We report on the g-factor measurement of the first isomeric state in (16)43S27 [Ex=320.5(5) keV, T1/2=415(5) ns, and g=0.317(4)]. The 7/2- spin-parity of the isomer and the intruder nature of the ground state of the nucleus are experimentally established for the first time, providing direct and unambiguous evidence of the collapse of the N=28 shell closure in neutron-rich nuclei. The shell model, beyond the mean-field and semiempirical calculations, provides a very consistent description of this nucleus showing that a well deformed prolate and quasispherical states coexist at low energy.

The β-decay half-lives of 110 neutron-rich isotopes of the elements from 37 Rb to 50 Sn were measured at the Radioactive Isotope Beam Factory. The 40 new half-lives follow robust systematics and highlight the persistence of shell effects. The new data have direct implications for r-process calculations and reinforce the notion that the second (A ≈ 130) and the rare-earth-element (A ≈ 160) abundance peaks may result from the freeze-out of an ðn; γÞ ⇄ ðγ; nÞ equilibrium. In such an equilibrium, the new half-lives are important factors determining the abundance of rare-earth elements, and allow for a more reliable discussion of the PRL 114, 192501 (2015) P H Y S I C A L R E V I E W L E T T E R S week ending 15 MAY 2015 0031-9007=15=114(19)=192501 (7) 192501-1 © 2015 American Physical Society r process universality. It is anticipated that universality may not extend to the elements Sn, Sb, I, and Cs, making the detection of these elements in metal-poor stars of the utmost importance to determine the exact conditions of individual r-process events. Introduction.-The origin of the heavy elements from iron to uranium is one of the main open questions in science. The slow neutron-capture (s) process of nucleosynthesis [1,2], occurring primarily in helium-burning zones of stars, produces about half of the heavy element abundance in the universe. The remaining half requires a more violent process known as the rapid neutron-capture (r) process [3][4][5][6]. During the r process, in environments of extreme temperatures and neutron densities, a reaction network of neutron captures and β decays synthesizes very neutron-rich isotopes in a fraction of a second. These isotopes, upon exhaustion of the supply of free neutrons, decay into the stable or semistable isotopes observed in the solar system. However, none of the proposed stellar models, including explosion of supernovae [7][8][9][10][11][12] and merging neutron stars [13][14][15][16], can fully explain abundance observations. The mechanism of the r process is also uncertain. At temperatures of one billion degrees or more, photons can excite unstable nuclei which then emit neutrons, thus, counteracting neutron captures in an ðn; γÞ ⇄ ðγ; nÞ equilibrium that determines the r process. These conditions may be found in the neutrino-driven wind following the collapse of a supernova core and the accreting torus formed around the black hole remnant of merging neutron stars. Alternatively, recent r-process models have shown that the r process is also possible at lower temperatures or higher neutron densities where the contribution from ðγ; nÞ reactions is minor. These conditions are expected in supersonically expanding neutrino-driven outflow in low-mass supernovae progenitors (e.g., 8 − 12 M ⊙ ) or prompt ejecta from neutron star mergers [17]. The final abundance distribution may also be dominated by postprocessing effects such as fission of heavy nuclei (A ≳ 280) possibly produced in merging neutron stars [18].New clues about the r process have come from the discovery of de...

The decay of excited states in the waiting-point nucleus 130 Cd 82 has been observed for the first time. An 8 two-quasiparticle isomer has been populated both in the fragmentation of a 136 Xe beam as well as in projectile fission of 238 U, making 130 Cd the most neutron-rich N 82 isotone for which information about excited states is available. The results, interpreted using state-of-the-art nuclear shell-model calculations, show no evidence of an N 82 shell quenching at Z 48. They allow us to follow nuclear isomerism throughout a full major neutron shell from 98 Cd 50 to 130 Cd 82 and reveal, in comparison with 76 Ni 48 one major proton shell below, an apparently abnormal scaling of nuclear two-body interactions. DOI: 10.1103/PhysRevLett.99.132501 PACS numbers: 21.60.Cs, 23.20.Lv, 26.30.+k, 27.60.+j The pioneering work of Goeppert-Mayer [1] and Haxel, Jensen, and Suess [2] in realizing that the experimental evidence for nuclear magic numbers could be explained by assuming a strong spin-orbit interaction constituted a major milestone in our understanding of the internal structure of the atomic nucleus. However, it has been recognized for more than 20 years that the single-particle ordering which underlies the shell structure (and with it the magic numbers) may change for nuclei approaching the neutron dripline. It has been argued that the neutron excess causes the central potential to become diffuse, leading to a modification of the single-particle spectrum of neutron-dripline nuclei [3,4]. In addition, a strong interaction between the energetically bound orbitals and the continuum also affects the level ordering. The consequence of these modifications can be a shell quenching; i.e., the shell gaps at magic neutron numbers are less pronounced in very neutronrich nuclei than in nuclei closer to stability. At the extreme, these gaps may even disappear. Alternatively, the tensor part of the nuclear force has been shown to cause shell reordering for very asymmetric proton and neutron numbers [5,6].The N 82 isotones below the doubly magic nucleus 132 Sn are crucial for stellar nucleosynthesis due to the close relation between the N 82 shell closure and the A 130 peak of the solar r-process abundance distribution. Based on the mass models available at that time, it was shown in the 1990s that the assumption of a quenching of the N 82 neutron shell closure leads to a considerable improvement in the global abundance fit in r-process calculations [7,8], in particular, a filling of the troughs around A 120 and 140. On the other hand, recently, alternative descriptions of the phenomenon have been given without invoking shell quenching at all [9,10]. Unfortunately, the very PRL 99,

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Rapid shape changes are observed for neutron-rich nuclei with A around 100. In particular, a sudden onset of ground-state deformation is observed in the Zr and Sr isotopic chains at N=60: low-lying states in N≤58 nuclei are nearly spherical, while those with N≥60 have a rotational character. Nuclear lifetimes as short as a few ps can be measured using fast-timing techniques with LaBr 3 (Ce)-scintillators, yielding a key ingredient in the systematic study of the shape evolution in this region. We used neutron-induced fission of 241 Pu and 235 U to study lifetimes of excited states in fission fragments in the A∼100 region with the EXILL-FATIMA array located at the PF1B cold neutron beam line at the Institut Laue-Langevin. In particular, we applied the generalized centroid difference method to deduce lifetimes of low-lying states for the nuclei 98 Zr (N=58), 100 Zr and 102 Zr (N≥60). The results are discussed in the context of the presumed phase transition in the Zr chain by comparing the experimental transition strengths with the theoretical calculations using the Interacting Boson Model and the Monte Carlo Shell Model.

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