M. Niikura

Shape Evolution in Neutron-rich Krypton Isotopes beyond N = 60 : First spectroscopy of 98;100Kr

We report on the first γ-ray spectroscopy of low-lying states in neutron-rich ^{98,100}Kr isotopes obtained from ^{99,101}Rb(p,2p) reactions at ∼220  MeV/nucleon. A reduction of the 2_{1}^{+} state energies beyond N=60 demonstrates a significant increase of deformation, shifted in neutron number compared to the sharper transition observed in strontium and zirconium isotopes. State-of-the-art beyond-mean-field calculations using the Gogny D1S interaction predict level energies in good agreement with experimental results. The identification of a low-lying (0_{2}^{+}, 2_{2}^{+}) state in ^{98}Kr provides the first experimental evidence of a competing configuration at low energy in neutron-rich krypton isotopes consistent with the oblate-prolate shape coexistence picture suggested by theory.

Nuclear structure and β -decay schemes for Te nuclides beyond N=82

We study for the first time the internal structure of 140Te through the beta-delayed gamma-ray spectroscopy of 140Sb. The very neutron-rich 140Sb, Z = 51 and N = 89, ions were produced by the in-flight fission of 238U beam on a 9Be target at 345 MeV per nucleon at the Radioactive Ion Beam Factory, RIKEN. The half-life and spin-parity of 140Sb are reported as 124(30) ms and (4-), respectively. In addition to the excited states of 140Te produced by the beta-decay branch, the beta-delayed one-neutron and two-neutron emission branches were also established. By identifying the first 2+ and 4+ excited states of 140Te, we found that Te isotopes persist their vibrator character with E(4+)/E(2+) = 2. We discuss the distinctive features manifest in this region, such as valence neutron symmetry and asymmetry, revealed in pairs of isotopes with the same neutron holes and particles with respect to N = 82.

Spectroscopic factor and proton formation probability for the d3/2 proton emitter 151mLu

Abstract The quenching of the experimental spectroscopic factor for proton emission from the short-lived d 3 / 2 isomeric state in 151mLu was a long-standing problem. In the present work, proton emission from this isomer has been reinvestigated in an experiment at the Accelerator Laboratory of the University of Jyvaskyla. The proton-decay energy and half-life of this isomer were measured to be 1295(5) keV and 15.4(8) μs, respectively, in agreement with another recent study. These new experimental data can resolve the discrepancy in the spectroscopic factor calculated using the spherical WKB approximation. Using the R-matrix approach it is found that the proton formation probability indicates no significant hindrance for the proton decay of 151mLu.

Is seniority a partial dynamic symmetry in the first ν g 9/2 shell?

Abstract The low-lying structures of the midshell ν g 9 / 2 Ni isotopes 72Ni and 74Ni have been investigated at the RIBF facility in RIKEN within the EURICA collaboration. Previously unobserved low-lying states were accessed for the first time following β decay of the mother nuclei 72Co and 74Co. As a result, we provide a complete picture in terms of the seniority scheme up to the first ( 8 + ) levels for both nuclei. The experimental results are compared to shell-model calculations in order to define to what extent the seniority quantum number is preserved in the first neutron g 9 / 2 shell. We find that the disappearance of the seniority isomerism in the ( 8 1 + ) states can be explained by a lowering of the seniority-four ( 6 + ) levels as predicted years ago. For 74Ni, the internal de-excitation pattern of the newly observed ( 6 2 + ) state supports a restoration of the normal seniority ordering up to spin J = 4 . This property, unexplained by the shell-model calculations, is in agreement with a dominance of the single-particle spherical regime near 78Ni.

Inelastic scattering of neutron-rich Ni and Zn isotopes off a proton target

Proton inelastic scattering of Ni72,74 and Zn76,80 ions at energies around 235 MeV/nucleon was performed at the Radioactive Isotope Beam Factory and studied using γ-ray spectroscopy. Angular integrated cross sections for direct inelastic scattering to the 21+ and 41+ states were measured. The Jeukenne-Lejeune-Mahaux folding model, extended beyond 200 MeV, was used together with neutron and proton densities stemming from quasiparticle random-phase approximation (QRPA) calculations to interpret the experimental cross sections and to infer neutron to proton matrix element ratios. In addition, coupled-channels calculations with a phenomenological potential were used to determine deformation lengths. For the Ni isotopes, correlations favor neutron excitations, thus conserving the Z=28 gap. A dominance of proton excitation, on the other hand, is observed in the Zn isotopes, pointing to the conservation of the N=50 gap approaching Ni78. These results are in agreement with QRPA and large-scale shell-model calculations.

