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Detailed γ-ray spectroscopy of the exotic neon isotope $^{28}$Ne has been performed for the first time using the one-neutron removal reaction from $^{29}$Ne 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 $^{28}$Ne 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.

17 pags., 11 figs., 3 tabs., Background: The nucleus Mg32 (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 Mg32, provide spin-parity assignments to excited states, and discuss the microscopic structure of each state through comparisons with theoretical calculations. Method: In-beam γ-ray spectroscopy of Mg32 was performed using two direct-reaction probes: one-neutron (two-proton) knockout reactions on Mg33 (Si34). 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 Mg32, 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 Mg32 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., N.K. acknowledges support of the Grant-in-Aid for JSPS Fellows (18J12542) from the Ministry of Education, Culture, Sports, Science, and Technology (MEXT), Japan. K.W. acknowledges support from the Ministerio de Ciencia e Innovación (Spain) through the “Ramón y Cajal” program RYC-2017- 22007. A.P. is supported by the Ministerio de Ciencia, Innovación y Universidades (Spain), Grant No. CEX2020- 001007-S funded by MCIN/AEI/10.13039/501100011033 and Grant No. PGC-2018-94583. The SDPF-M calculations were enabled by the CNS-RIKEN joint project for large-scale nuclear structure calculations and were performed mainly on the Oakforest-PACS supercomputer. N.S. acknowledges support from “Priority Issue on post-K computer” (hp190160) and “Program for Promoting Researches on the Supercomputer Fugaku” (JPMXP1020200105, hp200130, and hp210165) by JICFuS and MEXT, Japan. The IMSRG calculations were performed with an allocation of computing resources on Cedar at WestGrid and Compute Canada and on the Oak Cluster at TRIUMF managed by the University of British Columbia, Department of Advanced Research Computing. J.A.T. acknowledges support from the U.K. Science and Technology Facilities Council Grant No. ST/L005743/1. This work was supported by the U.S. Department of Energy (DOE), Office of Science, Office of Nuclear Physics, under Grant No. DE-SC0020451 and by the U.S. National Science Foundation (NSF) under Grant No. PHY-1306297. GRETINA was funded by the U.S. DOE, Office of Science. Operation of the array at NSCL is supported by the U.S. NSF under Cooperative Agreement No. PHY-1102511 (NSCL) and DOE under Grant No. DE-AC02-05CH11231 (LBNL).

A sequence of excited states has been established for the first time in the proton-rich nucleus 48Fe (Z=26, N=22). The technique of mirrored (i.e. analogue) one-nucleon knockout reactions was applied, in which the Tz= ±2 mirror pair, 48Fe/48Ti were populated via one-neutron/one-proton knockout from the secondary beams 49Fe/49V, respectively. The analogue properties of the reactions were used to help establish the new level scheme of 48Fe. The inclusive and exclusive cross sections were determined for the populated states. Large differences between the cross sections for the two mirrored reactions were observed and have been interpreted in terms of different degrees of binding of the mirror nuclei and in the context of the recent observations of suppression of spectroscopic strength as a function of nuclear binding, for knockout reactions on light solid targets. Mirror energy differences (MED) have been determined between the analogue T=2 states and compared with the shell model predictions. MED for this mirror pair, due to their location in the shell, are especially sensitive to excitations out of the f7/2 shell, and present a stringent test of the shell-model prescription.

Situated in the so-called “island of inversion,” the nucleus $^{32}$Mg is considered as an archetypal example of the disappearance of magicity at N=20. We report on high statistics in-beam spectroscopy of $^{32}$Mg 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 $^{32}$Mg were populated by knockout reactions starting from $^{33}$Mg and $^{34}$Si, 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 $^{32}$Mg and spin-parity assignments. By fully exploiting observables obtained in this measurement, a variety of structures coexisting in $^{32}$Mg 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.

