Lifetimes of higher-lying states ( 2_2^+ and 4_1^+ ) in ^16 C have been measured, employing the Gammasphere and Microball detector arrays, as key observables to test and refine ab initio calculations based on interactions developed within chiral Effective Field Theory. The presented experimental constraints to these lifetimes of τ (2_2^+) = [ 244, 446] fs and τ (4_1^+) = [ 1.8, 4] ps , combined with previous results on the lifetime of the 2_1^+ state of ^16 C, provide a rather complete set of key observables to benchmark the theoretical developments. We present No-Core Shell-Model calculations using state-of-the-art chiral 2- (NN) and 3-nucleon (3N) interactions at next-to-next-to-next-to-leading order for both the NN and the 3N contributions and a generalized natural-orbital basis (instead of the conventional harmonic-oscillator single-particle basis) which reproduce, for the first time, the experimental findings remarkably well. The level of agreement of the new calculations as compared to the CD-Bonn meson-exchange NN interaction is notable and presents a critical benchmark for theory.

The resonant states in 20Mg and 19Mg have been studied by the invariant-mass reconstruction of 18Ne +2p and 17Ne +2p events, respectively. Two new resolved states (4+1 and 2+2 ) in 20Mg are found to decay through 19Na intermediate states to the ground state of 18Ne from the examination of the decay-energy spectra for 18Ne +1p subsystems. The deduced R42 = E & lowast;(4+1 )/E & lowast;(2+1 ) ratio of 2.10(1) is close to the expected harmonic-vibrator value and reveals a spherical or weakly deformed shape for 20Mg. Systematic comparison of E & lowast;(2+1 ) and R42 along isotopic and isotonic chains with shell model calculations, using the YSOX interaction, suggest the persistence of N = 8 shell around the proton drip-line. The relatively higher 2+1 excitation energy in 18Mg relative to 20Mg can be ascribed to different excitation characters of the valence protons. The 1/2+1 state in 19Mg has been observed for the first time. The energy differences between the 1/2-1 and 1/2+1 states in 19Mg and its isotones manifest the magnitude of the N = 8 shell gap on the proton-rich side.

Near-threshold resonances play an important role in nucleosynthesis and applied nuclear science. The study of nuclei removed from stability has greatly extended the list of resonances close to decay thresholds. The no core shell model with continuum (NCSMC) recently predicted an s-wave resonance just above the proton -decay threshold of 7Li at an excitation energy of 10.2 MeV. This potential case of a near-threshold resonance is dependent on the quantum mechanics of the p + 6He fragments extended into the continuum. The 6He(d, n) 7Li* reaction was employed in an attempt to populate this resonance and search for its proton decay via the invariant -mass method. No evidence of this resonance was found. However, the data collected in this search led to better constraints on the energy and width of the isobaric analog state (IAS) with (J �, T ) = (3/2-, 3/2) and revealed a new weak resonance just above the IAS in energy, predicted by the NCSMC as a (Jr, T ) = (3/2-, 1/2) state and potentially a part of the antianalog structure.

The boundaries of the Chart of Nuclides contain exotic isotopes that possess extreme proton-toneutron asymmetries. Here we report on strong evidence of 9N, one of the most exotic proton-rich isotopes where more than one half of its constitute nucleons are unbound. With seven protons and two neutrons, this extremely proton-rich system would represent the first-known example of a ground-state five-proton emitter. The invariant-mass spectrum of its decay products can be fit with two peaks whose energies are consistent with the theoretical predictions of an open-quantum-system approach, however we cannot rule out the possibility that only a single resonance-like peak is present in the spectrum.

States in S-28 and Cl-30 have been studied using one- and two-proton decay spectroscopies. In the first spectrometer setting, states in( 28)S were populated following one-neutron knockout from a fast S-29 beam. Three new states are observed in S-28 from one- and two-proton decays. For the two-proton case the nature of the decay was investigated and found to proceed via sequential two-proton emission. For the second setting, states in Cl-30 were populated via one-proton knockout from a fast Ar-31 beam. The decay energy of the ground and first excited state were measured with the ground-state decay energy found to be in disagreement with a previous measurement. The spin and parity of these two Cl-30 states were inferred from shell-model calculations.

