The first high-resolution in-beam gamma -ray spectroscopy is reported for the neutron-rich nucleus 41Si, a tenant of the N = 28 island of inversion. Excited states were populated in the direct one-proton removal reaction from 42P projectiles and pn removal from 43P. Seven gamma -ray transitions were observed, only one of which had been reported previously in the literature. This makes 41Si the most neutron-rich odd-even N = 27 isotone with high-resolution excited-state information. For the one-proton removal, the measured partial cross-section distribution to all observed bound final states is contrasted with results from direct one-proton removal calculations that combine eikonal reaction dynamics with SDPF-MU shell-model spectroscopic factors and assume various possible initial states for the poorly known 42P projectile. Rather distinct calculated cross-section distributions emerge that, in comparison to the new data, imply that the initial state in 42P is most likely 3- or 2- rather than 1- or 0-, the predicted shell-model ground state of 42P. It is further shown that the level scheme from the novel VS-IMSRG calculation closely agrees with the one of SDPF-MU, the most successful phenomenological shell-model effective interaction in describing the much discussed neighboring isotope 42Si, perhaps cross-validating these complementary approaches on the quest to model rapid shell evolution away from the valley of beta stability.

The decay of excited states of the nucleus 135Sn, with three neutrons outside the doubly-magic 132Sn core, was studied in an experiment performed at the Radioactive Isotope Beam Factory at RIKEN. Several γ rays emitted from excited 135Sn ions were observed following one-neutron and one-neutron-one-proton removal from 136Sn and 137Sb beams, respectively, on a beryllium target at relativistic energies. Based on the analogy to 133Sn populated via one-neutron removal from 134Sn, an excitation energy of 695(15) keV is assigned to the 3/2− state with strongest single-particle character in 135Sn. This result provides the first direct information about the evolution of the neutron shell structure beyond N=82 and thus allows for a crucial test of shell-model calculations in this region. The experimental findings are in full agreement with calculations performed employing microscopic effective two-body interactions derived from CD-Bonn and N3LO nucleon-nucleon potentials, which do not predict a pronounced subshell gap at neutron number N=90. The occurrence of such a gap in 140Sn, i.e., when the 1f7/2 orbital is completely filled, had been proposed in the past, in analogy to the magicity of 48Ca, featuring a completely filled 0f7/2 orbital one harmonic oscillator shell below.

The recent discovery and spectroscopic measurements of O27 and O28 suggests the disappearance of the N=20 shell structure in these neutron-rich oxygen isotopes. We measured one- and two-proton removal cross sections from F27 and Ne29, respectively, extracting spectroscopic factors and comparing them to shell model overlap functions coupled with eikonal reaction model calculations. The invariant mass technique was used to reconstruct the two-body (O24+n) and three-body (O24+2n) decay energies from knockout reactions of F27 (106.2 MeV/u) and Ne29 (112.8 MeV/u) beams impinging on a Be9 target. The one-proton removal from F27 strongly populated the ground state of O26 and the extracted cross section of 3.4−1.5+0.3 mb agrees with eikonal model calculations that are normalized by the shell model spectroscopic factors and account for the systematic reduction factor observed for single nucleon removal reactions within the models used. For the two-proton removal reaction from Ne29 an upper limit of 0.08 mb was extracted for populating states in O27 decaying though the ground state of O26. The measured upper limit for the population of the ground state of O26 in the two-proton removal reaction from Ne29 indicates a significant difference in the underlying nuclear structure of F27 and Ne29.Published by the American Physical Society2024

Bound states of the neutron -deficient, near-dripline nucleus 36Ca were populated in two -neutron removal from the ground state of 38Ca, a direct reaction sensitive to the single -particle configurations and couplings of the removed neutrons in the projectile wave function. Final -state exclusive cross sections for the formation of 36Ca and the corresponding longitudinal momentum distributions, both determined through the combination of particle and gamma -ray spectroscopy, are compared to predictions combining eikonal reaction theory and shellmodel two -nucleon amplitudes from the USDB, USDC, and ZBM2 effective interactions. The final -state crosssection ratio sigma (2+1 )/sigma (0+) shows particular sensitivity and is approximately reproduced only with the twonucleon amplitudes from the ZBM2 effective interaction that includes proton cross -shell excitations into the pf shell. Characterizing the proton pf-shell occupancy locally and schematically, an increase of the sd-pf shell gap by 250 keV yields an improved description of this cross-section ratio and simultaneously enables a reproduction of the B(E2; 0+1 -> 2+1 ) excitation strength of 36Ca. This highlights an important aspect if a new shell -model effective interaction for the region was to be developed on the quest to model the neutron -deficient Ca isotopes and surrounding nuclei whose structure is impacted by proton cross -shell excitations.

