient thorium. Against all expectations, the symmetric scission in the light thorium isotopes shows a compact configuration, which is in total contrast to what is known in the fission of the heavier thorium isotopes and heavier actinides. This new main symmetric scission mode is characterized by a significant drop in deformation energy of the fission fragments of about 19 MeV, compared to the well-known symmetric scission in the uranium-plutonium region.

We measured missing mass spectrum of the ${}^{12}\mathrm{C}(\gamma ,p)$ reaction for the first time in coincidence with potential decay products from ${\eta }^{\prime }$ bound nuclei. We tagged an ($\eta +p$) pair associated with the ${\eta }^{\prime }N\to \eta N$ process in a nucleus. After applying kinematical selections to reduce backgrounds, no signal events were observed in the bound-state region. An upper limit of the signal cross section in the opening angle $\cos {\theta }_{\mathrm{lab}}^{\eta p}<-0.9$ was obtained to be $2.2\mathrm{nb}/\mathrm{sr}$ at the 90% confidence level. It is compared with theoretical cross sections, whose normalization ambiguity is suppressed by measuring a quasifree ${\eta }^{\prime }$ production rate. Our results indicate a small branching fraction of the ${\eta }^{\prime }N\to \eta N$ process and/or a shallow ${\eta }^{\prime }$-nucleus potential.