We report on the first in-beam γ-ray spectroscopy of the proton-dripline nucleus $^{40}$Sc using two-nucleon pickup onto an intermediate-energy rare-isotope beam of $^{38}$Ca. The $^{9}$Be($^{38}$Ca,$^{40}$Sc+γ)X reaction at 60.9 MeV/nucleon mid-target energy selectively populates states in $^{40}$Sc for which the transferred proton and neutron couple to high orbital angular momentum. In turn, due to angular-momentum selection rules in proton emission and the nuclear structure and energetics of $^{39}$Ca, such states in $^{40}$Sc then exhibit γ-decay branches although they are well above the proton separation energy. This work uniquely complements results from particle spectroscopy following charge-exchange reactions on $^{40}$Ca as well as $^{40}$Ti EC/β+ decay which both display very different selectivities. The population and γ-ray decay of the previously known first (5−) state at 892 keV and the observation of a new level at 2744 keV are discussed in comparison to the mirror nucleus and shell-model calculations. On the experimental side, this work shows that high-resolution in-beam γ-ray spectroscopy is possible with new generation Ge arrays for reactions induced by rare-isotope beams on the level of a few μb of cross section.

The single-particle structure of the N = 27 isotones provides insights into the shell evolution of neutron-rich nuclei from the doubly-magic 48Ca toward the drip line. 43S was studied employing the one-neutron knockout reaction from a radioactive 44S beam. Using a combination of prompt and delayed γ-ray spectroscopy the level structure of 43S was clarified. Momentum distributions were analyzed and allowed for spin and parity assignments. The deduced spectroscopic factors show that the 44S ground-state configuration has a strong intruder component. The results were confronted with shell model calculations using two effective interactions. General agreement was found between the calculations, but strong population of states originating from the removal of neutrons from the 2p3/2 orbital in the experiment indicates that the breakdown of the N = 28 magic number is more rapid than the theoretical calculations suggest.

The structure of ³³Si was studied by a one-neutron knockout reaction from a ³⁴Si beam at 98.5 MeV/u incident on a 9 Be target. The prompt γ rays following the de-excitation of ³³Si were detected using the GRETINA γ -ray tracking array while the reaction residues were identified on an event-by-event basis in the focal plane of the S800 spectrometer at the National Superconducting Cyclotron Laboratory. The presently derived spectroscopic factor values, C 2 S , for the 3 / 2 + and 1 / 2 + states, corresponding to a neutron removal from the 0 d 3 / 2 and 1 s 1 / 2 orbitals, agree with shell model calculations and point to a strong N = 20 shell closure. Three states arising from the more bound 0 d 5 / 2 orbital are proposed, one of which is unbound by about 930 keV. The sensitivity of this experiment has also confirmed a weak population of 9 / 2 − and 11 / 2 − 1 , 2 final states, which originate from a higher-order process. This mechanism may also have populated, to some fraction, the 3 / 2 − and 7 / 2 − negative-parity states, which hinders a determination of the C 2 S values for knockout from the normally unoccupied 1 p 3 / 2 and 0 f 7 / 2 orbits.

The astrophysical s-process is one of the two main processes forming elements heavier than iron. A key outstanding uncertainty surrounding s-process nucleosynthesis is the neutron flux generated by the Ne22(α,n)25Mg reaction during the He-core and C-shell burning phases of massive stars. This reaction, as well as the competing Ne22(α,γ)26Mg reaction, is not well constrained in the important temperature regime from ∼0.2–0.4 GK, owing to uncertainties in the nuclear properties of resonances lying within the Gamow window. To address these uncertainties, we have performed a new measurement of the Ne22(Li6,d)26Mg reaction in inverse kinematics, detecting the outgoing deuterons and Mg25,26 recoils in coincidence. We have established a new n/γ decay branching ratio of 1.14(26) for the key Ex=11.32 MeV resonance in Mg26, which results in a new (α,n) strength for this resonance of 42(11)μeV when combined with the well-established (α,γ) strength of this resonance. We have also determined new upper limits on the α partial widths of neutron-unbound resonances at Ex=11.112, 11.163, 11.169, and 11.171 MeV. Monte-Carlo calculations of the stellar Ne22(α,n)25Mg and Ne22(α,γ)26Mg rates, which incorporate these results, indicate that both rates are substantially lower than previously thought in the temperature range from ∼0.2–0.4 GK.

