M. K. Kabadiyski

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)

Seniority v=5 States in 89Nb

Yrast and yrare states in the nucleus [sup 89]Nb have been studied in the reaction [sup 58]Ni([sup 36]Ar,5[ital p]) by measuring [gamma][gamma] coincidences and directional correlation orientation with the OSIRIS array. In addition, [gamma]-ray singles angular distributions have been taken with the reaction [sup 58]Ni([sup 35]Cl,4[ital p]). Up to the highest spins observed (37/2[sup +],33/2[sup [minus]]), the states identified line up with the ones predicted by the spherical shell model, taking into account three proton particles and two neutron holes in the [ital g][sub 9/2] and [ital p][sub 1/2] single-particle orbits. The shell model also reproduces most of the measured branching and [ital E]2/[ital M]1 mixing ratios. The mean lifetime of the 2152 keV 17/2[sup [minus]] yrast state was determined as 0.74(7) ns.

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.

Determination of spins and mixing ratios from directional correlations measured with the OSIRIS array

From the general triple angular correlation of gamma-radiations from axially symmetric oriented nuclear states the limit of double correlations with unobserved intermediate transitions is derived. Expressions for DCO ratios are obtained for a detector geometry in which the OSIRIS array is placed perpendicular to the beam direction and an additional Ge detector is positioned close to the beam axis. Spin assignments to high-spin states in Mo-89,Mo-90 and Tc-90 populated in the Ni-58 + Ar-36 fusion reaction are reported. The relevance and precision of E2/M1 mixing ratios deduced from the DCO ratios is tested against those obtained from angular distributions which were taken for the same transitions, populated in the reaction Ni-58 + Cl-35.

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.

A Study of High-spin States in the Transitional Nucleus 90Mo

The reaction58Ni(36Ar, 4p)90Mo has been studied at a beam energy of 149 MeV. A detector array consisting of the OSIRIS spectrometer (12 Compton-suppressed Ge detectors), four charged-particleδE detectors and seven NE-213 neutron detectors at the VICKSI accelerator in Berlin has been used to measure the gamma radiation inγγ- and particle-γγ-coincidence mode. An additional Ge detector was placed at 162‡ to the beam direction to provide information on DCO ratios. The level scheme of90Mo has been extended up to an excitation energy of about 12 MeV and probable spin 23ħ. Some 70 transitions and 40 levels have been newly identified. Spin assignments have been proposed on the basis of measured DCO ratios. Shell model calculations in a model space consisting of the proton 1f5/2, 2p3/2, 2p1/2, and 1g9/2 orbits and the neutron 2p1/2, 1g9/2, 1g7/2, 2d5/2, 2d3/2, and 3s1/2 orbits with some truncation were made for states above 9ħ and the predicted structure of these states is discussed.

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.

Magnetic moments in 90Mo: a discriminating test of the (g9/2,p1/2) shell model

New magnetic moments presented here allow stringent tests of the truncated shell model calculations performed in the region below the N = 50 neutron shell. The experimental g-factors for selected medium-spin yrast states in Mo-90 were determined via the IMPAD technique using the (Ni(Cl,3p))-Ni-58-Cl-35 reaction. The results of g(5-, 2549 keV) = +1.10 +/- 0.28, g(11-, 4842 keV) = +0.42 +/- 0.13 and g(12+, 4556 keV) = +0.50 +/- 0.06 are used to show the that (g9/2,p1/2) configuration space with respect to a Sr-88 core is a rather successful truncation for the effective interactions considered but some shortcomings are exposed.