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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).

The region of neutron-rich Cr isotopes has garnered much attention in recent years due to a rapid onset of collectivity near neutron number N=40. We report here on the first γ-ray spectroscopy beyond the (41+) state in Cr62,64, using nucleon removal reactions from several projectiles within a rare-isotope beam cocktail. A candidate for the 6+ state in Cr64 is presented as well as one for, possibly, the second excited 0+ state in Cr62. The results are discussed in comparison to the LNPS shell-model predictions that allow for neutron excitations across the N=40 harmonic oscillator gap into the g9/2 and d5/2 orbitals. The calculated level schemes for Cr62,64 reveal intriguing collective structures. From the predicted neutron particle-hole character of the low-lying states in these Cr isotopes, Cr62 emerges as a transitional system on the path to the center of the N=40 island of inversion.

The shape and collectivity of 106Cd was investigated via a sub-barrier-energy Coulomb excitation experiment performed at the National Superconducting Cyclotron Laboratory Re-accelerator facility using the JANUS setup. Transition matrix elements between low-lying states were found to agree with adopted values, and information on the shape and collectivity of higher-lying states was extracted for the first time. Locally optimized large-scale shell-model calculations were found to describe well the B(E2) transition strengths but failed to reproduce the spectroscopic quadrupole moments Qs. An analysis of the E2 rotational invariants and the normalized quadrupole moment qs indicates that this may be due to a significant degree of triaxiality in 106Cd which is not captured by the present shell-model calculations. Analogous calculations for the Fe isotopes (two protons below the Z=28 magic number) reveal the critical role of high-j neutron configurations for the description of quadrupole moments in the heavy Fe and Cd isotopes (two protons below magic Z=50), but this effect is insufficient to explain the shape of 106Cd, posing a puzzle for the understanding of nuclear structure towards N=50.

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.

Simpson and Tostevin proposed that the width and shape of exclusive parallel momentum distributions of the A-2 residue in direct two-nucleon knockout reactions carry a measurable sensitivity to the nucleon single-particle configurations and their couplings within the wave functions of exotic nuclei. We report here on the first benchmarks and use of this new spectroscopic tool. Exclusive parallel momentum distributions for states in the neutron-deficient nuclei $^{22}$Mg, $^{23}$Al, and $^{24}$Si populated in such direct two-neutron removal reactions were extracted and compared to predictions combining eikonal reaction theory and shell-model calculations. For the well-known $^{22}$Mg and $^{23}$Al nuclei, measurements and calculations were found to agree, supporting the dependence of the parallel momentum distribution width on the angular momentum composition of the shell-model two-neutron amplitudes. In $^{24}$Si, a level at 3439(9) keV, of relevance for the important $^{23}$Al(p,$\gamma$)$^{24}$Si astrophysical reaction rate, was confirmed to be the $2^+_2$ state, while the $4^+_1$ state, expected to be strongly populated in two-neutron knockout, was not observed. This puzzle is resolved by theoretical considerations of the Thomas-Ehrman shift, which also suggest that a previously reported 3471-keV state in $^{24}$Si is in fact the ($0^+_2$) level with one of the largest experimental mirror-energy shifts ever observed., Comment: Accepted for publication in Phys. Rev. C as a Rapid Communication

The neutron-rich barium nuclei have been the subject of intense interest due to the enhanced octupole correlations they are predicted to exhibit. The observation of enhanced octupole collectivity in Ba144,146 as measured in sub-barrier Coulomb excitation, consistent with static octupole deformation, has further heightened this interest. In the present work, these studies are extended to the neighboring odd-mass Ba143 to investigate the interplay between single-particle and collective octupole degrees of freedom. A new measurement of the first 92 - state lifetime is also presented. Reflection-Asymmetric Triaxial Particle Rotor Model calculations indicate that the negative-parity bands in Ba143 can be understood as a decoupled structure of νh9/2 parentage, while the positive-parity bands are built on a decoupled octupole phonon. No evidence for E3 excitation is observed in this work, but an upper limit is placed on the E3 matrix element to the lowest octupole band.

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).

The development of advanced γ-ray tracking arrays allows for a sensitive new technique to investigate elusive states of exotic nuclei with fast rare-isotope beams. By taking advantage of the excellent energy and position resolution of the Gamma-Ray Energy Tracking In-beam Nuclear Array, we developed a novel technique to identify in-flight isomeric decays of the 0⁺₂ state in 32Mg populated in a two-proton removal reaction. We confirm the 0⁺₂→2+1γ-ray transition of ³²Mg and constrain the 0⁺₂ decay lifetime, suggesting a large collectivity. The small partial cross section populating the 0⁺₂ state in this reaction provides experimental evidence for the reduced occupancy of the normal configuration of the 0⁺₂ state, indicating the intruder dominance of this state.

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.

Excited states in the neutron-rich 70Fe nucleus were populated in a one-proton removal reaction from 71Co projectiles at 87 MeV/nucleon. A new transition was observed with the γ-ray tracking array GRETINA and shown to feed the previously assigned 4+ 1 state. In comparison to reaction theory calculations with shell-model spectroscopic factors, it is argued that the new γ ray possibly originates from the 6+ 1 state. It is further shown that the Doppler-reconstructed γ-ray spectra are sensitive to the very different lifetimes of the 2+ and 4+ states, enabling their approximate measurement. The emerging structure of 70Fe is discussed in comparison to LNPS-new large-scale shell-model calculations.