6 pags., 2 figs., 2 tabs., Spectroscopic factors of neutron-hole and proton-hole states in Sn131 and In131, respectively, were measured using one-nucleon removal reactions from doubly magic Sn132 at relativistic energies. For In131, a 2910(50)-keV γ ray was observed for the first time and tentatively assigned to a decay from a 5/2-state at 3275(50) keV to the known 1/2-level at 365 keV. The spectroscopic factors determined for this new excited state and three other single-hole states provide first evidence for a strong fragmentation of single-hole strength in Sn131 and In131. The experimental results are compared to theoretical calculations based on the relativistic particle-vibration coupling model and to experimental information for single-hole states in the stable doubly magic nucleus Pb208., We thank the staff of the RIKEN accelerator team for supplying a primary 238U beam with high intensity. This work was supported by the Spanish Ministerio de Economía y Competitividad under Contract No. FPA2017-84756-C4-2-P, the DFG via Sonderforschungsbereich SFB 1245, the GSI-TU Darmstadt cooperation agreement, the US-NSF Career Grants No. PHY-1654379 and No. PHY-1713857 and by INFN Italy. J. A. T. acknowledges support of the Science and Technology Facilities Council (UK) Grant No. ST/ L005314/1.

10 pags., 6 figs., 3 tabs., The single-particle structure of the N=27 isotones provides insights into the shell evolution of neutron-rich nuclei from the doubly magic Ca48 toward the drip line. S43 was studied employing the one-neutron knockout reaction from a radioactive S44 beam. Using a combination of prompt and delayed γ-ray spectroscopy the level structure of S43 was clarified. Momentum distributions were analyzed and allowed for spin and parity assignments. The deduced spectroscopic factors show that the S44 ground-state configuration has a strong intruder component. The results were confronted with shell-model calculations using two effective interactions. General agreement was found between the calculations, but strong population of states originating from the removal of neutrons from the 2p3/2 orbital in the experiment indicates that the breakdown of the N=28 magic number is more rapid than the theoretical calculations suggest., We would like to thank the NSCL staff for the preparation of the radioactive beam at the Coupled Cyclotron Facility. This work was supported by the US National Science Foundation under Grants No. PHY-1306297, No. PHY-1102511, and No. PHY-1565546, by the US Department of Energy (DOE) National Nuclear Security Administration through the Nuclear Science and Security Consortium under Award No. DE-NA0003180, and by the DOE-SC Office of Nuclear Physics under Grant No. DE-SC002045. GRETINA was funded by the DOE, Office of Science. Operation of the array at NSCL was supported by the DOE under Grants No. DESC0014537 (NSCL) and No. DE-AC02-05CH11231 (LBNL). S.M. acknowledges support from Japan Society for the Promotion of Science (JSPS) Grant-in-Aid for JSPS Research Fellow Grant No. JP15J08882. K.W. acknowledges support from the Spanish Ministerio de Economía y Competitividad RYC-2017-22007. J.A.T. acknowledges support from the Science and Technology Facilities Council (UK) Grant No. ST/L005743/1.

New experimental data obtained from γ-ray tagged one-neutron and one-proton knockout from 55Co is presented. A candidate for the sought-after T = 1, T z = 0, Jπ = 6+ state in 54Co is proposed based on a comparison to the new data on 54Fe, the corresponding observables predicted by large-scale-shell-model (LSSM) calculations in the full fp-model space employing charge-dependent contributions, and isospin-symmetry arguments. Furthermore, possible isospin-symmetry breaking in the A = 54, T = 1 triplet is studied by calculating the experimental c coefficients of the isobaric mass multiplet equation (IMME) up to the maximum possible spin J = 6 expected for the (1f7/2)-2 two-hole configuration relative to the doubly-magic nucleus 56Ni. The experimental quantities are compared to the theoretically predicted c coefficients from LSSM calculations using two-body matrix elements obtained from a realistic chiral effective field theory potential at next-to-next-to-next-to leading order (N3LO).