This work describes a silicon tracker system developed for experiments with proton-rich radioactive ion beams at the SAMURAI superconducting spectrometer of RIBF at RIKEN. The system is designed for accurate angular reconstruction and atomic number identification of relativistic heavy ions and protons which are simultaneously produced in reactions motivated by studies of proton capture reactions of interest for nuclear astrophysics. The technical characteristics of the tracking array are described in detail as are its performance in two pilot experiments. The physics justification for such a system is also presented.

States in $^{28}\mathrm{S}$ and $^{30}\mathrm{Cl}$ have been studied using one- and two-proton decay spectroscopies. In the first spectrometer setting, states in $^{28}\mathrm{S}$ were populated following one-neutron knockout from a fast $^{29}\mathrm{S}$ beam. Three new states are observed in $^{28}\mathrm{S}$ from one- and two-proton decays. For the two-proton case the nature of the decay was investigated and found to proceed via sequential two-proton emission. For the second setting, states in $^{30}\mathrm{Cl}$ were populated via one-proton knockout from a fast $^{31}\mathrm{Ar}$ beam. The decay energy of the ground and first excited state were measured with the ground-state decay energy found to be in disagreement with a previous measurement. The spin and parity of these two $^{30}\mathrm{Cl}$ states were inferred from shell-model calculations.

Excited states in O-13 were investigated using inelastic scattering of an E/A = 69.5 MeV O-13 beam off of a Be-9 target. The excited states were identified in the invariant-mass spectra of the decay products. Both single-proton and sequential two-proton decays of the excited states were examined. For a number of the excited states, the protons were emitted with strong anisotropy where emissions transverse to the beam axis are favored. The measured proton-decay angular distributions were compared to predictions from distorted-wave Born-approximation calculations of the spin alignment which was shown to be largely independent of the excitation mechanism. The deduced O-13 level scheme is compared to ab initio no-core shell model with continuum predictions. The lowest-energy excited states decay isotropically consistent with predictions of strong proton 1s(1/2) structure. Above these states in the level scheme, we observed a number of higher-spin states not predicted within the model. Possibly these are associated with rotational bands built on deformed cluster configurations predicted by antisymmetrized molecular dynamics calculations. The spin alignment mechanism is shown to be useful for making spin assignments and may have widespread use.

^{18}Mg was observed, for the first time, by the invariant-mass reconstruction of ^{14}O+4p events. The ground-state decay energy and width are E_{T}=4.865(34)  MeV and Γ=115(100)  keV, respectively. The observed momentum correlations between the five particles are consistent with two sequential steps of prompt 2p decay passing through the ground state of ^{16}Ne. The invariant-mass spectrum also provides evidence for an excited state at an excitation energy of 1.84(14) MeV, which is likely the first excited 2^{+} state. As this energy exceeds that for the 2^{+} state in ^{20}Mg, this observation provides an argument for the demise of the N=8 shell closure in nuclei far from stability. However, in open systems this classical argument for shell strength is compromised by Thomas-Ehrman shifts.

A discrepancy in the asymmetry dependence of spectroscopic factors extracted with different reaction probes calls into question whether the corresponding reaction models are properly understood. In this work, we present extracted spectroscopic factors from the Ar-46,Ar-34( p, d) Ar-45,Ar-33 transfer reactions in inverse kinematics at a beam energy of 70 MeV/nucleon. The results are consistent with previous measurements of these reactions at a lower beam energy [Lee et al., Phys. Rev. Lett. 104, 112701 (2010)], indicating that the transfer reaction is a reliable probe for the nuclear structure of exotic nuclei across a wide energy range. Results from a large body of transfer reaction measurements, (p, pN) measurements, and theoretical nuclear structure studies make a compelling case for much weaker asymmetry dependence than what is observed with single-nucleon knockout reactions on beryllium or carbon targets.

