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
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 half-lives of 20 neutron-rich nuclei with Z ¼ 27-30 have been measured at the RIBF, Atomic nuclei are quantum many-body systems consisting of two distinct types of fermions-protons and neutrons. Analogous to atomic physics, the concept of nuclear shell structure was triggered by the discovery of particularly stable nuclei with specific numbers of proton and neutron, such as 2, 8,20,28, 50, 82, and 126 along the β-stability line [1]. By assuming a strong spin-orbit interaction within a mean field potential, these magic numbers were correctly interpreted and regarded to be immutable throughout the nuclear chart [2,3]. However, with the development of experimental techniques exploiting radioactive ion beams, many nuclei with extreme neutron-to-proton ratios (N=Z), so-called exotic nuclei, have been produced and studied in the last few decades. The results obtained heretofore have demonstrated that the shell structure established for nuclei near the β-stability line may change drastically in these exotic nuclei. For instance, classical magic numbers in 12 Be (N ¼ 8), 32 Mg (N ¼ 20), and 42 Si (N ¼ 28) were found to disappear [4-6], whereas new magic numbers emerged in 24 O (N ¼ 16) and 54 Ca (N ¼ 34) [7][8][9]. To address the origins of shell evolution in heavier mass regions, it is of particular interest to investigate the properties of nuclei in the vicinity of 78 Ni, which has the proton number Z ¼ 28 and the neutron number N ¼ 50 with a large neutron excess N=Z ≈ 1.8.To study the shell evolution around 78 Ni, many experimental efforts have been made. One of the interesting phenomena related to the proton Z ¼ 28 shell gap is the monopole migration in Cu isotopes. A sudden drop of the excited 5=2− state relative to the ground 3=2 − state was observed in 71;73 Cu [10,11]. These two states are characterized by a single-particle nature [12] and their order was
Since October 2000, 56 hips in 48 patients with avascular necrosis of the femoral head were treated with free vascularized fibular transplants. The average follow-up was about 16 months. The Harris hip scores of all stages were improved during follow-up. Most femoral heads showed improvement (39 hips, 69.6%) or were at least unchanged (14 hips, 25.0%) on X-rays. The results show that a free vascularized fibular graft would be a valuable procedure for femoral head necrosis. By this method, we can avoid or delay progress of the disease, and improve the function of the hip and quality of life.
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
Tin is the chemical element with the largest number of stable isotopes. Its complete proton shell, comparable with the closed electron shells in the chemically inert noble gases, is not a mere precursor to extended stability; since the protons carry the nuclear charge, their spatial arrangement also drives the nuclear electromagnetism. We report high-precision measurements of the electromagnetic moments and isomeric differences in charge radii between the lowest 1/2+, 3/2+, and 11/2− states in 117–131Sn, obtained by collinear laser spectroscopy. Supported by state-of-the-art atomic-structure calculations, the data accurately show a considerable attenuation of the quadrupole moments in the closed-shell tin isotopes relative to those of cadmium, with two protons less. Linear and quadratic mass-dependent trends are observed. While microscopic density functional theory explains the global behaviour of the measured quantities, interpretation of the local patterns demands higher-fidelity modelling.
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