EURICA is a project at RIKEN Nishina Center aimed at studying a wide range of exotic nuclei through β-decay measurements and high-resolution γ-ray spectroscopy. The setup is located behind the ...BigRIPS fragment separator and the ZeroDegree spectrometer at the RIBF. EURICA consists of the HPGe cluster detectors from the previous Euroball and RISING projects, together with double-sided silicon-strip detectors for β-decay counting and lifetime measurements. In total, this setup provides us with the possibility to study several aspects of the exotic nuclei produced at the RIBF.
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 (n,γ)⇄(γ,n) 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 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.
The β-decay half-lives of 55 neutron-rich nuclei 134−139Sn, 134−142Sb, 137−144Te, 140−146I, 142−148Xe, 145−151Cs, 148−153Ba, and 151−155La were measured at the Radioactive Isotope Beam Factory ...employing the projectile fission fragments of 238U. The nuclear level structure, which relates to deformation, has a large effect on the half-lives. The impact of newly measured half-lives on modeling the astrophysical origin of the heavy elements is studied in the context of r process nucleosynthesis. For a wide variety of astrophysical conditions, including those in which fission recycling occurs, the half-lives have an important local impact on the second (A ≈ 130) peak.
Nuclei in the vicinity of 78Ni are important benchmarks for nuclear structure, which can reveal changes in the shell structure far from stability. Spectroscopy of the odd-odd isotope 78Cu was ...performed for the first time in an experiment with the EURICA setup at the Radioactive Isotope Beam Factory at RIKEN Nishina Center. Excited states in the neutron-rich isotope were populated following the β decay of 78Ni produced by in-flight fission and separated by the BigRIPS separator. A level scheme based on the analysis of γ−γ coincidences is presented. Tentative spin and parity assignments were made when possible based on the β-decay feeding intensities and γ-decay properties of the excited states. Time correlations between β and γ decay show clear indications of an isomeric state with a half-life of 3.8(4) ms. Large-scale Monte Carlo shell-model calculations were performed using the A3DA-m interaction and a valence space comprising the full fp shell and the 1g9/2 and 2d5/2 orbitals for both protons and neutrons. The comparison of the experimental results with the shell-model calculations allows interpreting the excited states in terms of spin multiplets arising from the proton-neutron interaction. The results provide further insight into the evolution of the proton single-particle orbitals as a function of neutron number, and quantitative information about the proton-neutron interaction outside the doubly magic 78Ni core.
Structure of the neutron-rich N=86 isotope 140Xe, located northeast of a doubly-magic nucleus 132Sn, is investigated by β−γ spectroscopy. Two β-decay isomers in 140I are newly found in the study of ...two different β decays of 140I which were produced by two reactions (i) direct in-flight fission at a primary target and (ii) β decay of 140Te at an active stopper. Half-lives of the β decays of the ground state, the low-spin isomer, and the high-spin isomer are determined to be 0.38(2), 0.91(5), and 0.47(4) s, respectively. Decay schemes of the β decay of the high-spin isomer and of the mixed β decays of the ground state and the low-spin isomer in 140I to 140Xe are constructed using the information on γ-ray coincidence relation and γ-ray intensity. Nuclear structures of the low-lying states in 140Xe and 140I are discussed by comparing the experimental results to two theoretical calculations based on a large-scale shell model and the deformed Skyrme Hartree-Fock-Bogoliubov plus deformed quasiparticle-random-phase approximation. Possible candidates for (quasi-)γ-band members of 2+ and 4+ states and the octupole collective 1− state are proposed in 140Xe. Increase of quadrupole, triaxial, and octupole collectivities is discussed with the increase of neutron and proton numbers.
The β-decay scheme of 138Te and the level structure of 138I is reported for the first time. The experiment was performed at the Radioactive Isotope Beam Factory of RIKEN, as one of the EUROBALL-RIKEN ...Cluster Array campaigns. Secondary radioactive ions, including 138Te and 138Sb, were produced by the in-flight fission of a 238U beam with the energy of 345 MeV per nucleon. From the β decay of 138Te, the level scheme of 138I was supplemented with new spin and parity assignments, such as the low-lying negative-parity states and a positive-parity 1+ state. This 1+ state can be interpreted as being associated with the π0h11/2⊗ν0h9/2 partner orbital configuration populated by the Gamow-Teller transition between a neutron in the 0h9/2 orbital and a proton in the 0h11/2 orbital. Details of the structure of 138I are discussed in terms of the proton-neutron interactions and Gamow-Teller transition strength within the theoretical context of shell-model calculations.
Newly observed decay schemes of the nuclei 137Sb and 138Sb are reported. The neutron-rich Sb isotopes were produced by the in-flight fragmentation of a 238U primary beam with an energy of 345 ...MeV/nucleon. Several new excited states of 137Te with tentatively assigned spin-parities of (5/2−), (9/2−), and (7/2) have been established which play an important role in the evolution of neutron levels beyond N=82. The study of the β decay of 138Sb led to a considerable extension of the level scheme of 138Te including the identification of several nonyrast states. The structure of 137Te and 138Te is discussed on the basis of large-scale shell-model calculations performed using two different effective interactions.
Neutron-rich Pm (Z=61) isotopes were studied by delayed γ-ray spectroscopy at RIBF, RIKEN Nishina Center using the in-flight fission of a 345 MeV/nucleon 238U beam. A cluster-type Ge detector array, ...EURICA, was used to measure the delayed γ rays from stopped ions. Isomeric γ decays were observed in 159Pm and 161Pm with half-lives of 4.97(12) μs and 0.79(4) μs, respectively. Level schemes for 159Pm and 161Pm were constructed in this study. The isomeric states of 159Pm and 161Pm could be interpreted as two quasiparticle excitations of neutrons with the configurations of ν(7/2633⊗5/2523) and ν(7/2633⊗1/2521), respectively. They are analogous to the isomers that have been observed systematically in other even-mass N=98 and N=100 isotones in this region. A projected shell model calculation was performed and it reproduced the order of three-quasiparticle states only if new Nilsson parameters with an N-dependent spin-orbit interaction were used. This work demonstrates that the strength of spin-orbit interactions in standard Nilsson parameters needs to be modified to study the properties of neutron-rich rare-earth nuclei around A=165, and provides new evidence supporting the existence of the deformed N=98 subshell gap in odd-mass nuclei for the first time.
Background: Detailed spectroscopy of neutron-rich odd-A Cu isotopes is of great importance for studying the shell evolution in the region of 78Ni. While there is experimental information on excited ...states in 69−73,77,79Cu isotopes, the information concerning 75Cu is very limited.
Purpose: Experimentally observed single-particle, core-coupling, and proton-hole intruder states in 75Cu, will complete the systematics of these states in the chain of isotopes.
Method: Excited states in 75Cu were populated in the β decay of 75Ni isotopes. The Ni nuclei were produced by the in-flight fission of 238U projectiles, and were separated, identified, and implanted in a highly segmented Si detector array for the detection of the β-decay electrons. The β-delayed γ rays were detected in a HPGe cluster array. Monte Carlo shell model calculations were performed using the A3DA interaction built on the pfg9/2d5/2 model space for both neutrons and protons.
Results: A level scheme of 75Cu was built up to ≈4 MeV by performing a γ−γ coincidence analysis. The excited states below 2 MeV were interpreted based on the systematics of neutron-rich odd-A Cu isotopes and the results of the shell model calculations.
Conclusions: The evolution of the single-particle, core-coupling, and proton-hole intruder states in the chain of neutron-rich odd-A Cu isotopes is discussed in the present work, in connection with the newly observed level structure of 75Cu.
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