A ^{13}F resonance was observed following a charge-exchange reaction between a fast ^{13}O beam and a ^{9}Be target. The resonance was found in the invariant-mass distribution of 3p+^{10}C events and probably corresponds to a 5/2^{+} excited state. The ground state was also expected to be populated, but was not resolved from the background. The observed level decays via initial proton emissions to both the ground and first 2^{+} state of ^{12}O, which subsequently undergo 2p decay. In addition, there may also be a significant proton decay branch to the second 2^{+} level in ^{12}O. The wave function associated with the observed level may be collectivized due to coupling to the continuum as is it located just above the threshold for proton decay to the 2_{2}^{+} state of ^{12}O.

The neutron total cross sections sigma(tot) of O-16,O-18, Ni-58,Ni-64, Rh-103, and Sn-112,Sn-124 have been measured at the Los Alamos Neutron Science Center from low to intermediate energies (3 <= E-lab 450 MeV) by leveraging wave-form-digitizer technology. The sigma tot relative differences between isotopes are presented, revealing additional information about the isovector components needed for an accurate optical-model description away from stability. Digitizer-enabled sigma(tot)-measurement techniques are discussed and a series of uncertainty-quantified dispersive optical model (DOM) analyses using these new data is presented, validating the use of the DOM for modeling light systems (O-16,O-18) and systems with open neutron shells (Ni-58,Ni-64 and Sn-112,Sn-124). The valence-nucleon spectroscopic factors extracted for each isotope reaffirm the usefulness of high-energy proton reaction cross sections for characterizing depletion from the mean-field expectation.

The invariant-mass distribution of O fragments formed in two-neutron-knockout reactions with a O projectile and Be target has been further examined. Gating on events where the C decay fragments produced following 2p emission are recoiled transversely in the projectile’s frame improves the overall invariant-mass resolution. The observed peak is now shown to have contributions from at least two O levels, in contradiction with the suggestion of Fortune [Phys. Rev. C 99, 051302(R) (2019)]. The data, however, do not differentiate between the ground-state properties obtained by the Gamow coupled-channels calculation and the prompt 2p-decay model of Fortune. The ground-state 2p-decay energy is 4.25(6) MeV in a two-level fit to the data, but lower values are possible if more states contribute to the observed spectrum, as suggested in the previous analysis.

Received 7 April 2020DOI:https://doi.org/10.1103/PhysRevC.102.019904©2020 American Physical SocietyPhysics Subject Headings (PhySH)Proton emissionResearch AreasNuclear structure & decaysProton emissionResearch AreasNuclear structure & decaysProton emissionRare & new isotopesProperties6 ≤ A ≤ 19TechniquesRadioactive beamsNuclear Physics

The invariant-mass distribution of O-11 fragments formed in two-neutron-knockout reactions with a O-13 projectile and Be-9 target has been further examined. Gating on events where the C-9 decay fragments produced following 2p emission are recoiled transversely in the projectile's frame improves the overall invariant-mass resolution. The observed peak is now shown to have contributions from at least two O-11 levels, in contradiction with the suggestion of Fortune [Phys. Rev. C 99, 051302(R) (2019)]. The data, however, do not differentiate between the ground-state properties obtained by the Gamow coupled-channels calculation and the prompt 2p-decay model of Fortune. The ground-state 2p-decay energy is 4.25(6) MeV in a two-level fit to the data, but lower values are possible if more states contribute to the observed spectrum, as suggested in the previous analysis.