An experiment with the aim to obtain information on the excited states of neutron-rich nuclei with N~82 was performed at RIBF/RIKEN as part of the HiCARI campaign. The method to identify nuclei on ion-by-ion basis, including charge-state identification, is presented. The Doppler correction technique was validated using the test case of 131 In, based on the prompt π p 3/2 → πp 1/2 transition at 988 keV. Preliminary analysis of the 130 Cd spectrum is also presented.

Detailed γ-ray spectroscopy of the exotic neon isotope ^28Ne has been performed for the first time using the one-neutron removal reaction from ^29Ne on a liquid hydrogen target at 240 MeV/nucleon. Based on an analysis of parallel momentum distributions, a level scheme with spin-parity assignments has been constructed for ^28Ne and the negative-parity states are identified for the first time. The measured partial cross sections and momentum distributions reveal a significant intruder p-wave strength providing evidence of the breakdown of the N=20 and N=28 shell gaps. Only a weak, possible f-wave strength was observed to bound final states. Large-scale shell-model calculations with different effective interactions do not reproduce the large p-wave and small f-wave strength observed experimentally, indicating an ongoing challenge for a complete theoretical description of the transition into the island of inversion along the Ne isotopic chain.

The neutron-deficient Ca isotopes continue to attract attention due to their importance for testing isospin symmetry and their relevance in capture reactions of interest for nova nucleosynthesis and the shape of light curves in Type I X-ray bursts. To date, spectroscopic information on 38,39 Ca is largely limited to data on lower-spin excited states. Here, we report in-beam γ-ray spectroscopy of complementary higher-spin, complex-structure states in 39 Ca populated in fast-beam-induced, momentum-dissipative processes leading to neutron pickup onto excited configurations of the projectile, 9 Be(38 Ca , 39 Ca + γ)X. Such a dissipative reaction was recently characterized for the case of inelastic scattering of 38 Ca off 9 Be, 9 Be(38 Ca, 38 Ca + γ)X. Additional data and discussion on the nuclear structure of 38 Ca is also presented. An explanation for the more-complex-structure states, populated with small cross sections in one-nucleon knockout reactions, and observed in the tails of their longitudinal momentum distributions, is also offered.

The $^{25}$F($5/2^+) (-1p) ^{24}$O reaction was studied at the NSCL using the S800 spectrometer. The experimental spectroscopic factor for the ground-state to ground-state transition indicates a substantial depletion of the proton $d_{5/2}$ strength compared to shell-model expectations. Our result supports the findings reported by Tang \textit{et al.}, from their study of the $(p,2p)$ reaction at RIBF. The overlap between the $^{25}$F and $^{24}$O ground-states is considerably less than anticipated if $^{24}$O acted as a robust and rigid doubly-magic core in $^{25}$F. We interpret the results within the framework of the Particle-Vibration Coupling (PVC) of a $d_{5/2}$ proton coupled to a quadrupole phonon of an effective core. This approach provides a good description of the experimental data by requiring an effective $^{24}$O* core with a phonon energy of $\hbar\omega_2$= 3.2 MeV, and a $B(E2) ~ 2.7$ W.u., softer and more collective than a bare $^{24}$O. Both the Nilsson deformed mean field and the PVC models appear to capture the properties of the effective core of $^{25}$F, suggesting that the additional proton tends to polarize the free, doubly magic $^{24}$O in such a way that it becomes either slightly deformed or a quadrupole vibrator.

