The Coulomb excitation experiment to study electromagnetic properties of the heaviest stable Mo isotope, 100 Mo, was performed using a 76 MeV 32 S beam from the Warsaw cyclotron U-200P. Magnitudes and relative signs of 26 E1, E2, E3, and M1 matrix elements coupling nine low-lying states in 100 Mo were determined using the least-squares code GOSIA. Diagonal matrix elements (related to the spectroscopic quadrupole moments) of the 2 + 1 , 2 + 2 , and 2 + 3 states as well as the 4 + 1 state were extracted. The resulting set of reduced E2 matrix elements was complete and precise enough to obtain, using the quadrupole sum rules approach, quadrupole deformation parameters of 100 Mo in its two lowest 0 + states: ground and excited. The overall deformation of the 0 + 1 and 0 + 2 states in 100 Mo is of similar magnitude, in both cases larger compared to what was found for the neighboring isotopes 96 Mo and 98 Mo. At the same time, the asymetry parameters obtained for both states strongly differ, indicating a triaxial shape of the 100 Mo nucleus in the ground state and a prolate shape in the excited 0 + state. Low-energy quadrupole excitations of the 100 Mo nucleus were studied in the frame of the general quadrupole collective Bohr Hamiltonian model (GBH). The potential energy and inertial functions were calculated using the adiabatic time-dependent Hartree-Fock-Bogoliubov (ATDHFB) method starting from two possible variants of the Skyrme effective interaction: SIII and Sly4. The overall quadrupole deformation parameters resulting from the GBH calculations with the SLy4 variant of the Skyrme interaction are slightly closer to the experimentally obtained values than those obtained using SIII.
Characterisation of the few doubly magic nuclei, known and predicted, provides a benchmark for our knowledge of the fundamental forces that drive the evolution of shell closures with proton-to-neutron asymme
In-beam γ-ray spectroscopy of 79 Cu is performed at the Radioactive Isotope Beam Factory of RIKEN. The nucleus of interest is produced through proton knockout from a 80Zn beam at 270 MeV=nucleon. The level scheme up to 4.6 MeV is established for the first time and the results are compared to Monte Carlo shell-model calculations. We do not observe significant knockout feeding to the excited states below 2.2 MeV, which indicates that the Z ¼ 28 gap at N ¼ 50 remains large. The results show that the 79 Cu nucleus can be described in terms of a valence proton outside a 78Ni core, implying the magic character of the latter. DOI: 10.1103/PhysRevLett.119.192501 The shell model constitutes one of the main building blocks of our understanding of nuclear structure. Its robustness is well proven for nuclei close to the valley of stability, where it successfully predicts and explains the occurrence of magic numbers [1,2]. However, these magic numbers are not universal throughout the nuclear chart and their evolution far from stability, observed experimentally over the last decades, has generated much interest [3]. For example, the magic numbers N ¼ 20 and 28 may disappear [4][5][6][7] while new magic numbers arise at N ¼ 14, 16 and 32, 34, respectively [8][9][10][11][12][13]. Although shell gaps, defined within a given theoretical framework as differences of effective single-particle energies (ESPE), are not observables [14], they are useful quantities to assess the underlying structure of nuclei [15][16][17]. The nuclear potential acting on nuclei far from stability can induce drifts of the single-particle orbitals and their behavior as a function of isospin can be understood within the shell model [18][19][20][21][22]. Difficulties arise, however, when the single-particle properties are masked by correlations that stem from residual interactions and discriminating between the two effects is nontrivial.In the shell model as it was initially formulated, the proton πf 7=2 orbital separates from the 3ℏω harmonic oscillator shell because of the spin-orbit splitting and forms the Z ¼ 28 gap. The neutron νg 9=2 orbital splits off from the 4ℏω shell to join the 3ℏω orbits and creates a magic number at N ¼ 50. With 28 protons and 50 neutrons, the 78 Ni nucleus is thus expected to be one of the most neutronrich doubly magic nuclei, making it of great interest for nuclear structure. Up to now, no evidence has been found for the disappearance of the shell closures at Z ¼ 28
Previously published particle-γ coincidence data on the 64 Ni(p, p γ) 64 Ni and 64 Ni(d, pγ) 65 Ni reactions were further analyzed to study the statistical properties of γ decay in 64,65 Ni. To do so, the γ decay to the quasicontinuum region and discrete low-lying states was investigated at γ-ray energies of 2.0-9.6 and 1.6-6.1 MeV in 64 Ni and 65 Ni, respectively. In particular, the dependence of the γ-strength function with initial and final excitation energy was studied to test the validity of the generalized Brink-Axel hypothesis. Finally, the role of fluctuations in transition strengths was estimated as a function of γ-ray and excitation energy. The γ-strength function is consistent with the hypothesis of the independence of initial excitation energy, in accordance with the generalized Brink-Axel hypothesis. The results show that the γ decay to low-lying levels displays large fluctuations for low initial excitation energies.
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