Deformation versus Sphericity in the Ground States of the Lightest Gold Isotopes.

The changes in mean-squared charge radii of neutron-deficient gold nuclei have been determined using the in-source, resonance-ionization laser spectroscopy technique, at the ISOLDE facility (CERN). From these new data, nuclear deformations are inferred, revealing a competition between deformed and spherical configurations. The isotopes ^{180,181,182}Au are observed to possess well-deformed ground states and, when moving to lighter masses, a sudden transition to near-spherical shapes is seen in the extremely neutron-deficient nuclides, ^{176,177,179}Au. A case of shape coexistence and shape staggering is identified in ^{178}Au which has a ground and isomeric state with different deformations. These new data reveal a pattern in ground-state deformation unique to the gold isotopes, whereby, when moving from the heavy to light masses, a plateau of well-deformed isotopes exists around the neutron midshell, flanked by near-spherical shapes in the heavier and lighter isotopes-a trend hitherto unseen elsewhere in the nuclear chart. The experimental charge radii are compared to those from Hartree-Fock-Bogoliubov calculations using the D1M Gogny interaction and configuration mixing between states of different deformation. The calculations are constrained by the known spins, parities, and magnetic moments of the ground states in gold nuclei and show a good agreement with the experimental results.

Large Shape Staggering in Neutron-Deficient Bi Isotopes.

The changes in the mean-square charge radius (relative to ^{209}Bi), magnetic dipole, and electric quadrupole moments of ^{187,188,189,191}Bi were measured using the in-source resonance-ionization spectroscopy technique at ISOLDE (CERN). A large staggering in radii was found in ^{187,188,189}Bi^{g}, manifested by a sharp radius increase for the ground state of ^{188}Bi relative to the neighboring ^{187,189}Bi^{g}. A large isomer shift was also observed for ^{188}Bi^{m}. Both effects happen at the same neutron number, N=105, where the shape staggering and a similar isomer shift were observed in the mercury isotopes. Experimental results are reproduced by mean-field calculations where the ground or isomeric states were identified by the blocked quasiparticle configuration compatible with the observed spin, parity, and magnetic moment.

78Ni revealed as a doubly magic stronghold against nuclear deformation.

Nuclear magic numbers correspond to fully occupied energy shells of protons or neutrons inside atomic nuclei. Doubly magic nuclei, with magic numbers for both protons and neutrons, are spherical and extremely rare across the nuclear landscape. Although the sequence of magic numbers is well established for stable nuclei, experimental evidence has revealed modifications for nuclei with a large asymmetry between proton and neutron numbers. Here we provide a spectroscopic study of the doubly magic nucleus 78Ni, which contains fourteen neutrons more than the heaviest stable nickel isotope. We provide direct evidence of its doubly magic nature, which is also predicted by ab initio calculations based on chiral effective-field theory interactions and the quasi-particle random-phase approximation. Our results also indicate the breakdown of the neutron magic number 50 and proton magic number 28 beyond this stronghold, caused by a competing deformed structure. State-of-the-art phenomenological shell-model calculations reproduce this shape coexistence, predicting a rapid transition from spherical to deformed ground states, with 78Ni as the turning point.

Erratum: Spectroscopic Quadrupole Moments in

This corrects the article DOI: 10.1103/PhysRevLett.116.022701.

Spectroscopic Quadrupole Moments in {96,98}Sr: Evidence for Shape Coexistence in Neutron-Rich Strontium Isotopes at N=60.

Neutron-rich {96,98}Sr isotopes have been investigated by safe Coulomb excitation of radioactive beams at the REX-ISOLDE facility. Reduced transition probabilities and spectroscopic quadrupole moments have been extracted from the differential Coulomb excitation cross sections. These results allow, for the first time, the drawing of definite conclusions about the shape coexistence of highly deformed prolate and spherical configurations. In particular, a very small mixing between the coexisting states is observed, contrary to other mass regions where strong mixing is present. Experimental results have been compared to beyond-mean-field calculations using the Gogny D1S interaction in a five-dimensional collective Hamiltonian formalism, which reproduce the shape change at N=60.

Primer reporte del dinoflagelado potencialmente tóxico Alexandrium minutum Halim 1960 en el litoral peruano

En el presente trabajo se registra por primera vez al dinoflagelado potencialmente tóxico Alexandrium minutum Halim 1960 causante de las floraciones algales de marzo del 2006 y febrero del 2009, en el en el litoral del Callao, Perú. La identificación de A. minutum se realizó mediante un examen morfo-taxonómico detallando sus placas con microscopía de luz y epifluorescencia. Para su cuantificación se utilizaron cámaras de sedimentación. Nuestros resultados reportan por primera vez la presencia de A. minutum en el lado sudamericano del Pacífico, frente al Callao. Las características en tamaño, forma y morfología tecal que exhibieron los especímenes fueron muy similares a las descripciones clásicas de esta especie.

Herein, we report the first record of the potentially toxic dinoflagellate Alexandrium minutum Halim 1960 from the Peruvian littoral. Alexandrium minutum produced the algae bloom in March 2006 and February 2009, in the Callao bay. Its identification was carried out by a morpho-taxonomic examination, detailing their plates with light and epifluorescence microscopy, moreover its quantification was realized in sedimentation chambers. This is the first report of A. minutum for Southeast Pacific. The characteristics in size, shape and thecal morphology were similarly to original descriptions of this species.

First Gogny-Hartree-Fock-Bogoliubov nuclear mass model.

We present the first Gogny-Hartree-Fock-Bogoliubov (HFB) model which reproduces nuclear masses with an accuracy comparable with the best mass formulas. In contrast with the Skyrme-HFB nuclear-mass models, an explicit and self-consistent account of all the quadrupole correlation energies are included within the 5D collective Hamiltonian approach. The final rms deviation with respect to the 2149 measured masses is 798 keV. In addition, the new Gogny force is shown to predict nuclear and neutron matter properties in agreement with microscopic calculations based on realistic two- and three-body forces.

M1 gamma strength for zirconium nuclei in the photoneutron channel.

Photoneutron cross sections were measured for 91Zr, 92Zr, and 94Zr near the neutron separation energy with quasimonochromatic gamma rays. The data exhibit some extra components around the neutron threshold. A coherent analysis of the photoneutron data for 92Zr together with the neutron capture on 91Zr based on the microscopic Hartree-Fock-Bogoliubov plus quasiparticle random-phase approximation model for the E1 strength has revealed the presence of an M1 resonance at 9 MeV. The microscopic approach systematically shows the same M1 strength in the photoneutron cross section for 91Zr and 94Zr. The total M1 strength is about 75% larger than the strength predicted by the systematics, being qualitatively consistent with the giant M1 resonance observed in the inelastic proton scattering.

Systematics of the first 2+ excitation with the Gogny interaction.

We report the first comprehensive calculations of 2(+) excitations with a microscopic theory applicable to over 90% of the known nuclei. The theory uses a quantal collective Hamiltonian in five dimensions. The only parameters in theory are those of the finite-range, density-dependent Gogny D1S interaction. The following properties of the lowest 2(+) excitations are calculated: excitation energy, reduced transition probability, and spectroscopic quadrupole moment. We find that the theory is very reliable to classify the nuclei by shape. For deformed nuclei, average excitation energies and transition quadrupole moments are within 5% of the experimental values, and the dispersion about the averages are roughly 20% and 10%, respectively. Including all nuclei in the performance evaluation, the average transition quadrupole moment is 11% too high and the average energy is 13% too high.