The $^{9}\mathrm{Be}(^{25}\phantom{{}^{B}e}\mathrm{F}(5/{2}^{+}),^{24}\phantom{{}^{B}e}\mathrm{O}$)X proton-removal reaction was studied at the NSCL using the S800 spectrometer. The experimental spectroscopic factor for the ground-state to ground-state transition indicates a substantial depletion of the proton ${d}_{5/2}$ strength compared to shell-model expectations, similar to the findings of an inverse-kinematics $(p,2p)$ measurement performed at RIBF. The $^{25}\phantom{{}^{B}e}\mathrm{F}$ to $^{24}\phantom{{}^{B}e}\mathrm{O}$ ground-states overlap is considerably less than anticipated if the core nucleons behaved as rigid, doubly-magic $^{24}\phantom{{}^{B}e}\mathrm{O}$ within $^{25}\phantom{{}^{B}e}\mathrm{F}$. We interpret the new results within the framework of the Particle-Vibration Coupling (PVC) model, of a ${d}_{5/2}$ proton coupled to a quadrupole phonon of an effective core. This approach provides a good description of the experimental data, requiring an effective $^{24}\phantom{{}^{B}e}\mathrm{O}^{*}$ core with a phonon energy of $\ensuremath{\hbar}{\ensuremath{\omega}}_{2}$= 3.2 MeV and a $B(E2)\ensuremath{\approx}2.7$ W.u. -- softer and more collective than a bare $^{24}\phantom{{}^{B}e}\mathrm{O}$. Both the Nilsson deformed mean field and the PVC models appear to capture the properties of the effective core of $^{25}\phantom{{}^{B}e}\mathrm{F}$, suggesting that the additional proton polarizes $^{24}\phantom{{}^{B}e}\mathrm{O}$ in such a way that it becomes either slightly deformed or a quadrupole vibrator.

The 9Be(25F(5/2+), 24O)X proton-removal reaction was studied at the NSCL using the S800 spectrometer. The experimental spectroscopic factor for the ground-state to ground-state transition indicates a substantial depletion of the proton d5/2 strength compared to shell-model expectations, similar to the findings of an inverse-kinematics (p, 2p) measurement performed at RIBF. The 25F to 24O ground-states overlap is considerably less than anticipated if the core nucleons behaved as rigid, doubly-magic 24O within 25F. We interpret the new results within the framework of the Particle-Vibration Coupling (PVC) model, of a d5/2 proton coupled to a quadrupole phonon of an effective core. This approach provides a good description of the experimental data, requiring an effective 24O* core with a phonon energy of h over bar omega 2= 3.2 MeV and a B(E2) approximate to 2.7 W.u. - softer and more collective than a bare 24O. Both the Nilsson deformed mean field and the PVC models appear to capture the properties of the effective core of 25F, suggesting that the additional proton polarizes 24O in such a way that it becomes either slightly deformed or a quadrupole vibrator.

Background: The nucleus 32Mg (N = 20 and Z = 12) plays a central role in the so-called "island of inversion," where in the ground states sd-shell neutrons are promoted to the fp-shell orbitals across the shell gap, resulting in the disappearance of the canonical neutron magic number N = 20. Purpose: The primary goals of this work are to extend the level scheme of 32Mg, provide spin-parity assignments to excited states, and discuss the microscopic structure of each state through comparisons with theoretical calculations. Method: In-beam gamma -ray spectroscopy of 32Mg was performed using two direct-reaction probes: one-neutron (two-proton) knockout reactions on 33Mg (34Si). Final-state exclusive cross sections and parallel momentum distributions were extracted from the experimental data and compared with eikonal-based reaction model calculations combined with shell-model overlap functions. Results: Owing to the remarkable selectivity of the one-neutron and two-proton knockout reactions, a significantly updated level scheme for 32Mg, which exhibits negative-parity intruder and positive-parity normal states, was constructed. The experimental results were confronted with four different nuclear structure models. Conclusions: In some of these models, different aspects of 32Mg and the transition into the island of inversion are well described. However, unexplained discrepancies remain, and, even with the help of these state-of-the-art theoretical approaches, the structure of this key nucleus is not yet fully captured.

In many instances, single nucleon removal reactions from neutron-proton asymmetric projectile nuclei populate final states in the residual nuclei that are very weakly bound. Familiar examples include neutron removal reactions from neutron-rich $^{11}$Be and $^{12}$Be, the latter populating the well-known $1/2^+$ halo ground-state and $1/2^-$ excited-state of $^{11}$Be - both states less than 1 MeV from the first neutron-decay threshold. Numerous additional examples arise in reactions of asymmetric $p$- and $sd$-shell nuclei. The importance of this weak residue binding upon calculated single-nucleon removal reaction cross sections is quantified by means of model calculations that neglect or include the dissociation degree of freedom of the residual nuclei. The calculated removal-reaction cross sections for two representative $p$-shell projectiles indicate that an explicit treatment of these residue break-up effects is unnecessary and that the differences between the break-up and no break-up calculations are small provided a consistent description of the residue structure and density is used.