Shape coexistence and isospin symmetry in A = 70 nuclei: Spectroscopy of the Tz = −1 nucleus 70Kr

Abstract Excited states in the T z = − 1 nucleus 70 Kr have been populated using inelastic scattering of a radioactive 70 Kr beam as well as one- and two-neutron removal reactions from 71,72 Kr at intermediate beam energies. The level scheme of 70 Kr was constructed from the observed γ -ray transitions and coincidences. Tentative spin and parity assignments were made based on comparison with the mirror nucleus 70 Se. A second 2 + state and a candidate for the corresponding 4 2 + state suggest shape coexistence in 70 Kr.

Signatures of triaxiality in low-spin spectra of 86Ge

Low-spin states of neutron-rich 84,86,88Ge were measured by in-flight γ-ray spectroscopy at 270 MeV/u at the RIKEN-RIBF facility. The exotic beams have been produced by primary 238U in-flight fission reactions and impinged on the MINOS device. MINOS combines a 10-cm long LH2 target with a Time Projection Chamber (TPC) to reconstruct the reaction vertices. The reactions were selected by the BigRIPS and the ZeroDegree spectrometers for the incoming and outgoing channels, respectively. Emitted γ radiation was detected by the NaI-array DALI2. De-excitations from the , , and states of 84,86Ge and and states of 88Ge were observed. The data are compared to state-of-the-art shell model and beyond-mean-field calculations. Furthermore, a candidate for a state of 86Ge was identified. This state plays a key role in the discussion of ground-state triaxiality of 86Ge, along with other features of the low-energy level scheme. This work was published in [1].

Inelastic scattering of 72,74Ni off a proton target

Inelastic scattering of 72,74Ni off a proton target was performed at RIBF, RIKEN, Japan. The isotopes were produced by the fission of 238U on a thick Beryllium target and were then selected and identified on an event-by-event basis using the BigRIPS separator. Selected isotopes were focused onto the liquid hydrogen target of the MINOS device and gamma rays from the reactions were measured with the DALI2 array. The energy of the ions in the middle of the target was 213 MeV/u. Outgoing particles were identified using the ZeroDegree spectrometer. Here, we report on the current status of the data analysis and preliminary results for the proton inelastic scattering cross sections for both isotopes.

Systematic Study of β-Decay Half-Lives in the Vicinity of 78Ni

Experimental study of β-decay properties for nuclei around Ni is of great importance as Ni is probably the most neutron-rich nuclide known so far. Observables like β-decay half-life (T1/2), β-decay Q value (Qβ), neutron emission probability (Pn) can serve as benchmarks for one to understand the nuclear structure far from the valley of stability. Results from the experiment allow for systematic study and comparison between different global mass models or various theories including shell model calculations, which will largely extend the present knowledge of nuclear structure towards neutron drip-line. In order to produce the secondary beam containing very neutron-rich nuclei around Ni, the experiment was performed as part of EURICA campaign at Radioactive Isotope beam factory (RIBF) operated by RIKEN Nishina Center (RNC) and Center of Nuclear Study (CNS), University of Tokyo. A high intensity U beam was extracted from accelerator complex at RIBF with an energy of 345 MeV/nucleon. The secondary beam was generated by the in-flight fission of U in a 3 mm Be target. The fragments from fission of U were analyzed in the BigRIPS and ZeroDegree with an event-by-event mode. A highly segmented active stopper, WAS3ABi, was mounted after the last focal plane of ZeroDegree spectrometer, F11, for the implantation of fragments of interest transimitted through BigRIPS and ZeroDegree. The ion-β-γ correlation was obtained to study the half-lives as well as β-delayed γ-ray spectroscopy of exotic nuclei around Ni. In this presentation, new half-lives of Co, Co, Ni, Cu, Cu, Zn, Ga, and Ge, which was measured for the first time experimentally, will be reported. Half-life of nuclide such as Ni, which had been measured in previous experiment but resulted in a large uncertainty, was re-measured in this work with a much higher precision. The shell gap effect on half-lives and magicity at Z = 28 and N = 50 will be discussed based on the data from this experiment together with previous ones from literature. APPC12 The 12th Asia Pacific Physics Conference

25Al+p Elastic Scattering with CRIB

J. Pearson, A. A. Chen, S. Kubono, H. Yamaguchi, J. J. He, L. H. Khiem, A. Saito, G. Amadio, H. Fujikawa, M. Niikura, Y. Wakabayashi, T. Teranishi, S. Nishimura, Y. Togano, A. Odahara, f J. Y. Moon, Y. K. Kwon,, S. Cherubini, R. Pizzone, M. La Cognata, Department of Physics and Astronomy, McMaster University, Canada Centre for Nuclear Study, University of Tokyo, Japan Department of Physics, Kyushu University, Japan RIKEN, Japan Rikkyo University, Tokyo, Japan f Nishinippon Institute of Technology, Japan Department of Physics, Chung-Ang University, Korea University of Catania, Italy