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Self-conjugate nuclei in the A≈70–80 region have attracted a great deal of attention due to phenomena such as shape coexistence and increasing collectivity along the N=Z line. We investigate the structure of nuclei in this region through lifetime measurements using the GRETINA array. The first implementation of the Differential Recoil Distance Doppler Shift technique with fast radioactive beams is demonstrated and verified through a measurement of the well-known B(E2;21+→01+) transition strength in 74Kr. The method is then applied to determine the B(E2;21+→01+) transition strength in 74Rb, the heaviest odd–odd N=Z nucleus for which this quantity has been determined. This result and extended systematics along N=Z suggest the dominance of the isoscalar part of the quadrupole transition strengths in self-conjugate nuclei, as well as the possible presence of shape coexistence in 74Rb. Keywords: Lifetimes, Nuclear transition probabilities, Isospin

The low-spin structure of the semimagic ^{64}Ni nucleus has been considerably expanded: combining four experiments, several 0^{+} and 2^{+} excited states were identified below 4.5 MeV, and their properties established. The Monte Carlo shell model accounts for the results and unveils an unexpectedly complex landscape of coexisting shapes: a prolate 0^{+} excitation is located at a surprisingly high energy (3463 keV), with a collective 2^{+} state 286 keV above it, the first such observation in Ni isotopes. The evolution in excitation energy of the prolate minimum across the neutron N=40 subshell gap highlights the impact of the monopole interaction and its variation in strength with N. ispartof: PHYSICAL REVIEW LETTERS vol:125 issue:10 ispartof: location:United States status: published

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.

© 2019 American Physical Society. One of the most exotic light neutron-rich nuclei currently accessible for experimental study is Mg40, which lies at the intersection of the nucleon magic number N=28 and the neutron drip line. Low-lying excited states of Mg40 have been studied for the first time following a one-proton removal reaction from Al41, performed at the Radioactive Isotope Beam Factory of RIKEN Nishina Center with the DALI2 γ-ray array and the ZeroDegree spectrometer. Two γ-ray transitions were observed, suggesting an excitation spectrum that shows unexpected properties as compared to both the systematics along the Z=12, N≥20 Mg isotopes and available state-of-the-art theoretical model predictions. A possible explanation for the observed structure involves weak-binding effects in the low-lying excitation spectrum.

Background: In the late stages of stellar core collapse just prior to core bounce, electron captures on medium-heavy nuclei drive deleptonization. Therefore, simulations require the use of accurate reaction rates. Nuclei with neutron number near N=50 above atomic number Z=28 play an important role. Rates presently used in astrophysical simulations rely primarily on a relatively simple single-state approximation. In order to improve the accuracy of the astrophysical simulations, experimental data are needed to test the electron-capture rates and to guide the development of better theoretical models and astrophysical simulations. Purpose: The purpose of the present work was to measure the Gamow-Teller transition strength from 86Kr to 86Br, to derive the stellar electron-capture rates based on the extracted strengths, and to compare the derived rates with rates based on shell-model and quasiparticle random-phase approximation (QRPA) Gamow-Teller strengths calculations, as well as the single-state approximation. An additional purpose was to test the impact of using improved electron-capture rates on the late evolution of core-collapse supernovae. Method: The Gamow-Teller strengths from 86Kr were extracted from the 86Kr(t, 3He+γ) charge-exchange reaction at 115MeV/u. The electron-capture rates were calculated as a function of stellar density and temperature. Besides the case of 86Kr, the electron-capture rates based on the QRPA calculations were calculated for 78 additional isotopes near N=50 above Z=28. The impact of using these rates instead of those based on the single-state approximation is studied in a spherically symmetrical simulation of core collapse just prior to bounce. Results: The derived electron-capture rates on 86Kr from the experimental Gamow-Teller strength distribution are much smaller than the rates estimated based on the single-state approximation. Rates based on Gamow-Teller strengths estimated in shell-model and QRPA calculations are more accurate. The core-collapse supernova simulation with electron-capture rates based on the QRPA calculations indicate a significant reduction in the deleptonization during the collapse phase. Conclusions: It is important to utilize microscopic theoretical models that are tested by experimental data to constrain and estimate Gamow-Teller strengths and derived electron-capture rates for nuclei near N=50 that are inputs for astrophysical simulations of core-collapse supernovae and their multimessenger signals, such as the emission of neutrinos and gravitational waves.

