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The nuclear structure of $^{66}$Se, nucleus beyond the N=Z line on the proton-rich side of the valley of stability, was investigated by the neutron knock-out reaction $^{67}$Se($^{12}$C,X)$^{66}$Se using a $^{12}$C target. The analysis of the singles spectrum of the γ-rays emitted during the de-excitation of the populated low-lying excited states revealed two previously detected (927(4) keV, 1460(32) keV) and three new (744(6) keV, 1210(17) keV, 1661(23) keV) transitions. The 744-keV, the 1210-keV, and the 1460-keV transitions were found to be in coincidence with the one at 927 keV. The spectrum coincident with the 927-keV transition showed a further possible transition at 299(35) keV, which was obscured by significant atomic background in the singles spectrum. This transition might correspond to a peak previously reported at 273(5) keV that could not be assigned to $^{66}$Se unambiguously. Based on a comparison of the experimental data to theoretical calculations, four new excited states are proposed which suggest that $^{66}$Se exhibits shape coexistence.

The structure of $^{74}$Se at low energy was investigated via spectroscopy of internal conversion electrons at the INFN Legnaro National Laboratories (LNL). A set of internal K-conversion coefficients and monopole transition strengths was measured. A large ρ2(E0;22+→21+)⋅103=210(130) value was deduced. This result, in addition to a low upper limit for the 03+→02+ electron transition, casts in doubt a simple interpretation of the $^{74}$Se low-lying structure, in particular the recently proposed spherical, vibrational character. New microscopic beyond-mean-field calculations generally agree with the experimental results and are capable of producing a large ρ2(E0;22+→21+) value, even if still a factor ≈7 smaller than the experiment. Triaxiality and a complex shape-coexistence and mixing scenario seem responsible for this unexpected experimental result.

International audience; The Coulomb excitation of Ru102 was performed with beams of C12 and O16 ions. The beam particles scattered at forward angles were momentum analyzed with a magnetic spectrograph. The resolution achieved enabled the populations of the 21+ state, the unresolved 22+/41+, and 24+/31−, doublets of states, and the 32− state to be determined as a function of the scattering angle. These populations are compared with gosia calculations, yielding B(E2;21+→01+)=41.5±2.3 W.u., B(E2;22+→01+)=1.75±0.11 W.u., B(E3;31−→01+)=31.5±3.5 W.u., and B(E3;32−→01+)=6.8±0.5 W.u. The B(E3;31−→01+) value is significantly larger than previously measured. The weakly populated 23+ state, presumed to be a member of the band built on the 02+ state, was observed clearly for a single angle only, and a fit to its population results in B(E2;23+→01+)=0.053±0.011 W.u. Using the known γ-ray branching ratios for the 23+ level, the B(E2;23+→02+) value is calculated to be 18±4 W.u., substantially less than the B(E2;21+→01+). This suggests that the deformation of the 02+ state is lower than that of the 01+ state. The results are compared with beyond-mean-field calculations with the Gogny-D1S interaction using the symmetry-conserving configuration-mixing method.

International audience; The electromagnetic structure of Zn66 at low excitation energy was investigated via low-energy Coulomb excitation at INFN Legnaro National Laboratories, using the Gamma Array of Legnaro Infn Laboratories for nuclEar spectrOscopy (GALILEO) γ-ray spectrometer coupled to the SPIDER (Silicon PIe DEtectoR). A set of reduced E2, E3, and M1 matrix elements was extracted from the collected data using the gosia code, yielding 12 reduced transition probabilities between the low-spin states and the spectroscopic quadrupole moment of the 21+ state. The B(E2) values for transitions depopulating the 02+ state have been determined for the first time, allowing for the lifetime of this state to be deduced and, consequently, the ρ2(E0;02+→01+) monopole transition strength to be extracted. In addition, the B(E3;31−→01+) value has been determined for the first time in a Coulomb excitation experiment. The obtained results resolve the existing discrepancies between literature lifetimes and demonstrate that Zn66 cannot be described by using simple collective models. Therefore, new state-of-the-art beyond-mean-field and large-scale shell-model calculations were performed in order to interpret the structure of this nucleus. Both the experimental and theoretical results suggest that the triaxial degree of freedom has an important impact on electromagnetic properties of Zn66, while the unique features of the 02+ state indicate its distinct and rather isolated structure.

