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A search for isomeric γ-decays among fission fragments from 345 MeV/nucleon 238 U has been performed at the RIKEN Nishina Center RI Beam Factory. Fission fragments were selected and identified using the superconducting in-flight separator BigRIPS and were implanted in an aluminum stopper. Delayed γ-rays were detected using three clover-type high-purity germanium detectors located at the focal plane within a time window of 20 μs following the implantation. We identified a total of 54 microsecond isomers with half-lives of ~ 0. on the obtained spectroscopic information and the systematics in neighboring nuclei. Nature of the nuclear isomerism is discussed in relation to evolution of nuclear structure.KEYWORDS: Nuclear reactions Be( 238 U, x) and Pb( 238 U, x) E = 345 MeV/nucleon, in-flight fission, fission fragments, in-flight RI beam separator, short-lived isomers, new isomers, half-life, γ-ray relative intensity, γγ coincidence, proposed level schemes DOI: PACS number(s): 23.35.+g, 23.20.Lv, 29.38.Db _____________________ *

A search for new isotopes using in-flight fission of a 345 MeV/nucleon 238 U beam has been carried out at the RI Beam Factory at the RIKEN Nishina Center. Fission fragments were analyzed and identified by using the superconducting in-flight separator BigRIPS. We observed 45 new neutron-rich isotopes: Since the pioneering production of radioactive isotope (RI) beams in the 1980s, 1) studies of exotic nuclei far from stability have been attracting much attention. Neutron-rich exotic nuclei are of particular interest, because new phenomena such as neutron halos, neutron skins, and modifications of shell structure have been discovered.2-5) Furthermore these neutron-rich nuclei are important in relation to astrophysical interests, 6) because many of them play a role in the astrophysical r-process. 7) To make further advances in nuclear science and nuclear astrophysics, it is essential to expand the region of accessible exotic nuclei towards the neutron dripline. In-flight fission of a uranium beam is known to be an excellent mechanism for this purpose, having large production cross sections for neutron-rich exotic nuclei. became operational, in which the superconducting in-flight separator BigRIPS 10,11) has been used for the production of RI beams. The BigRIPS separator is designed as a two-stage separator with large acceptance, so that excellent features of in-flight fission can be exploited. In May 2007, right after the commissioning of the BigRIPS separator, we performed an experiment to search for new isotopes using in-flight fission of a 345 MeV/nucleon 238 U beam, aiming to expand the LETTERS Ã

The β-decay half-lives of 38 neutron-rich isotopes from 36 Kr to 43 Mo and 116,117 Tc are reported here for the first time. These results when compared to previous standard models indicate an overestimation in the predicted half-lives by a factor of two or more in the A ≈ 110 region. A revised model based on the second generation gross theory of β decay better predicts the measured half-lives and suggests a more rapid flow of the rapid neutron-capture process (r-matter flow) through this region than previously predicted.About half of the elements heavier than Fe are thought to be produced in rapid neutron-capture process (rprocess) nucleosynthesis, a sequence of neutron-capture and β-decay processes. Although the astronomical site and the mechanism of the r-process are not yet fully understood, it is generally agreed that the process must occur in environments with extreme neutron densities. The study of the elemental distribution along the r-process path requires sensitive β-decay related information such as β-decay half-lives, β-delayed neutron-emission probabilities, and nuclear masses. In particular, determination of the timescale that governs matter flow from the r-process "seeds" to the heavy nuclei, as well as the distribution in the r-process peaks, depends sensitively on decay half-lives [1,2].Isotopes with extreme neutron-to-proton ratios in the mass region A = 110 − 125 have attracted special attention since theoretical r-process yields are found to underestimate isotopic abundances observed in the predicted global abundances by an order of magnitude or more [1,3,4]. This discrepancy has been investigated using numerous mass formulae that differ mainly in the strength of the nuclear shell closures [5,6]. The results indicate that considerable improvements in the global abundances of the isotopes can be achieved by assuming a quenching of the N = 82 shell gap. The properties of most of these crucial r-process nuclei are, however, currently unknown due to their extremely low production yields in the laboratory.A number of experimental studies on nuclei around neutron-rich krypton to technetium have been performed to investigate the region of the r-process path near N = 82 [7][8][9]. In the current work, we report on a first systematic study of the β-decay properties of very exotic, neutron-rich 36 Kr to 43 Tc nuclides that contribute to the r-process.Decay spectroscopy of very neutron-rich nuclei around A = 110 was performed at the recently-commissioned RIBF facility at RIKEN. A secondary beam, comprised of a cocktail of neutron-rich nuclei, was produced by inflight fission of a 345-MeV/nucleon 238 U beam in a 550-mg/cm 2 Be target. The primary beam was produced by the RIKEN cyclotron accelerator complex with a typical intensity ∼ 0.3 pnA at the production target posi-

The unbound excited states of the neutron drip-line isotope 24 O have been investigated via the 24 O(p,p ) 23 O+n reaction in inverse kinematics at a beam energy of 62 MeV/nucleon. The decay energy spectrum of 24 O * was reconstructed from the momenta of 23 O and the neutron. The spinparity of the first excited state, observed at Ex = 4.65 ± 0.14 MeV, was determined to be J π = 2 + from the angular distribution of the cross section. Higher-lying states were also observed. The quadrupole transition parameter β2 of the 2 Otsuka et al. have investigated theoretically the structural evolution of the oxygen isotopes with increasing neutron number (N ) and attributed the development of the shell closure at N = 16 to the strong neutron-proton tensor interaction [10,11].In this Letter we report on the first spectroscopic study of 24 O by proton inelastic scattering. In addition to the excitation energies of the states populated, the wellknown character of proton inelastic scattering also permits the spins-parities, as well as the quadrupole transition parameter (β 2 ) of the first 2 + state to be deduced. As described below, we have been able to provide a firm 2 + assignment for the state at E x = 4.65 ± 0.14 MeV and determine the β 2 , the small value of which is indicative of the spherical closed-shell character of 24 O. A comparison of the E x (2 + 1 ) and β 2 for the chain of oxygen isotopes shows strong evidence for a large shell gap at N = 16.The experiment was performed at the RIPS facility [17] at RIKEN. A schematic view of the downstream section of RIPS and the experimental setup is shown in Fig. 1