Excited states in the neutron-rich 70Fe nucleus were populated in a one-proton removal reaction from 71Co projectiles at 87 MeV/nucleon. A new transition was observed with the γ-ray tracking array GRETINA and shown to feed the previously assigned 4+1 state. In comparison to reaction theory calculations with shell-model spectroscopic factors, it is argued that the new γ ray possibly originates from the 6+1 state. It is further shown that the Doppler-reconstructed γ-ray spectra are sensitive to the very different lifetimes of the 2+ and 4+ states, enabling their approximate measurement. The emerging structure of 70Fe is discussed in comparison to LNPS-new large-scale shell-model calculations.

The calcium isotopes have emerged as a critical testing ground for new microscopically derived shell-model interactions, and a great deal of experimental and theoretical focus has been directed toward this region. We investigate the relative spectroscopic strengths associated with 1f7=2 neutron hole states in 47;49Ca following one-neutron knockout reactions from 48;50Ca. The observed reduction of strength populating the 7/2

Comprehensive spectroscopy of the N = 29 nucleus 47Ar is presented, based on two complemen-tary direct reaction mechanisms: one-neutron pickup onto 46Ar projectiles and one-proton removal from the 1− ground state of 48K. The results are compared to shell-model calculations that use the state-of-the-art SDPF-U and SDPF-MU effective interactions. Also, from the 9Be(46Cl,45S+γ)X one-proton removal reaction, we report the first γ-ray transitions observed from 45S. Using compar-isons with shell-model calculations, and from the observed intensities and energy sums, we propose a first tentative level scheme for 45S.

Comprehensive spectroscopy of the N=29 nucleus Ar47 is presented, based on two complementary direct reaction mechanisms: one-neutron pickup onto Ar46 projectiles and one-proton removal from the 1- ground state of K48. The results are compared with shell-model calculations that use the state-of-the-art SDPF-U and SDPF-MU effective interactions. Also, from the Be9(Cl46,S45+γ)X one-proton-removal reaction, we report the first γ-ray transitions observed from S45. By using comparisons with shell-model calculations, and from the observed intensities and energy sums, we propose a first tentative level scheme for S45.

Excited states in the neutron-rich N=38, 36 nuclei Ti60 and Ti58 were populated in nucleon-removal reactions from V61 projectiles at 90 MeV/nucleon. The γ-ray transitions from such states in these Ti isotopes were detected with the advanced γ-ray tracking array GRETINA and were corrected event by event for large Doppler shifts (v/c∼0.4) using the γ-ray interaction points deduced from online signal decomposition. The new data indicate that a steep decrease in quadrupole collectivity occurs when moving from neutron-rich N=36, 38 Fe and Cr toward the Ti and Ca isotones. In fact, Ti58,60 provide some of the most neutron-rich benchmarks accessible today for calculations attempting to determine the structure of the potentially doubly magic nucleus Ca60.

Intermediate-energy Coulomb excitation measurements were performed on the neutron-rich isotopes 58,60,62Cr. The electric quadrupole excitation strengths, B(E2; 01+→21+), of 60,62Cr are determined for the first time. The results quantify the trend of increasing quadrupole collectivity in the Cr isotopes approaching neutron number N=40. The results are confronted with large-scale shell-model calculations in the fpgd shell using the state-of-the-art LNPS effective interaction. Different sets of effective charges are discussed that provide an improved and robust description of the B(E2) values of the neutron-rich Fe and Cr isotopes in this region of rapid shell evolution. The ratio of the neutron and proton transition matrix elements, |Mn/Mp|, is proposed as an effective tool to discriminate between the various choices of effective charges.

Intermediate-energy Coulomb excitation measurements were performed on the neutron-rich isotopes 58,60,62Cr. The electric quadrupole excitation strengths, B(E2; 0+ 1 → 2+ 1 ), of 60,62Cr are determined for the first time. The results quantify the trend of increasing quadrupole collectivity in the Cr isotopes approaching neutron number N = 40. The results are confronted with large-scale shell-model calculations in the fpgd shell using the state-of-the-art LNPS effective interaction. Different sets of effective charges are discussed that provide an improved and robust description of the B(E2) values of the neutron-rich Fe and Cr isotopes in this region of rapid shell evolution. The ratio of the neutron and proton transition matrix elements, |Mn/Mp|, is proposed as an effective tool to discriminate between the various choices of effective charges, This work was funded by the National Science Foundation under Contract No. PHY-0606007; by the US Department of Energy, Office of Nuclear Physics, under Contracts No. DE-AC02-06CH11357 and No. DE-FG02-08ER41556, and by the United Kingdom Science and Technology Facilities Council (STFC). A.P. is supported by the MICINN (Spain) (Grant No. FPA2011-29854) and by the Comunidad de Madrid (Spain) (Grant No. HEPHACOS S2009-ESP-1473)

Be9-induced inelastic scattering of Fe62,64,66 and Cr60,62,64 was performed at intermediate beam energies. Excited states in Cr64 were measured for the first time. Energies and population patterns of excited states in these neutron-rich Fe and Cr nuclei are compared and interpreted in the framework of large-scale shell-model calculations in different model spaces. Evidence for increased collectivity and for distinct structural changes between the neighboring Fe and Cr isotopic chains near N=40 is presented.