Particle-decaying states of the light nuclei ^11,12N and ^12O were studied using the invariant-mass method. The decay energies and intrinsic widths of a number of states were measured, and the momentum correlations of three-body decaying states were considered. A second 2p-decaying 2^+ state of ^12O was observed for the first time, and a higher energy ^12O state was observed in the 4p+2α decay channel. This 4p+2α channel also contains contributions from fission-like decay paths, including ^6Be_g.s.+^6Be_g.s.. Analogs to these states in ^12O were found in ^12N in the 2p+^10B and 2p+α+^6Li channels. The momentum correlations for the prompt 2p decay of ^12O_g.s. were found to be nearly identical to those of ^16Ne_g.s., and the correlations for the new 2^+ state were found to be consistent with sequential decay through excited states in ^11N. The momentum correlations for the 2^+_1 state in ^12O provide a new value for the ^11N ground-state energy. The states in ^12N/^12O that belong to the A=12 isobaric sextet do not deviate from the quadratic isobaric multiplet mass equation (IMME) form.

The structure of the extremely proton-rich nucleus _{8}^{11}O_{3}, the mirror of the two-neutron halo nucleus _{3}^{11}Li_{8}, has been studied experimentally for the first time. Following two-neutron knockout reactions with a ^{13}O beam, the ^{11}O decay products were detected after two-proton emission and used to construct an invariant-mass spectrum. A broad peak of width ∼3.4  MeV was observed. Within the Gamow coupled-channel approach, it was concluded that this peak is a multiplet with contributions from the four lowest ^{11}O resonant states: J^{π}=3/2_{1}^{-}, 3/2_{2}^{-}, 5/2_{1}^{+}, and 5/2_{2}^{+}. The widths and configurations of these states show strong, nonmonotonic dependencies on the depth of the p-^{9}C potential. This unusual behavior is due to the presence of a broad threshold resonant state in ^{10}N, which is an analog of the virtual state in ^{10}Li in the presence of the Coulomb potential. After optimizing the model to the data, only a moderate isospin asymmetry between ground states of ^{11}O and ^{11}Li was found.

The structure of the extremely proton-rich nucleus $^{11}_{~8}$O$_3$, the mirror of the two-neutron halo nucleus $^{11}_{~3}$Li$_8$, has been studied experimentally for the first time. Following two-neutron knockout reactions with a $^{13}$O beam, the $^{11}$O decay products were detected after two-proton emission and used to construct an invariant-mass spectrum. A broad peak of width $\sim$3\,MeV was observed. Within the Gamow coupled-channel approach, it was concluded that this peak is a multiplet with contributions from the four-lowest $^{11}$O resonant states: $J^{\pi}$=3/2$^-_1$, 3/2$^-_2$, 5/2$^+_1$, and 5/2$^+_2$. The widths and configurations of these states show strong, non-monotonic dependencies on the depth of the $p$-$^9$C potential. This unusual behavior is due to the presence of a broad threshold resonant state in $^{10}$N, which is an analog of the virtual state in $^{10}$Li in the presence of the Coulomb potential. After optimizing the model to the data, only a moderate isospin asymmetry between ground states of $^{11}$O and $^{11}$Li was found.

Particle-decaying states of the light nuclei N-11,N-12 and O-12 were studied using the invariant-mass method. The decay energies and intrinsic widths of a number of states were measured, and the momentum correlations of three-body decaying states were considered. A second 2p-decaying 2(+) state of O-12 was observed for the first time, and a higher-energy O-12 state was observed in the 4p + 2 alpha decay channel. This 4p + 2 alpha channel also contains contributions from fissionlike decay paths, including Be-6(g.s.) + Be-6(g.s.). Analogs to these states in O-12 were found in N-12 in the 2p + B-10 and 2p + alpha + Li-6 channels. The momentum correlations for the prompt 2p decay of O-12(g.s.) were found to be nearly identical to those of Ne-16(g.s.), and the correlations for the new 2(+) state were found to be consistent with sequential decay through excited states in N-11. The momentum correlations for the 2(1)(+) state in O-12 provide a new value for the N-11 ground-state energy. The states in N-12/O-12 that belong to the A = 12 isobaric sextet do not deviate from the quadratic isobaric multiplet mass equation form.