Z. Y. Xu

Extension of the N = 40 Island of Inversion towards N =50: Spectroscopy of 66Cr,70,72Fe

We report on the measurement of the first 2(+) and 4(+) states of (66)Cr and (70,72)Fe via in-beam γ-ray spectroscopy. The nuclei of interest were produced by (p,2p) reactions at incident energies of 260  MeV/nucleon. The experiment was performed at the Radioactive Isotope Beam Factory, RIKEN, using the DALI 2γ-ray detector array and the novel MINOS device, a thick liquid hydrogen target combined with a vertex tracker. A low-energy plateau of 2(1)(+) and 4(1)(+) energies as a function of the neutron number was observed for N≥38 and N≥40 for even-even Cr and Fe isotopes, respectively. State-of-the-art shell model calculations with a modified Lenzi-Nowacki-Poves-Sieja (LNPS) interaction in the pfg(9/2)d(5/2) valence space reproduce the observations. Interpretation within the shell model shows an extension of the island of inversion at N=40 for more neutron-rich isotopes towards N=50.

94β -Decay Half-Lives of Neutron-Rich Cs55 to Ho67 : Experimental Feedback and Evaluation of the r -Process Rare-Earth Peak Formation

The β-decay half-lives of 94 neutron-rich nuclei ^{144-151}Cs, ^{146-154}Ba, ^{148-156}La, ^{150-158}Ce, ^{153-160}Pr, ^{156-162}Nd, ^{159-163}Pm, ^{160-166}Sm, ^{161-168}Eu, ^{165-170}Gd, ^{166-172}Tb, ^{169-173}Dy, ^{172-175}Ho, and two isomeric states ^{174m}Er, ^{172m}Dy were measured at the Radioactive Isotope Beam Factory, providing a new experimental basis to test theoretical models. Strikingly large drops of β-decay half-lives are observed at neutron-number N=97 for _{58}Ce, _{59}Pr, _{60}Nd, and _{62}Sm, and N=105 for _{63}Eu, _{64}Gd, _{65}Tb, and _{66}Dy. Features in the data mirror the interplay between pairing effects and microscopic structure. r-process network calculations performed for a range of mass models and astrophysical conditions show that the 57 half-lives measured for the first time play an important role in shaping the abundance pattern of rare-earth elements in the solar system.

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.

New Isotopes and Proton Emitters Crossing the Drip Line in the Vicinity of

Several new isotopes, ^{96}In, ^{94}Cd, ^{92}Ag, and ^{90}Pd, have been identified at the RIKEN Nishina Center. The study of proton drip-line nuclei in the vicinity of ^{100}Sn led to the discovery of new proton emitters ^{93}Ag and ^{89}Rh with half-lives in the submicrosecond range. The systematics of the half-lives of odd-Z nuclei with T_{z}=-1/2 toward ^{99}Sn shows a stabilizing effect of the Z=50 shell closure. Production cross sections for nuclei in the vicinity of ^{100}Sn measured at different energies and target thicknesses were compared to the cross sections calculated by epax taking into account contributions of secondary reactions in the primary target.

^{100}

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.

Sn

In this contribution the low-excitation structural properties of the doubly mid-shell nucleus 170Dy are discussed, with a special empasis on the evolution of the ground state rotational band within the dysprosium isotopic chain. Recent results from an experiment with the EURICA setup at RIKEN are shown in the context of previous measurements at the PRISMA+CLARA as well as the PRISMA+AGATA setups at Laboratori Nazionali di Legnaro. A brief outlook on future planned measurements is also given.

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

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].

Heavy rotation – evolution of quadrupole collectivity centred at the neutron-rich doubly mid-shell nucleus 170Dy

J. Park et al. -- 1 pag. -- Original Article: Properties of γ-decaying isomers and isomeric ratios in the 100 Sn region. Phys. Rev. C 96, 044311 (2017)

Signatures of triaxiality in low-spin spectra of 86Ge

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.

Publisher's Note: Properties of γ -decaying isomers and isomeric ratios in the Sn100 region [Phys. Rev. C 96 , 044311 (2017)]

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