We present a high-resolution in-beam γ-ray spectroscopy study of excited states in the mirror nuclei 55Co and 56Ni following one-nucleon knockout from a projectile beam of 56Ni. The newly determined partial cross sections and the γ-decay properties of excited states provide a test of state-of-the-art nuclear structure models and probe mirror symmetry in unique ways. The new experimental data are compared to large-scale shell-model calculations in the full pf space and including charge-dependent contributions. A mirror asymmetry for the partial cross sections leading to the two lowest 3/2− states in the A = 55 mirror pair was identified as well as a significant difference in the E1 decays from the 1/21+ state to the same two 3/2− states. The mirror asymmetry in the partial cross sections cannot be reconciled with the present shell-model picture or small mixing introduced in a two-state model. The observed mirror asymmetry in the E decay pattern, however, points at stronger mixing between the two lowest 3/2− states in 55Co than in its mirror 55Ni.

Low lying states of neutron-rich 32Ne were populated by means of one- and two-proton knock- out reactions at the RIKEN Radioactive Isotope Beam Factory. A new transition is observed at 1410(15) keV and assigned to the 4+ 1 → 2+ 1 decay. With this energy the R4/2 ratio is calculated to be 2.99(6), lying close to the rigid rotor limit and suggests a high degree of collectivity and strongest deformation among neutron-rich Neon isotopes. Comparisons of experimental inclusive and exclu- sive reaction cross sections with shell model and eikonal reaction dynamical calculations reveals considerable quenching for this highly asymmetric system and contributes to systematic trends.

In atomic nuclei, the spin-orbit interaction originates from the coupling of the orbital motion of a nucleon with its intrinsic spin. Recent experimental and theoretical works have suggested a weakening of the spin-orbit interaction in neutron-rich nuclei far from stability. To study this phenomenon, we have investigated the spin-orbit energy splittings of single-hole and single-particle valence neutron orbits of $^{132}$Sn. The spectroscopic strength of single-hole states in $^{131}$Sn was determined from the measured differential cross sections of the tritons from the neutron-removing $^{132}$Sn(d, t)$^{131}$Sn reaction, which was studied in inverse kinematics at the Holifield Radioactive Ion Beam Facility at Oak Ridge National Laboratory. The spectroscopic factors of the lowest 3/2+, 1/2+ and 5/2+ states were found to be consistent with their maximal values of (2j+1), confirming the robust N=82 shell closure at $^{132}$Sn. We compared the spin-orbit splitting of neutron single-hole states in $^{131}$Sn to those of single-particle states in $^{133}$Sn determined in a recent measurement of the $^{132}$Sn(d, p)$^{133}$Sn reaction. We found a significant reduction of the energy splitting of the weakly bound 3p orbits compared to the well-bound 2d orbits, and that all the observed energy splittings can be reproduced remarkably well by calculations using a one-body spin-orbit interaction and a Woods–Saxon potential of standard radius and diffuseness. The observed reduction of spin-orbit splitting can be explained by the extended radial wavefunctions of the weakly bound orbits, without invoking a weakening of the spin-orbit strength.

International audience; The (Be10,B*10[1.74 MeV]) charge-exchange reaction at 100 AMeV is presented as a new probe for isolating the isovector (ΔT=1) nonspin-transfer (ΔS=0) response of nuclei, with Si28 being the first nucleus studied. By using a secondary Be10 beam produced by fast fragmentation of O18 nuclei at the NSCL Coupled Cyclotron Facility, applying the dispersion-matching technique with the S800 magnetic spectrometer to determine the excitation energy in Al28, and performing high-resolution γ-ray tracking with the Gamma-Ray Energy Tracking In-beam Nuclear Array (GRETINA) to identify the 1022-keV γ ray associated with the decay from the 1.74-MeV T=1 isobaric analog state in B10, a ΔS=0 excitation-energy spectrum in Al28 was extracted. Monopole and dipole contributions were determined through a multipole-decomposition analysis, and the isovector giant dipole resonance and isovector giant monopole resonance (IVGMR) were identified. The results show that this probe is a powerful tool for studying the elusive IVGMR, which is of interest for performing stringent tests of modern density functional theories at high excitation energies and for constraining the bulk properties of nuclei and nuclear matter. The extracted distributions were compared with theoretical calculations based on the normal-modes formalism and the proton-neutron relativistic time-blocking approximation. Calculated cross sections based on these strengths underestimate the data by about a factor of 2, which likely indicates deficiencies in the reaction calculations based on the distorted wave Born approximation.