The heaviest N=Z doubly-magic nucleus, $^{100}$Sn, and the neighboring nuclei offer unique opportunities to investigate the properties of nuclear interaction in extreme conditions. In particular, the Cd isotopes are expected to present features similar to those found in the Sn isotopic chain, since they have only two proton holes in the Z=50 shell. In this manuscript, the lifetime measurements of low-lying states in the even-mass $^{102-108}$Cd is presented. Thanks to the powerful detection capabilities of AGATA array and VAMOS++ spectrometer, the unusual employment of multi-nucleon transfer reactions permitted to investigate the first 2$^+$ and 4$^+$ states in all these nuclei, together with various deformed bands in $^{106}$Cd. The results were interpreted in the context of new state-of-the-art beyond-mean-field calculations, using the symmetry-conserving configuration-mixing approach. Despite the similarities in the electromagnetic properties of the low-lying states, there is a fundamental structural difference between the ground-state bands in the Z=48 and Z=50 isotopes. The comparison between experimental and theoretical results revealed a rotational character of the Cd nuclei, which have prolate-deformed ground states with $\beta_2 \approx 0.2$. At this deformation Z=48 becomes a closed-shell configuration, which is favored with respect to the spherical one., Comment: V1 : 14 pages, 12 figures. V2 : 16 pages, 11 figures. V3 : 16 pages, 11 figures (figure 11 corrected); triaxiality evaluated with "quadrupole sum rule". submitted to Phys.Rev.C

Excited states in $^{98}$Ru were investigated using γ-ray spectroscopy following the β-decay of $^{98}$Rh, and via the $^{100}$Ru(p,t) reaction. Combining the results from the two experiments, two states were revised to have spin-parity of 4+ and subsequently assigned to the 02+ and “γ” bands, respectively. The observed structures in $^{98}$Ru are suggested to be deformed and rotational, rather than spherical and vibrational, and fit well into the systematics of these excitations in the Ru isotopes. The 02+ excitation is suggested as a shape coexisting configuration. This observation eliminates some of the last remaining candidates for nearly harmonic vibrational nuclei in the Z≈50 region. Beyond-mean-field calculations are presented that support shape coexistence throughout the Ru isotopes with N=52–62, and suggest a smooth evolution of the shapes.

6 pags., 4 figs., Here we present new information on the shape evolution of the very neutron-rich ^{92,94}Se nuclei from an isomer-decay spectroscopy experiment at the Radioactive Isotope Beam Factory at RIKEN. High-resolution germanium detectors were used to identify delayed γ rays emitted following the decay of their isomers. New transitions are reported extending the previously known level schemes. The isomeric levels are interpreted as originating from high-K quasineutron states with an oblate deformation of β∼0.25, with the high-K state in ^{94}Se being metastable and K hindered. Following this, ^{94}Se is the lowest-mass neutron-rich nucleus known to date with such a substantial K hindrance. Furthermore, it is the first observation of an oblate K isomer in a deformed nucleus. This opens up the possibility for a new region of K isomers at low Z and at oblate deformation, involving the same neutron orbitals as the prolate orbitals within the classic Z∼72 deformed hafnium region. From an interpretation of the level scheme guided by theoretical calculations, an oblate deformation is also suggested for the ^{94}Se_{60} ground-state band., This work was carried out at the RIBF operated by RIKEN Nishina Center, RIKEN and CNS, University of Tokyo. We acknowledge the EUROBALL Owners Committee for the loan of germanium detectors and the PreSpec Collaboration for the readout electronics of the cluster detectors. The authors thank the RIBF and BigRIPS teams for providing a stable high-intensity uranium beam and operating the secondary beams. We acknowledge support from the German BMBF Grants No. 05P15RDFN1, No. 05P19RDFN1, No. 05P15PKFNA, and No. 05P19PKFNA, the ERC Grant No. MINOS-258567, the Spanish MEC under Contracts No. FPA2014-57196-C5-4-P and No. FIS2014-53434, the National Key R&D Program of China (Contract No. 2018YFA0404402), the National Natural Science Foundation of China (Grants No. 11961141004, No. 11735017, No. 11675225, No. 11635003), the Vietnam MOST via the Physics Development Program Grant No. ĐTĐLCN.25/18, as well as from the Science and Technology Facilities Council (STFC). We further thank GSI for providing computing facilities.

