Testing microscopically derived descriptions of nuclear collectivity : Coulomb excitation of 22Mg

Many-body nuclear theory utilizing microscopic or chiral potentials has developed to the point that collectivity might be studied within a microscopic or ab initio framework without the use of effective charges; for example with the proper evolution of the E2 operator, or alternatively, through the use of an appropriate and manageable subset of particle–hole excitations. We present a precise determination of E2 strength in 22Mg and its mirror 22Ne by Coulomb excitation, allowing for rigorous comparisons with theory. No-core symplectic shell-model calculations were performed and agree with the new B(E2) values while in-medium similarity-renormalization-group calculations consistently underpredict the absolute strength, with the missing strength found to have both isoscalar and isovector components. The discrepancy between two microscopic models demonstrates the sensitivity of E2 strength to the choice of many-body approximation employed

In-beam γ -ray spectroscopy of Mn 63

Background: Neutron-rich, even-mass chromium and iron isotopes approaching neutron number N=40 have been important benchmarks in the development of shell-model effective interactions incorporating the effects of shell evolution in the exotic regime. Odd-mass manganese nuclei have received less attention, but provide important and complementary sensitivity to these interactions. Purpose: We report the observation of two new γ-ray transitions in Mn63, which establish the (9/2-) and (11/2-) levels on top of the previously known (7/2-) first-excited state. The lifetime for the (7/2-) and (9/2-) excited states were determined for the first time, while an upper limit could be established for the (11/2-) level. Method: Excited states in Mn63 have been populated in inelastic scattering from a Be9 target and in the fragmentation of Fe65. γγ coincidence relationships were used to establish the decay level scheme. A Doppler line-shape analysis for the Doppler-broadened (7/2-)→5/2-, (9/2-)→(7/2-), and (11/2-)→(9/2-) transitions was used to determine (limits for) the corresponding excited-state lifetimes. Results: The low-lying level scheme and the excited-state lifetimes were compared with large-scale shell-model calculations using different model spaces and effective interactions in order to isolate important aspects of shell evolution in this region of structural change. Conclusions: While the theoretical (7/2-) and (9/2-) excitation energies show little dependence on the model space, the calculated lifetime of the (7/2-) level and calculated energy of the (11/2-) level reveal the importance of including the neutron g9/2 and d5/2 orbitals in the model space. The LNPS effective shell-model interaction provides the best overall agreement with the new data

Collectivity at N=40 in neutron-rich Cr64

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

Neutron single-particle strengths at N=40, 42: Neutron knockout from Ni 68,70 ground and isomeric states

The distribution of single-particle strength in Ni67,69 was characterized with one-neutron knockout reactions from intermediate-energy Ni68,70 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 Ni69 compares well with shell-model predictions, the results for Ni67 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

Neutron knockout from 68,70Ni ground and isomeric states.

Neutron-rich isotopes are an important source of new information on nuclear physics. Specifically, the spin-isospin components in the nucleon-nucleon (NN) interaction, e.g., the proton-neutron tensor force, are expected to modify shell structure in exotic nuclei. These potential changes in the intrinsic shell structure are of fundamental interest. The study of the excitation energy of states corresponding to specific configurations in even-even isotopes, together with the single-particle character of the first excited states of odd-A, neutron-rich Ni isotopes, probes the evolution of the neutron orbitals around the Fermi surface as a function of the neutron number a step forward in the understanding of the region and the nature of the NN interaction at large N/Z ratios. In an experiment carried out at the National Superconducting Cyclotron Laboratory [1], new spectroscopic information was obtained for 68Ni and the distribution of single-particle strengths in 67,69Ni was characterized by means of single-neutron knockout from 68,70Ni secondary beams. The spectroscopic strengths, deduced from the measured partial cross sections to the individual states tagged by their de-exciting gamma rays, is used to identify and quantify configurations that involve neutron excitations across the N = 40 harmonic oscillator shell closure. The de-excitation γ rays were measured with the GRETINA tracking array [2]. The results challenge the validity of the most current shell-model Hamiltonians and effective interactions, highlighting shortcomings that cannot yet be explained. These results suggest that our understanding of the low-energy states in such nuclei is not complete and requires further investigation

Nuclear structure towards N=40 Ca 60: In-Beam γ-ray spectroscopy of Ti 58,60

Excited states in the neutron-rich N=38, 36 nuclei Ti60 and Ti58 were populated in nucleon-removal reactions from V61 projectiles at 90MeV/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

Unexpected distribution of ν1f7/2 strength in Ca 49

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 Ca47,49 following one-neutron knockout reactions from Ca48,50. The observed reduction of strength populating the 7/21- state in Ca49, as compared to Ca47, is inconsistent with shell-model calculations using both phenomenological interactions such as GXPF1, and interactions derived from microscopically based two- and three-nucleon forces. The result suggests a fragmentation of the l=3 strength to higher-lying states as suggested by the microscopic calculations, but the observed magnitude of the reduction is not reproduced in any shell-model description

Configuration mixing and relative transition rates between low-spin states in 68Ni

The low-spin level scheme of 68Ni was investigated following two-neutron-knockout and multinucleon-transfer reactions. The energy of the first excited state was determined to be Ex(02+)=1603.5(3) keV. Relative B(E2) transition probabilities were deduced and compared with shell-model calculations using several modern effective interactions. Theory reproduces the data well, but indicates substantial mixing of multi-particle, multi-hole configurations for the lowest observed 0+ and 2 + states

Identification of deformed intruder states in semi-magic Ni 70

The structure of semi-magic 2870Ni42 was investigated following complementary multinucleon-transfer and secondary fragmentation reactions. Changes to the higher-spin, presumed negative-parity states based on observed γ-ray coincidence relationships result in better agreement with shell-model calculations using effective interactions in the neutron f5/2pg9/2 model space. The second 2+ and (4+) states, however, can only be successfully described when proton excitations across the Z=28 shell gap are included. Monte Carlo shell-model calculations suggest that the latter two states are part of a prolate-deformed intruder sequence, establishing an instance of shape coexistence at low excitation energies similar to that observed recently in neighboring Ni68