A novel pathway for the formation of multi-particle-multi-hole (np-mh) excited states in rare isotopes is reported from highly energy- and momentum-dissipative inelastic-scattering events measured in reactions of an intermediate-energy beam of 38Ca on a Be target. The negative-parity,complex-structure final states in 38Ca were observed following the in-beam gamma-ray spectroscopy of events in the 9Be(38Ca,38Ca+gamma)X reaction in which the scattered projectile lost longitudinal momentum of order p = 700 MeV/c. The characteristics of the observed final states are discussed and found to be consistent with the formation of excited states involving the rearrangement of multiple nucleons in a single, highly-energetic projectile-target collision. Unlike the far-less dissipative, surface-grazing reactions usually exploited for the in-beam gamma-ray spectroscopy of rare isotopes, these more energetic collisions appear to offer a practical pathway to nuclear-structure studies of more complex multi-particle configurations in rare isotopes - final states conventionally thought to be out of reach with high-luminosity fast-beam-induced reactions.

Level schemes of the proton-rich nuclei, Mn-47 (Z = 25, N = 22) and Cr-45 (Z = 24, N = 21), have been established for the first time. The technique of mirrored one- and two-nucleon knockout reactions was applied to the secondary beams of V-48/Mn-48 and V-47/Cr-47 to populate states in Ti-47/Mn-47 and Sc-45/Cr-45, respectively. Mirror energy differences (MED) have been studied between the mirrored T = 3/2 states for both mirror pairs and interpreted using both a shell-model approach and a density-functional-theory approach using the no-core configuration-interaction method. MED in this mass region provide a stringent test of the model prescriptions since both integral p- and sd-shell orbitals are active and, in Cr-45, spherical and well-deformed structures coexist near the ground state. The inclusive and exclusive one-nucleon removal cross sections have been determined for the populated states in Ti-47/Mn-47 and compared with results from reaction-model calculations.

Level schemes of the proton-rich nuclei, 47 Mn (Z = 25, N = 22) and 45 Cr (Z = 24, N = 21), have been established for the first time.The technique of mirrored one-and two-nucleon knockout reactions was applied to the secondary beams of 48 V/ 48 Mn and 47 V/ 47 Cr to populate states in 47 Ti/ 47 Mn and 45 Sc/ 45 Cr, respectively.Mirror energy differences (MED) have been studied between the mirrored T = 3 2 states for both mirror pairs and interpreted using both a shell-model approach and a density-functional-theory approach using the no-core configuration-interaction method.MED in this mass region provide a stringent test of the model prescriptions since both fp-and sd-shell orbitals are active and, in 45 Cr, spherical and well-deformed structures coexist near the ground state.The inclusive and exclusive one-nucleon removal cross sections have been determined for the populated states in 47 Ti/ 47 Mn and compared with results from reaction-model calculations.

The nucleus Ne-29 is situated at the border of the island of inversion. Despite significant efforts, no bound low-lying intruder f(7/2) state, which would place Ne-29 firmly inside the island of inversion, has yet been observed. Here, the first investigation of unbound states of Ne-29 is reported. The states were populated in Ne-30(p, pn) and Na-30(p, 2p) reactions at a beam energy of around 230 MeV/nucleon, and analyzed in terms of their resonance properties, partial cross sections, and momentum distributions. The momentum distributions are compared to calculations using the eikonal, direct reaction model, allowing assignments for the observed states. The lowest lying resonance at an excitation energy of 1.48(4) MeV shows clear signs of a significant l = 3 component, giving first evidence for f(7/2) single particle strength in Ne-29. The excitation energies and strengths of the observed states are compared to shell-model calculations using the SDPF-U-MIX interaction.

A. Gade, 2 R.V.F. Janssens, D. Bazin, R. Broda, B.A. Brown, 2 C.M. Campbell, 2 M.P. Carpenter, J.M. Cook, 2 A.N. Deacon, D.-C. Dinca, 2 B. Fornal, S. J. Freeman, T. Glasmacher, 2 P.G. Hansen, 2 B.P. Kay, P.F. Mantica, 6 W.F. Mueller, J.R. Terry, 2 J.A. Tostevin, and S. Zhu National Superconducting Cyclotron Laboratory, Michigan State University, East Lansing, Michigan 48824 Department of Physics and Astronomy, Michigan State University, East Lansing, Michigan 48824 Physics Division, Argonne National Laboratory, Argonne, IL 60439 Institute of Nuclear Physics, Polish Academy of Science, PL-31342 Cracow, Poland School of Physics and Astronomy, Schuster Laboratory, University of Manchester, Manchester M13 9PL, United Kingdom Department of Chemistry, Michigan State University, East Lansing, MI 48824 Department of Physics, School of Electronics and Physical Sciences, University of Surrey, Guildford, Surrey GU2 7XH, United Kingdom (Dated: March 30, 2022)