International audience; Here we present new information on the shape evolution of the very neutron-rich Se92,94 nuclei from an isomer-decay spectroscopy experiment at the Radioactive Isotope Beam Factory at RIKEN. High-resolution germanium detectors were used to identify delayed γ rays emitted following the decay of their isomers. New transitions are reported extending the previously known level schemes. The isomeric levels are interpreted as originating from high-K quasineutron states with an oblate deformation of β∼0.25, with the high-K state in Se94 being metastable and K hindered. Following this, Se94 is the lowest-mass neutron-rich nucleus known to date with such a substantial K hindrance. Furthermore, it is the first observation of an oblate K isomer in a deformed nucleus. This opens up the possibility for a new region of K isomers at low Z and at oblate deformation, involving the same neutron orbitals as the prolate orbitals within the classic Z∼72 deformed hafnium region. From an interpretation of the level scheme guided by theoretical calculations, an oblate deformation is also suggested for the Se6094 ground-state band.

Excited states in the N=40 isotone $^{62}$Ti were populated via the $^{63}$V(p,2p)$^{62}$Ti reaction at ∼200 MeV/nucleon at the Radioactive Isotope Beam Factory and studied using γ-ray spectroscopy. The energies of the 21+→0gs+ and 41+→21+ transitions, observed here for the first time, indicate a deformed $^{62}$Ti ground state. These energies are increased compared to the neighboring $^{64}$Cr and $^{66}$Fe isotones, suggesting a small decrease of quadrupole collectivity. The present measurement is well reproduced by large-scale shell-model calculations based on effective interactions, while ab initio and beyond mean-field calculations do not yet reproduce our findings. The shell-model calculations for $^{62}$Ti show a dominant configuration with four neutrons excited across the N=40 gap. Likewise, they indicate that the N=40 island of inversion extends down to Z=20, disfavoring a possible doubly magic character of the elusive $^{60}$Ca.

International audience; Background: The neutron-deficient osmium isotopic chain provides a great laboratory for the study of shape evolution, with the transition from the soft triaxial rotor in Os168 to the well-deformed prolate rotor in Os180, while shape coexistence appears around N=96 in Os172. Therefore, the study of the Os isotopic chain should provide a better understanding of shape changes in nuclei and a detailed scrutiny of nuclear structure calculations. In this paper, the lifetimes of the low-lying yrast states of Os170 have been measured for the first time to investigate the shape evolution with neutron number. Purpose: Lifetimes of excited states in the ground-state band of Os170 are measured to investigate the shape evolution with neutron number in osmium isotopes and compare with state-of-the-art calculations. Methods: The states of interest were populated via the fusion-evaporation reaction Nd142(S32,4n) at a bombarding energy of 170 MeV at the ALTO facility from IPN (Orsay, France). Lifetimes of the 21+ and 41+ states in Os170 were measured with the recoil-distance Doppler-shift method using the Orsay universal plunger system. Results: Lifetimes of the two first excited states in Os170 were measured for the first time. A very small B(E2;41+→21+)/B(E2;21+→01+)=0.38(11) was found, which is very uncharacteristic for collective nuclei. These results were compared to state-of-the-art beyond-mean-field calculations. Conclusions: Although theoretical results give satisfactory results for the energy of the first few excited states in Os170 and the B(E2;21+→01+) they fail to reproduce the very small B(E2;41+→21+), which remains a puzzle.

