Dirk Rudolph

Multiparticle-hole states of high spin in N < 50, A ≈ 90 nuclei: 4. Systematics of level energies and electromagnetic properties

Abstract The large body of recently measured energies and electromagnetic properties of high-spin states in the Z = 40–45, N = 46–50 isotopes is consistently accommodated in the shell model using empirical and schematic two-body matrix elements in the ( 1g 9 2 , 2p 1 2 ) model space. In particular, we test the purity of seniority, the effects of an enlarged model space (including the 1f 5 2 , 2p 3 2 orbits) and the onset of quadrupole collectivity near N = 46. Furthermore, we give a summary of observed and predicted magnetic moments and lifetimes of long-lived high-spin states.

First identification of excited states in 84–86Nb

We report on the observation of excited states in the niobium isotope with masses 84, 85 and 86. These nuclei lie in the shape transition region known to occur in nuclei with 35 less-than-or-equal-to Z less-than-or-equal-to 40 and N = 45. Unambiguous identification was achieved by detecting gamma-rays in coincidence with mass-separated recoils produced in heavy-ion fusion reactions. Rotational bands were observed in 84Nb and 85Nb which show decoupling schemes similar to those in the neighbouring nuclei. A rotational model was used to interpret these bands and the quasiparticle configurations involved. Two sequences of transitions were observed in 86Nb, at least one of which appears to be a rotational band.

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.

Heavy-ion In-beam Studies of the Nucleus 87Nb

High spin states in the transitional nucleus87Nb up to 14 MeV excitation have been established for the first time via the reactions40Ca(50Cr, 3p)87Nb and58Ni (32S, 3p)87Nb. The87Nbγ-radiations have been identified throughγ-ray spectra taken in coincidence with the evaporation residues detected in the Daresbury recoil separator or with multiple proton emission. Gamma-gamma coincidences, DCO ratios,γ-ray angular distributions and lifetimes have been measured. A total of some 100 transitions have been placed into a level scheme comprising of sixty states. The one-quasiparticle (1qp) bands of either parity and several other band-like structures have been identified, some containing alignedg9/2 nucleons. Moderately enhancedE2 in-band transitions of 13–48 W.u. as well as several weakE2 yrast transitions connecting bands with different quasiparticle numbers have been found. Similarities with respect to theN=46 isotones83Rb,84Sr,85Y,86Zr and88Mo are discussed.

Multiparticle-hole states of high spin in N < 50, A ≈ 90 nuclei: 1. The transitional nucleus 4389Tc46

Abstract High spin states in the nucleus 89 Tc have been studied via the fusion evaporation reaction 58 Ni( 40 Ca,2αp) 89 Tc at 180 MeV beam energy. The NORDBALL γ-ray spectrometer equipped with auxiliary detectors for light particle selection was used to measure γγ- and particle-γγ coin-cidences. Some 60 transitions were placed into a level scheme comprising 38 levels reaching up to 9.2 MeV excitation energy and a possible spin of I = 45/2 h . The level scheme is compared to those of neighbouring nuclei and interpreted in terms of the spherical shell model. The calculations were performed with different sets of parameters within a restricted π(p 1 2 ) , π(g 9 2 ) , ν(p 1 2 ) and ν(g 9 2 ) configuration space. States above 2.3 MeV excitation energy are well reproduced by shell model calculations based on an empirical residual interaction, whereas collective excitations are suggested to contribute to the wave functions of lower lying states.

First Identification and Shell Model Structure of 92Rh

Excited states in theN=47 nucleus92Rh were populated via the fusion evaporation reaction58Ni(40Ca,αpn)92Rh at 180 MeV beam energy. A level scheme reaching up to about 7 MeV and probable spins of (19+) and (21−) was established and interpreted with the shell model in the (p1/2,g9/2) model space.

Multiparticle-hole States of High Spin in N

High spin states in the nucleus Tc-89 have been studied via the fusion evaporation reaction Ni-58(Ca-40,2 alpha p)Tc-89 at 180 MeV beam energy. The NORDBALL gamma-ray spectrometer equipped with auxiliary detectors for light particle selection was used to measure gamma gamma- and particle-gamma gamma coincidences. Some 60 transitions were placed into a level scheme comprising 38 levels reaching up to 9.2 MeV excitation energy and a possible spin of I = 45/2HBAR The level scheme is compared to those of neighbouring nuclei and interpreted in terms of the spherical shell model. The calculations were performed with different sets of parameters within a restricted pi(p(1/2)), pi(g(9/2)), nu(p(1/2)) and nu(g(9/2)) configuration space. States above 2.3 MeV excitation energy are well reproduced by shell model calculations based on an empirical residual interaction, whereas collective excitations are suggested to contribute to the wave functions of lower lying states. (Less)

Investigation of shape changes in 87Mo

The level scheme of 87Mo was investigated with the reaction Ni-58(S-32, 2pn) at projectile energies of 110-130 MeV and the reaction Ca-40(Cr-50, 2pn) at 170 MeV. Extensive spectroscopic information was collected including gamma-gamma and neutron-gated gamma-gamma-coincidences as well as coincidences with identified 87Mo residues. Level lifetimes were studied with the RDDS and DSA methods. The angular distributions of low-lying transitions were measured. The results indicate a sudden transition from single-particle-like to collective behavior which is caused by g9/2 particle alignment. The level scheme was constructed up to 12 MeV excitation energy and a probable spin of 49/2 HBAR.

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.