μ s isomers of Nd 158,160

The neutron-rich nuclei Nd158,160 have been studied via delayed γ-ray spectroscopy of μs isomeric states at the RIBF facility, RIKEN. These nuclei were produced following the projectile fission of a 345 AMeVU238 beam and delayed γ rays were detected by the EURICA cluster Ge array. The isomeric states have measured half-lives of 339(20) ns and 1.63(21) μs for Nd158 and Nd160, respectively. From the observed γ decays and the systematics of levels in the neighboring Nd isotopes first level schemes were constructed for these nuclei. The isomeric states of Nd158,160 have been assigned spins of (6-) and (4-), with proposed ν5/2[523] - ν7/2[633] and ν1/2[521] - ν7/2[633] configurations, respectively. © 2016 American Physical Society

Observation of Anomalous Internal Pair Creation in 8^8Be: A Possible Signature of a Light, Neutral Boson

Electron-positron angular correlations were measured for the isovector magnetic dipole 17.6 MeV state ($J^\pi=1^+$, $T=1$) $\rightarrow$ ground state ($J^\pi=0^+$, $T=0$) and the isoscalar magnetic dipole 18.15 MeV ($J^\pi=1^+$, $T=0$) state $\rightarrow$ ground state transitions in $^{8}$Be. Significant deviation from the internal pair creation was observed at large angles in the angular correlation for the isoscalar transition with a confidence level of $> 5\sigma$. This observation might indicate that, in an intermediate step, a neutral isoscalar particle with a mass of 16.70$\pm0.35$ (stat)$\pm 0.5$ (sys) MeV$/c^2$ and $J^\pi = 1^+$ was created.Comment: 5 pages, 5 figure

Shell Evolution towards Ni 78: Low-Lying States in Cu 77

The level structure of the neutron-rich Cu77 nucleus is investigated through β-delayed γ-ray spectroscopy at the Radioactive Isotope Beam Factory of the RIKEN Nishina Center. Ions of Ni77 are produced by in-flight fission, separated and identified in the BigRIPS fragment separator, and implanted in the WAS3ABi silicon detector array, surrounded by Ge cluster detectors of the EURICA array. A large number of excited states in Cu77 are identified for the first time by correlating γ rays with the β decay of Ni77, and a level scheme is constructed by utilizing their coincidence relationships. The good agreement between large-scale Monte Carlo shell model calculations and experimental results allows for the evaluation of the single-particle structure near Ni78 and suggests a single-particle nature for both the 5/21- and 3/21- states in Cu77, leading to doubly magic Ni78. © 2017 American Physical Society

β-Decay Half-Lives of 110 Neutron-Rich Nuclei across the N = 82 Shell Gap: Implications for the Mechanism and Universality of the Astrophysical r Process

The β-decay half-lives of 110 neutron-rich isotopes of the elements from 37Rb to 50Sn were measured at the Radioactive Isotope Beam Factory. The 40 new half-lives follow robust systematics and highlight the persistence of shell eff

β decay of 129 Cd and excited states in 129 In

The β decay of Cd129, produced in the relativistic fission of a U238 beam, was experimentally studied at the RIBF facility at the RIKEN Nishina Center. From the γ radiation emitted after the β decays, a level scheme of In129 was established comprisin

Low-lying excitations in Ni 72

Low-lying excited states in Ni72 have been investigated in an in-flight fission experiment at the RIBF facility of the RIKEN Nishina Center. The combination of the state-of-the-art BigRIPS and EURICA setups has allowed for a very accurate study of the β decay from Co72 to Ni72, and has provided first experimental information on the decay sequence Fe72→Co72→Ni72 and on the delayed neutron-emission branch Co73→Ni72. Accordingly, we report nearly 60 previously unobserved γ transitions which deexcite 21 new levels in Ni72. Evidence for the location of the so-sought-after (42+),(62+), and (81+) seniority states is provided. As well, the existence of a low-spin β-decaying isomer in odd-odd neutron-rich Co isotopes is confirmed for mass A=72. The new experimental information is compared to simple shell-model calculations including only neutron excitations across the fpg shells. It is shown that, in general, the calculations reproduce well the observed states. © 2016 American Physical Society