International audience; Algebraic collective model (ACM) calculations for the even-even selenium isotopes Se86,88 have been performed to describe collective properties of these two transitional nuclei with only two and four valence neutrons, respectively. From the perspective of the ACM the nuclei turn out to be only weakly deformed, described by a very γ-soft potential. This is confirmed by Skyrme and Gogny energy density functional calculations. Additionally, the fully microscopic symmetry conserving configuration mixing method with the Gogny interaction has been applied to the study of these isotopes. The results show the transitional character of Se86 and the relevant role of the triaxial degree of freedom in the structure of these nuclei.

12 pags., 11 figs., 3 tabs. -- Open Access funded by Creative Commons Atribution Licence 4.0, The occurrence of octupolar shapes in the Ba isotopic chain was recently established experimentally up to N = 90. To further extend the systematics, the evolution of shapes in the most neutron-rich members of the Z = 56 isotopic chain accessible at present, Ba-148,Ba-150, has been studied via beta decay at the ISOLDE Decay Station. This paper reports on the first measurement of the positive-and negative-parity low-spin excited states of 150Ba and presents an extension of the beta-decay scheme of Cs-148. Employing the fast timing technique, half-lives for the 2(1)(+) level in both nuclei have been determined, resulting in T-1/2 = 1.51(1) ns for Ba-148 and T-1/2 = 3.4(2) ns for Ba-150. The systematics of low-spin states, together with the experimental determination of the B(E2 : 2(+) -> 0(+)) transition probabilities, indicate an increasing collectivity in Ba148-150, towards prolate deformed shapes. The experimental data are compared to symmetry conserving configuration mixing (SCCM) calculations, confirming an evolution of increasingly quadrupole deformed shapes with a definite octupolar character., The IDS Collaboration acknowledges financial support from Istituto Nazionale di Fisica Nucleare, the Italian “Programmi di Ricerca Scientifica di Rilevante Interesse Nazionale” (PRIN), contract 2001024324 01302; the European Union seventh framework through ENSAR, contract 262010; the European Unions Horizon 2020 Framework research and innovation program under grant agreement 654002 (ENSAR2); the FATIMA-NuPNET network via the PRI-PIMNUP-2011-1338 project; the Romanian IFA grant CERN/ISOLDE and Romanian PN-II-RU-TE-2014-4-2003; the Spanish MINECO projects, reference numbers FPA2013- 41467-P, FPA2015-64969-P, FPA2015-65929, and FIS2015- 63770; Spanish grants FIS-2014-53434-P MINECO and Programa Ramon y Cajal 2012 No. 11420, MINECO grant IJCI-2014-19172, and the MINECO project FPA2014-52823- C2-1-P; the German BMBF under contract 05P15PKCIA, contract 05P15PKFNA, and “Verbundprojekt 05P2015,” the FWO-Vlaanderen (Belgium); and the IAP Belgian Science Policy (BriX network P7/12). V.Ch. and Z.P. acknowledge support by the Polish grant of Narodowe Centrum Nauki, no. 2015/18/M/ST2/00523.

International audience; The neutron-rich nucleus Pt200 is investigated via in-beam γ-ray spectroscopy to study the shape evolution in the neutron-rich platinum isotopes towards the N=126 shell closure. The two-neutron transfer reaction Pt198(Se82, Se80)Pt200 is used to populate excited states of Pt200. The Advanced Gamma Ray Tracking Array (AGATA) demonstrator coupled with the PRISMA spectrometer detects γ rays coincident with the Se80 recoils, the binary partner of Pt200. The binary partner method is applied to extract the γ-ray transitions and build the level scheme of Pt200. The level at 1884 keV reported by Yates et al. [S. W. Yates, E. M. Baum, E. A. Henry, L. G. Mann, N. Roy, A. Aprahamian, R. A. Meyer, and R. Estep, Phys. Rev. C 37, 1889 (1988)] was confirmed to be at 1882.1 keV and assigned as the (61+) state. An additional γ ray was found and it presumably deexcites the (81+) state. The results are compared with state-of-the-art beyond mean-field calculations, performed for the even-even 190−204Pt isotopes, revealing that Pt200 marks the transition from the γ-unstable behavior of lighter Pt nuclei towards a more spherical one when approaching the N=126 shell closure.