M. Weiszflog

Proton and Neutron g9/2 Alignments in the N=46 Isotones 86Zr and 87Nb

The magnetic moments of the 8(+) yrast state in Zr-86 and the 17/2(-) and 21/2(+) yrast states in Nb-87 have been measured via the IMPAD technique, The nuclei were produced in the Ni-58(S-32,Xp) reaction; the time-integral Larmor precessions were measured after recoil implantation into a polarised Fe foil. The g-factors are used to deduce the character of the g(9/2) Shell alignments within the states and their structures are interpreted with the shell model in the (g(9/2),p(1/2)) single-particle space, using the Gross-Frenkel two-body matrix elements. A survey on the various alignment schemes in these neutron-deficient isotopes is given. We find that neutron-pair alignment is favoured for even-parity states in the near-spherical nuclei with N greater than or equal to 46 but a major component of the odd-parity 17/2(1)(-) state arises from the alignment of a neutron-proton pair in the g(9/2) shell coupled to an unpaired neutron in the p(1/2) shell. (Less)

High-spin States in the Transitional Nucleus 88Mo

The reaction58Ni(36Ar,α qρ)88Mo has been studied at 145 MeV beam energy. A detector array consisting of the OSIRIS spectrometer, four charged-particleΔE detectors and seven NE213 neutron detectors has been used to meaure the gamma radiation inγγ- and particle-γγ-coincidence mode. The level scheme of88Mo has been extended up to 11.6 MeV excitation energy and probable spin 23ħ; some 70 transitions and 40 levels have been identified. Spin assignments have been proposed on the basis of measured DCO ratios. Hartree Fock cranking calculations of the Total Routhians and shell model calculations of the high spin states are presented which imply near-sphericity of the yrast line up to the highest spins found. A classification of the high spin states according to their leading seniority is proposed.

Picosecond lifetime measurement of neutron core-excited states in the N = 50 nucleus 95Rh

Abstract Lifetimes of high-spin states in the N = 50 nucleus 95Rh were measured using the recoil-distance Doppler-shift technique. The nuclei were produced via the reaction 58Ni(40Ca,3p) at 145 MeV beam energy and the γ radiation was detected in six EUROBALL cluster detectors. Reduced transition probabilities for seventeen γ transitions and limits for nine further transitions were extracted and compared with the predictions of the spherical shell model based on the extended configuration space, ( 0f 5 2 , 1p 3 2 , 1p 1 2 , 0g 9 2 ) for protons and ( 0g 9 2 , 1d 5 2 ) for neutrons relative to a hypothetical 68Ni core. The results indicate that all observed states with 27 2 ⩽ I ⩽ 39 2 are built from a neutron g 9 2 → d 5 2 excitation across the N = 50 shell closure, coupled to up to five valence protons in different configurations. Evidence of stretched dipole cascades having large M1 strengths was found and explained by spin recoupling within the ν( g 9 2 ) −1 ν( d 5 2 ) neutron part of the wavefunctions.

Multiparticle-hole states of high spin in N < 50, A ≈ 90 nuclei: 3. The odd-odd nucleus 88Nb

Abstract High-spin states of the neutron deficient nucleus 88 Nb have been studied following the reaction 56 Fe ( 35 Cl, 2pn) 88 Nb at 120 and 123 MeV bombarding energy. Neutron gated and pure γγ-coincidences were measured with a neutron-γγ-coincidence setup; in addition γγ-coincidences were detected with the OSIRIS-cube at the Cologne tandem accelerator. Via the analysis of γγ-coincidence spectra the level scheme of 88 Nb has been extended up to 9.7 MeV excitation energy. About 88 transitions and 40 new levels have been placed in the level scheme. The experimental level energies and branching ratios are compared to predictions of shell model calculations in the restricted ( p 1/2 , g 9/2 ) configuration space.

Identification and Structure of High-spin Particle-hole States in 89Mo

Particle-γ andγ-coincidences of the reaction58Ni(36Ar, 4pn)89Mo have been used to gain more information about high spin states in89Mo and to establish the yrast sequence up to 7.6 MeV excitation energy and probable spin 37/2 ħ. Spins and parities were assigned on the basis of DCO-ratios measured with the OSIRIS spectrometer and a large volume Ge detector placed at 162° to the beam. Furthermore, aγ-ray angular distribution experiment was carried out using the reaction58Ni(35Cl, 3pn)89Mo. As in the neighboring isotopes88Mo and90Mo, the positive-parity high-spin states can be grouped into shell model multiplets characterized by increasing seniorities of proton particles and neutron holes in the 1g9/2 shell. The negative-parity states can be explained with one nucleon moving in thep1/2 orbit. The energies and wave functions of these states have been deduced by means of the shell model code RITSSCHIL. The 2584 keV (21/2+) is an isomeric state the mean life of which has been estimated from delayedγγ-coincidences.

Deformations and Shape Changes in 81Y

High spin states in81Y up to the probable 33/2+ and 29/2− yrast states have been measured via the reaction58Ni(28Si,αp, using the Cologne tandem accelerator. Directional correlations of oriented nuclei were determined and the level scheme was extended with lifetimes measured by means of DSA and recoil distance techniques. Reduced transition strengths inferred from lifetimes were consistent with the results of Hartree-Fock-Bogolyubov calculations, which predict a nearly axially symmetric deformation ofβ2≈0.37 for theπ=+ band. Predictions reveal an alignment driving the system to a smaller and triaxial deformation withβ2=0.23 andγ=−30°, although the data show this crossing to be somewhat delayed. Particle-rotor calculations for the one-quasiparticle bands corroborate the predictions of deformation parameters based on the cranking model.

Excited States in the Transitional N=45 Nucleus 85Zr

Excited states in the transitional nucleus85Zr were studied via the fusion evaporation reaction56Fe(35Cl,αpn)85Zr at 120 MeV. A level scheme reaching up to probable spins of (33/2+) at 6.2 MeV and (27/2−) at 4.9 MeV was established and compared to neighbouring nuclei.

Transitional Quadrupole Moments in 83Y

Doppler shift attenuation lifetime measurements have been carried out for the 3qp and isomeric rotational bands in83Y using the reaction58Ni(28Si,3p). While the transitional quadrupole moments in the 3qp bands are slightly lower than in the 1qp bands, we found evidence for a strong reduction in ¦Qt¦ for the isomeric band.

Proton alignment in the 21/2+ yrast state of89Mo

The magnetic moment of the isomeric 2584 keV 21/2+ yrast state in89Mo was measured via the time differential perturbed angular distribution method after recoil implantation (IMPAD) into a polarized Ni target. A g-factor of +0.79(4) was extracted, indicating a π(g9/2)2 v(g9/2)−1 structure of this state. The result is placed in a systematic survey of g-factors in this mass region as well as compared with the prediction of shell model calculations.

Electromagnetic Decay Strengths in the Isobars 89Nb, 89Mo and 89Tc

Lifetimes of high spin states in the threeA=89 isobars89Nb,89Mo and89Tc have been measured via the Recoil Distance Doppler Shift technique and the fusion evaporation reactions58Ni(35Cl,xnypzα) at 120 MeV beam energy. Reduced transition probabilities for about 50γ transitions were extracted. The experimental branching ratios in89Mo and reduced transition probabilities in all three isobars are compared with the predictions of the spherical shell model based on the restrictedπ(p1/2),π(g9/2), ν(p1/2),ν(g9/2) configuration space with two different types of residual interaction.