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 02+ state in Mg32 populated in a two-proton removal reaction. We confirm the 02+→21+γ-ray transition of Mg32 and constrain the 02+ decay lifetime, suggesting a large collectivity. The small partial cross section populating the 02+ state in this reaction provides experimental evidence for the reduced occupancy of the normal configuration of the 02+ state, indicating the intruder dominance of this state.

© 2018 American Physical Society. A model-independent technique was used to determine the γ-ray strength function (γSF) of Fe56 down to γ-ray energies less than 1 MeV for the first time with GRETINA using the (p,p′) reaction at 16 MeV. No difference was observed in the energy dependence of the γSF built on 2+ and 4+ final states, supporting the Brink hypothesis. In addition, angular distribution and polarization measurements were performed. The angular distributions are consistent with dipole radiation. The polarization results show a small bias towards magnetic character in the region of the enhancement.

© 2017 The Authors Recent results from RIKEN/RIBF on the low-lying level structure of 29F are interpreted within the Particle-Rotor Model. We show that the experimental data can be understood in the Rotation-aligned Coupling Scheme, with the 5/2+ ground state as the bandhead of a decoupled band. In this picture, the energy of the observed 1/21+ state correlates strongly with the rotational energy of the core and provides an estimate of the 2+ energy in 28O. Our analysis suggests a moderate deformation, ϵ2∼0.16, and places the 2+ in 28O at ∼ 2.5 MeV.

© 2017 The Author(s) The structure of 71Zn was investigated by one-neutron transfer and heavy-ion induced complex (deep-inelastic) reactions using the GRETINA-CHICO2 and the Gammasphere setups, respectively. The observed inversion between the 9/2+ and 1/2− states is explained in terms of the role of neutron pairing correlations. Non-collective sequences of levels were delineated above the 9/2+ isomeric state. These are interpreted as being associated with a modest oblate deformation in the framework of Monte-Carlo shell-model calculations carried out with the A3DA-m Hamiltonian in the pfg9/2d5/2 valence space. Similarities with the structure of Ni402868 were observed and the shape-coexistence mechanism in the N=40 region of neutron-rich nuclei is discussed in terms of the so-called Type-II shell evolution, with an emphasis on proton–neutron correlations between valence nucleons, especially those involving the shape-driving g9/2 neutron orbital. ispartof: Physics Letters B vol:775 pages:271-276 status: published

© 2017 The Author(s) The structure of71Zn was investigated by one-neutron transfer and heavy-ion induced complex (deep-inelastic) reactions using the GRETINA-CHICO2 and the Gammasphere setups, respectively. The observed inversion between the 9/2+and 1/2−states is explained in terms of the role of neutron pairing correlations. Non-collective sequences of levels were delineated above the 9/2+isomeric state. These are interpreted as being associated with a modest oblate deformation in the framework of Monte-Carlo shell-model calculations carried out with the A3DA-m Hamiltonian in the pfg9/2d5/2valence space. Similarities with the structure of Ni402868 were observed and the shape-coexistence mechanism in the N=40 region of neutron-rich nuclei is discussed in terms of the so-called Type-II shell evolution, with an emphasis on proton–neutron correlations between valence nucleons, especially those involving the shape-driving g9/2neutron orbital.

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

© 2017 American Physical Society. Despite the more than 1 order of magnitude difference between the measured dipole moments in Ba144 and Ba146, the octupole correlations in Ba146 are found to be as strong as those in Ba144 with a similarly large value of B(E3;3-→0+) determined as 48(+21-29) W.u. The new results not only establish unambiguously the presence of a region of octupole deformation centered on these neutron-rich Ba isotopes, but also manifest the dependence of the electric dipole moments on the occupancy of different neutron orbitals in nuclei with enhanced octupole strength, as revealed by fully microscopic calculations.