International audience; The distribution of single-particle strength in $^{67,69}$Ni was characterized with one-neutron knockout reactions from intermediate-energy $^{68,70}$Ni secondary beams, selectively populating neutron-hole configurations at N=39 and 41, respectively. The spectroscopic strengths deduced from the measured partial cross sections to the individual final states, as tagged by their γ-ray decays, are used to identify and quantify neutron configurations in the wave functions. While $^{69}$Ni compares well with shell-model predictions, the results for $^{67}$Ni challenge the validity of current effective shell-model Hamiltonians by revealing discrepancies that cannot be explained so far. These results suggest that our understanding of the low-lying states in the neutron-rich, semimagic Ni isotopes may be incomplete and requires further investigation on both the experimental and theoretical sides.

Cross sections of 1n-removal reactions from the neutron-rich nucleus (37)Mg on C and Pb targets and the parallel momentum distributions of the (37)Mg residues from the C target have been measured at 240 MeV/nucleon. A combined analysis of these distinct nuclear- and Coulomb-dominated reaction data shows that the (37)Mg ground state has a small 1n separation energy of 0.22(-0.09)(+0.12) MeV and an appreciable p-wave neutron single-particle strength. These results confirm that (37)Mg lies near the edge of the "island of inversion" and has a sizable p-wave neutron halo component, the heaviest such system identified to date.

The one-proton knockout reaction Be9(Ti54,Sc53+γ)X at 72 MeV/nucleon has been measured. The location of the first 3/2- state at 2110(3) keV was confirmed, and new γ-ray transitions were observed at 1111(2), 1273(2), 1539(4), and 2495(5) keV. Large spectroscopic strength to excited states in Sc53 was found and attributed to the knockout of sd-shell protons.

The one-neutron knockout reaction Be9(Cr57,Cr56+γ)X has been measured in inverse kinematics with an intermediate-energy beam. Cross sections to individual states in Cr56 were partially untangled through the detection of the characteristic γ-ray transitions in coincidence with the reaction residues. The experimental inclusive longitudinal momentum distribution and the yields to individual states are compared to calculations that combine spectroscopic factors from the full fp shell model and nucleon-removal cross sections computed in a few-body eikonal approach.

International audience; Data have been obtained on exclusive single neutron knockout cross sections from 12Be to study its ground state structure. The cross sections for the production of 11Be in its ground state (1/2+) and first excited state (0.32 MeV, 1/2−) have previously been measured, indicating a strong (2s½)2 component to the 12Be ground state. In the present experiment, performed at the GANIL laboratory, cross sections for the first (0.32 MeV, 1/2−) and second (1.78 MeV, 5/2+, unbound) excited states in 11Be were measured, which gives information on the admixture of (1p½)2 and (1math)2 components in the ground state of 12Be. A fragmentation beam of 12Be of ∼10000 pps (95% pure) was incident on a carbon target at 41 MeV/u. The beam particles were tracked onto the target, and their energies were measured event‐by‐event. The beam‐like residues were measured in a position sensitive telescope mounted at zero degrees, and neutrons were measured in the DéMoN array. The 1/2− state of 11Be was identified by measuring coincident 320 keV γ‐rays, using four NaI detectors. Full kinematic reconstruction of unbound states in 11Be was performed using coincident neutrons and 10Be ions. Detailed simulations were performed in order to interpret the data, and spectroscopic factors were calculated, using preliminary single particle removal cross sections calculated using a Glauber model. © 2005 American Institute of Physics

Partial cross sections and corresponding momentum distributions have been studied in the: one-neutron knockout reaction (Be-12. Be-11 + gamma) on a Be-9 target at 78 MeV/nucleon. The resulting spectroscopic factors for the only two bound starts of Be-11 an 0.42 +/- 0.06 (1/2(+)) and 0.37 +/- 0.06 (1/2(-)), where the errors an experimental only. This result shows that N = 8 is not a good closed shell in the neutron-rich Be-12 and that the last neutron pair is two-thirds in the (1s(2) + 0d(2)) intruder configuration.

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