Nucleon removal reactions have been shown to be an effective tool for studying the single particle structure of nuclei. This work continues efforts to experimentally probe and benchmark the reaction and structure models used to calculate the removal reaction cross sections when using microscopic nuclear structure inputs. Three different single nucleon removal reactions were performed, from p-shell nuclei with masses A = 7, 9, and 10. The residual nuclei from the reactions were detected in coincidence with gamma rays to determine partial cross sections to individual final states. The eikonal direct-reaction model is combined with overlap functions and residual nucleus densities from microscopic, variational Monte Carlo calculations to provide consistent nuclear structure input to the partial cross section calculations. Comparisons of measured and calculated cross sections, including for mirror reactions, are presented. The analysis of the partial cross sections leading to the ground states shows a similar behavior to the one observed from analyses of inclusive cross sections using shell model nuclear structure input: the theoretical description of the removal process is in better agreement with the data when removing weakly bound nucleons, than when removing well-bound ones. The two mirror reaction pairs presented here show consistent results between the respective members of the pairs. The results obtained for the population of the excited states, however, show a systematically different trend that appears connected to the structure part of the calculation. Additional cases are needed to better understand the respective roles of structure and dynamical effects in the deviations.

S. Ota, 2, ∗ G. Christian, 3, 4, 5 W. N. Catford, G. Lotay, M. Pignatari, 8, 9, 2 U. Battino, 2 E. A. Bennett, 4 S. Dede, 4 D. T. Doherty, S. Hallam, F. Herwig, 7, 2 J. Hooker, 4 C. Hunt, 4 H. Jayatissa, 4 A. Matta, M. Mouhkaddam, E. Rao, 12 G. V. Rogachev, 4, 5 A. Saastamoinen, D. Scriven, 4 J. A. Tostevin, S. Upadhyayula, 4 and R. Wilkinson Cyclotron Institute, Texas A&M University, College Station, TX 77843, USA NuGrid Collaboration, http: // nugridstars. org Department of Astronomy & Physics, Saint Mary’s University, Halifax, NS B3H 3C3, Canada Department of Physics & Astronomy, Texas A&M University, College Station, TX 77843, USA Nuclear Solutions Institute, Texas A&M University, College Station, TX 77843, USA Department of Physics, University of Surrey, Guildford GU2 7XH, UK Joint Institute for Nuclear Astrophysics Center for the Evolution of the Elements, East Lansing, 48823, USA E. A. Milne Centre for Astrophysics, Department of Physics and Mathematics, University of Hull, Hull HU6 7RX, UK Konkoly Observatory, Research Centre for Astronomy and Earth Sciences, Hungarian Academy of Sciences, Konkoly Thege M. t 15-17, 1121, Budapest, Hungary School of Physics and Astronomy, University of Edinburgh, EH9 3FD, UK Department of Physics and Astronomy, University of Victoria, Victoria, BC V8P5C2, Canada Department of Physics & Astronomy, Rutgers University, New Brunswick, NJ, USA (Dated: July 2, 2021)

Situated in the so-called "island of inversion," the nucleus ^32Mg is considered as an archetypal example of the disappearance of magicity at N=20. We report on high statistics in-beam spectroscopy of ^32Mg with a unique approach, in that two direct reaction probes with different sensitivities to the underlying nuclear structure are employed at the same time. More specifically, states in ^32Mg were populated by knockout reactions starting from ^33Mg and ^34Si, lying inside and outside the island of inversion, respectively. The momentum distributions of the reaction residues and the cross sections leading to the individual final states were confronted with eikonal-based reaction calculations, yielding a significantly updated level scheme for ^32Mg and spin-parity assignments. By fully exploiting observables obtained in this measurement, a variety of structures coexisting in 32Mg was unraveled. Comparisons with theoretical predictions based on shell-model overlaps allowed for clear discrimination between different structural models, revealing that the complete theoretical description of this key nucleus is yet to be achieved.