© 2016 Elsevier B.V. In this paper, we provide a formalism for the characterization of tracking arrays with emphasis on the proper corrections required to extract their photopeak efficiencies and peak-to-total ratios. The methods are first applied to Gammasphere, a well characterized 4π array based on the principle of Compton suppression, and subsequently to GRETINA. The tracking efficiencies are then discussed and some guidelines as to what clustering angle to use in the tracking algorithm are presented. It was possible, using GEANT4 simulations, to scale the measured efficiencies up to the expected values for the full 4π implementation of GRETA.

©=0.65(+17-23) eb3/2, corresponds to a reduced B(E3) transition probability of 48(+25-34) W.u. This result represents an unambiguous determination of the octupole collectivity, is larger than any available theoretical prediction, and is consistent with octupole deformation. © 2016 American Physical Society. The neutron-rich nucleus Ba144 (t1/2=11.5 s) is expected to exhibit some of the strongest octupole correlations among nuclei with mass numbers A less than 200. Until now, indirect evidence for such strong correlations has been inferred from observations such as enhanced E1 transitions and interleaving positive- and negative-parity levels in the ground-state band. In this experiment, the octupole strength was measured directly by sub-barrier, multistep Coulomb excitation of a post-accelerated 650-MeV Ba144 beam on a 1.0-mg/cm2 Pb208 target. The measured value of the matrix element, 31-M(E3)01+

© 2015 The Authors. The odd-odd nucleus 30Na is studied via a one-proton, one-proton-one-neutron and one-neutron removal reaction using an intermediate-energy 31Mg, 32Mg and 31Na radioactive ion beam, respectively. Combining high-resolution γ-ray spectroscopy with the selectivity of the three reaction mechanisms, we are able to distinguish multiple particle-hole configurations. Negative-parity states in 30Na are observed for the first time, providing an important measure of the excitation of the 1p1h/3p3h configuration and hence the sd-pf shell gap. The extracted band structures and level energies serve as invaluable input for the theoretical refinement of the effective interactions used in this region.

A measurement of the direct two-proton removal from Si42 has provided the first structural information on the N=28 isotone Mg40. The value for the inclusive cross section for two-proton removal from Si42 of 40+27−17 μb is significantly lower than that predicted by structure calculations using the recent SDPF-MU shell-model effective interaction combined with eikonal reaction theory. This observed discrepancy is consistent with the interpretation that only one of the predicted low-lying 0+ states in Mg40 is bound. A two-state mixing analysis describing two-proton knockout cross sections along N=28 provides support for the interpretation of a prolate-deformed Mg40 ground state.

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.

Excited states in the very neutron-rich nuclei 35 Mg and 33 Na were populated in the fragmentation of a 38 Si projectile beam on a Be target at 83 MeV / u beam energy. We report on the first observation of γ -ray transitions in 35 Mg, the odd- N neighbor of 34 Mg and 36 Mg, which are known to be part of the ‘island of inversion” around N = 20. The results are discussed in the framework of large-scale shell-model calculations. For the A = 3 Z nucleus 33 Na, a new γ -ray transition was observed that is suggested to complete the γ -ray cascade 7 / 2 + → 5 / 2 + → 3 / 2 + gs connecting three neutron two-particle–two-hole intruder states that are predicted to form a close-to-ideal K = 3 / 2 rotational band in the strong-coupling limit. ispartof: Physical Review C, Nuclear Physics vol:83 issue:4 status: published

The neutron-rich N = 50 isotones 82 Ge and 84 Se were investigated using intermediate-energy Coulomb excitation on a 197 Au target and inelastic scattering on 9 Be. As typical for intermediate-energy Coulomb excitation with projectile energies exceeding 70 MeV / nucleon, only the first 2 + states were excited in 82 Ge and 84 Se. However, in the inelastic scattering on a 9 Be target, a strong population of the first 4 + state was observed for 84 Se, while there is no indication of a similarly strong excitation of the corresponding state in the neighboring even-even isotone 82 Ge. The results are discussed in the framework of systematics and shell-model calculations using three different effective interactions ispartof: Physical Review C, Nuclear Physics vol:81 issue:6 status: published

