J. Heese

Band crossings and near-rigid rotation in 76Kr and 78Kr

Abstract High-spin states in 78 Kr and 76 Kr were studied via the reactions 58 Ni( 24 Mg, 4p) 78 Kr and 58 Ni( 24 Mg, α2p) 76 Kr at 110 MeV beam energy. An array of fifteen Compton suppressed Ge detectors was used in the POLYTESSA framework to collect γγ-coincidence spectra. Rotational bands were observed up to probable spins of 24 + and 22 − in 76 Kr, and 24 + and 21 in 78 Kr. While the positive-parity yrast bands show strong variations in the moments of inertia caused by the alignments of pairs of ( g 9 2 ) proton and neutron quasiparticles, the negative-parity bands display a constant moment of inertia throughout most of their frequency range. The band crossings and alignments in both nuclei are discussed in the context of the cranked shell model. From an examination of the systematics of crossing frequencies along the Kr isotopes, the previous suggestion that protons align first is confirmed. The intrinsic structures of the bands are also analyzed with the Woods-Saxon-Strutinsky cranking model. The interplay between neutron and proton excitations, the shape changes induced by quasiparticle alignment, and the possible reduction of the static proton pairing in the negative-parity bands are discussed in detail. Three lifetimes in the 76 Kr ground band were remeasured by using the Doppler shift attenuation method, indicating a constant deformation of β 2 = 0.33 up to spin 10 + .

Shell model structure at100Sn — The nuclides98Ag,103In, and104, 105Sn

The neutron deficient nuclei98Ag,104Sn withTz=2 and103In,105Sn (Tz=5/2) were studied in-beam following the reaction of 250 MeV58Ni+50Cr. Neutron and charged particle (p, α) gatedγγ-coincidence spectra were used to identify these nuclei, which are populated with yields between 0.05% and 2% of the total residue cross section, and to determine their level schemes. In a comprehensive shell model study various approaches for the residual interaction were used to describe these newly and several previously studied neutron deficient nuclei. As a result predictions for the neutron single particle energies for100Sn are obtained and used to discuss the next generation of experiments.

Study of the high-spin structure of 144Gd

Abstract High-spin states in 144 Gd have been investigated through the 108 Pd( 40 Ar, 4n) reaction. An extended level scheme is proposed. A ridge structure has been observed in a γγ coincidence matrix, consistent with energy spacings in the superdeformed bands of neighbouring nuclei. Several discrete high-spin sequences have been observed. They are interpreted in the context of cranked-Strutinsky-type calculations in terms of different coexisting shapes.

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.

Rotational bands in73Br: The disappearance of shape coexistence in72Se

By means of the reaction40Ca (36Ar, 3p), the band structure of73Br connected with the g9/2 single-particle proton orbit has been investigated up to Iπ=33/2+. In contrast to the core spectrum in72Se, which is dominated by the coexistance of different shapes, the nucleus73Br features rotational bands with slowly increasing moments of inertia. Their deformation β2=0.40(2) has been deduced from recoil distanc lifetime measurements.

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

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.

Nature of the three-quasiparticle 17 22 - state in 83Y

Abstract Information on the nature of the 17 2 2 − state in 83 is deduced from measurements of g-factor. The measurements were carried out using the IMPAD technique in an iron host. The g-factor, g( 17 2 − )= 0.29 (6) , excludes a structure for the state arising from pure configurations of a rotationally aligned g 9 2 proton, neutron, or proton-neutron pair. Satisfactory agreement is only found if the state has both protons and neutrons active in the g 9 2 shell.

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.