High-spin structures of 88Kr and 89Rb: Evolution from collective to single-particle behaviors

The high-spin states of the two neutron-rich nuclei, 88Kr and 89R have been studied from the 18O + 208Pb fusion-fission reaction. Their level schemes were built from triple gamma-ray coincidence data and gamma-gamma angular correlations were analyzed in order to assign spin and parity values to most of the observed states. The two levels schemes evolve from collective structures to single-particle excitations as a function of the excitation energy. Comparison with results of shell-model calculations gives the specific proton and neutron configurations which are involved to generate the angular momentum along the yrast lines.Comment: 12 pages, 9 figures, Physical Review C (2013) in pres

Evolution of shell structure in neutron-rich calcium isotopes

We employ interactions from chiral effective field theory and compute the binding energies and low-lying excitations of calcium isotopes with the coupled-cluster method. Effects of three-nucleon forces are included phenomenologically as in-medium two-nucleon interactions, and the coupling to the particle continuum is taken into account using a Berggren basis. The computed ground-state energies and the low-lying 2+ states for the isotopes 42,48,50,52Ca are in good agreement with data, and we predict the excitation energy of the first 2+ state in 54Ca at 1.9 MeV, displaying only a weak sub-shell closure. In the odd-mass nuclei 53,55,61Ca we find that the positive parity states deviate strongly from the naive shell model.Comment: 5 pages, 4 figures; small correction of effective 3NF and slight change of the corresponding parameters; updated figures and tables; main results and conclusions unchange

Medium-spin states in neutron-rich 83As and 81As

The 83,81 As nuclei have been produced as fission fragments in the fusion reaction 18O + 208Pb at 85 MeV bombarding energy and studied with the Euroball array. Medium-spin states of 83,81 As have been established up to ∼3.5 MeV excitation energy. From angular correlation analysis, spin values have been assigned to most of the 81 As excited states. The behaviors of the yrast structures identified in this work are discussed in comparison with the general features known in the mass region. Then they are compared to the results of two theoretical approaches: the "rotor + quasiparticle" for 81 As and the shell model using the effective interactions JUN45 for 83,81 As

High-spin structures of five N=82 isotones: 136Xe, 137Cs, 138Ba, 139La, and 140Ce

Five N=82 isotones have been produced in two fusion-fission reactions and their gamma-rays studied with the Euroball array. The high-spin states of 139La have been identified for the first time, while the high-spin yrast and near-to-yrast structures of the four others have been greatly extended. From angular correlation analysis,spin values have been assigned to some states of 136Xe and 137Cs. Several cascades involving gamma-rays of 139La have been found to be delayed, they deexcite an isomeric state with T1/2= 315(35) ns located at 1800-keV excitation energy. The excited states of these five N=82 isotones are expected to be due to various proton excitations involving the three high-j subshells located above the Z=50 shell closure. This is confirmed by the results of shell-model calculations performed in this work. In addition, high-spin states corresponding to the excitation of the neutron core have been unambiguously identified in 136Xe, 137Cs, and 138Ba.Comment: 18 pages, 18 figures, 10 tables, accepted for publication in Phys. Rev.

A new high-spin level scheme for

A new high-spin level scheme of 149Nd has been identified, which is at variance with the one previously published. The states have been populated in the 12C + 238U fusion-fission reaction at 90MeV bombarding energy and the emitted γ-transitions were detected using the Euroball III array. The yrast band is built on the νi 13/2 sub-shell as in the neighbouring isotones. Moreover, sign of octupole vibrations has been found in 149Nd, as well as in 147Ce, where several new states decaying to the yrast band have been newly observed

Half-life measurements of

A fast-timing setup has been developed to measure the nuclear-state half-lives of neutron-rich nuclei. The first on-line measurements were performed on 137, 139Cs obtained from the β decay of 137, 139Xe. These neutron-rich Xe nuclei were produced by the 238U photofission induced by the 50MeV electron beam delivered by the ALTO facility. The half-lives of the first excited state in 137Cs and of six excited states in 139Cs were measured for the first time. Taking into account new information given by the deduced transition probabilities, we discuss the structure of the first excited states in 137, 139Cs : it is shown that, in both nuclei, the first excited $$\tfrac{5} {2}^ +$$ level corresponds mainly to the one-proton state issued from the π2d 5/2 orbital and that the most probable spin values for the states located at 289.8, 393.5, and 393.8keV in 139Cs are $$\tfrac{5} {2}^ +$$, $$\tfrac{3} {2}^ +$$, and $$\tfrac{1} {2}^ +$$, respectively

Fast timing: Lifetime measurements with LaBr3 scintillators

In order to perform half-life measurements at Alto, we developed a fast-timing setup using fast scintillators : LaBr3. The main components of our setup and their characteristics (timing properties) are presented. The resulting time resolution, obtained after optimization, and measured off-line with radioactive sources is discussed. This set-up was tested on-line by measuring using the slope method half-lives of 137-139Cs levels fed by the β- decay of 137-139Xe. The neutron-rich Xe nuclei were produced by 238U photofission using, at low intensity, the electron beam and the line of the future facility ALTO. Our results show the ability of such a fast-timing system to measure half-lives lower than the total time resolution. Off-line and on-line results are discussed.Fil: Deloncle, I.. Université Paris Sud; Francia. Centre National de la Recherche Scientifique; FranciaFil: Roussière, B.. Centre National de la Recherche Scientifique; Francia. Université Paris Sud; FranciaFil: Cardona, Maria Angelica. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Comision Nacional de Energia Atomica. Gerencia D/area Invest y Aplicaciones No Nucleares. Gerencia Fisica (cac). Departamento de Física de la Materia Condensada.; ArgentinaFil: Hojman, Daniel Leonardo. Comision Nacional de Energia Atomica. Gerencia D/area Invest y Aplicaciones No Nucleares. Gerencia Fisica (cac). Departamento de Física de la Materia Condensada.; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Kiener, J.. Université Paris Sud; Francia. Centre National de la Recherche Scientifique; FranciaFil: Petkov, P.. Institue of Nuclear Research and Nuclear Energy; BulgariaFil: Tonev, D.. Institue of Nuclear Research and Nuclear Energy; BulgariaFil: Venkova, Ts.. Institue of Nuclear Research and Nuclear Energy; Bulgari