γ -ray decays from excited states in 30 Ne and inclusive and exclusive cross sections were measured in the 9 Be( 32 Mg, 30 Ne + γ ) X two-proton knockout reaction at incident beam energies of 99.7 and 86.7 MeV / nucleon. The measured cross section is suppressed compared to calculations and is indicative of a reduced overlap of initial and final state wave functions in 32 Mg and 30 Ne. We interpret this reduction as due to large 4 p 4 h amplitudes present in the 30 Ne ground state wave function, but not 32 Mg. Such large intruder components are predicted to help stabilize the heavier fluorine isotopes against neutron emission. ispartof: Physical Review C, Nuclear Physics vol:81 issue:4 status: published

In-beam gamma-ray spectroscopy of Fe-66,Fe-68 following intermediate-energy one- and two-proton knockout from cobalt and nickel secondary beams has been performed at the National Superconducting Cyclotron Laboratory. New transitions have been observed in Fe-66 and Fe-68. This is the first observation of gamma-ray transitions in Fe-68. In addition, Cr-64 was produced using the Be-9(Fe-66,Cr-64)X two-proton knockout reaction. An unexpectedly low inclusive cross section is observed for this reaction, an order of magnitude smaller than for the Be-9(Ni-68,Fe-66)X reaction. This observation is discussed in terms of a significant structural difference between Fe-66 and Cr-64 and considerable admixtures of nu(pf)(n-2) (g(9/2))(+2) configurations in the ground and excited states of Cr-64 at N = 40.

The breakdown of the N = 20 magic number in the so-called island of inversion around 32 Mg is well established. Recently developed large-scale shell-model calculations suggest a transitional region between normal- and intruder-dominated nuclear ground states, thus modifying the boundary of the island of inversion. In particular, a dramatic change in single-particle structure is predicted between the ground states of 30 Mg and 32 Mg, with the latter consisting nearly purely of 2p-2h N = 20 cross-shell configurations. Single-neutron knockout experiments on 30 , 32 Mg projectiles have been performed. We report on a first direct observation of intruder configurations in the ground states of these very neutron-rich nuclei. Spectroscopic factors to low-lying negative-parity states in the knockout residues are deduced and compare well with shell-model predictions ispartof: Physical Review C, Nuclear Physics vol:77 issue:1 status: published

The even 52 – 56 Ti isotopes have been studied with intermediate-energy Coulomb excitation and absolute B ( E 2; 0 + → 2 + 1 ) transition rates have been obtained. These data confirm the presence of a subshell closure at neutron number N = 32 in neutron-rich nuclei above the doubly magic nucleus 48 Ca and provide no direct evidence for the predicted N = 34 closure. Large-scale shell model calculations with the most recent effective interactions are unable to reproduce the magnitude of the measured strengths in the semimagic Ti nuclei and their strong variation with neutron number. ispartof: Physical Review C, Nuclear Physics vol:71 issue:4 status: published

Low-lying excited states of 46 Ar have been studied via inverse-kinematics proton scattering with a thick target. Coupled-channels calculations have been used to extract the deformation length of the 2 + 1 state. This result, combined with existing Coulomb excitation data, yields a ratio of the neutron-to-proton transition matrix elements of M n /M p = 1 . 19(25) N/Z , showing a departure from the proton dominance observed in the N = 28 isotones above 48 Ca. The status of the N = 28 shell below 48 Ca is discussed ispartof: Physical Review C, Nuclear Physics vol:72 issue:2 status: published

The collectivity of the odd-mass nucleus 55 Ni was explored via intermediate-energy Coulomb excitation using a powerful combination of particle and g -ray spectroscopy. A g -ray at 2879 ( 18 ) keV was observed and is interpreted to deexcite a member of the core-coupled quintuplet 2 1 + s 56 Ni d ^ n f 7/2 −1 at the same energy. By similarity with the mirror nucleus 55 Co, transition probabilities were calculated assuming J p =9/2 − and J p =11/2 − for this state. Both assumptions lead to a transition strength higher than predicted by a large-scale shell-model calculation using the GXPF1 effective interaction and exceed the value predicted within a simple weak-coupling approach ispartof: Physical Review C, Nuclear Physics vol:70 issue:6 status: published