H. Ryde

Nuclear moment of inertia at high rotational frequencies

Abstract Members of the ground state rotational bands in the doubly even well-deformed nucleides 158Dy, 160Dy, 162Er and 168Yb are populated in (α, x n) reactions and the variation of the moments of inertia with the rotational angular velocity is investigated. In three of these nucleides abrupt changes in the moment of inertia are observed at high rotational frequencies. This may be interpreted as a phase transition from the superfluid state to a non-superfluid state with no pairing due to the Mottelson-Valatin Coriolis antipairing effect.

High-spin spectroscopic study of 167Lu

Abstract Three decay sequences associated with the 7 2 + [404], 9 2 − [514] and 1 2 − [541] Nilsson states of 167 Lu have been established to high spin using the 123 Sb( 48 Ca,4n) 167 Lu reaction. The quasineutron and quasiproton band crossings, the energy signature splittings, and the B (M1)/ B (E2) values are discussed and compared with similar quantities for the lighter lutetium isotopes. Configuration-and isotopic-dependent deformations, reflected in such comparisons, indicate that 167 Lu has a larger and more stable prolate deformation than the lighter isotopes and that the 1 2 − [541] configuration has a larger quadrupole deformation than the 9 2 − [514] configuration. The shift of the AB neutron band crossing occurring in the 1 2 − [541] sequences to a larger rotational frequency cannot be understood in terms only of deformation differences. The configuration and neutron-number dependent shapes are also discussed in a “gauge”-space analysis.

Proton configuration dependence of neutron alignments and transition probabilities in 165Lu

Abstract High-spin states in 165 Lu have been populated in the 153 Eu( 16 O, 4n) reaction. An excitation function measurement was made to assign correct γ-rays to transitions in 165 Lu. A level scheme is constructed from the studies of γγ coincidences, and the measurement of γ-ray angular distributions. Nine decay sequences are assigned to 165 Lu. At low spin, these sequences are identified with the 5 2 + [402], 1 2 + [411], 1 2 − [541], 7 2 + [404] and 9 2 − [514] Nilss determined and the variation in the crossing frequency is discussed. ΔI = 1 transitions observed between the favoured and unfavoured decay sequences of four of the five configurations are also discussed.

High-spin properties of 163Yb: Band crossings and signature-splitting in Yb nuclei

Abstract High-spin states in 163Yb have been studied by the 149Sm(18O, 4n)163Yb reaction with 84 MeV 18O ions from the NBI tandem accelerator. Gamma-gamma coincidence data were accumulated with an array of five Compton-suppressed Ge(Li) detectors resulting in an extension of our knowledge of high spin members of familiar rotational bands as well as of weakly populated side bands. A γγ time coincidence experiment, conversion electrons and angular distribution measurements were performed. Four side bands are observed besides the yrast cascade, which is followed up to its 49 2 + member. The yrast band of the 163Yb nucleus has been interpreted, following the systematics in this region of neutron deficient rare-earth nuclei, as the favoured member of a decoupled i 3 2 neutron band. The negative-parity band ( −, +)1 based on the 3 2 − [521] state was reestablished. The other signature of this band is the newly introduced band ( −, − )1. The signature splitting in the negative-parity band is observed to be small; however, for large zω, band ( −, − )1 lies lower in energy than band ( −, +)1; which is a unique observation in this mass region. The quasiparticle routhians and alignments obtained from these data are compared with results of cranked shell-model calculations and similar data for the heavier and lighter Yb nuclei. The rotational effects on M1 transitions connecting unfavoured members of the band, based on the i 13 2 orbital, with the favoured members are demonstrated.

High-Spin Band Structure in 166Yb

High-spin states in 166Yb have been studied via the 154Sm(16O, 4n)166Yb reaction with 80 MeV 16O ions from the NBI tandem accelerator. γ-γ coincidence data were accumulated with an array of four Ge(Li) and four NaI(Tl) detectors to favour high multiplicity events. Totally about 3 × 108 events were accumulated in the coincidence experiment. Measurements of the angular distribution of the γ-radiation and the conversion coefficients were furthermore performed. Four weakly populated side bands are observed besides the yrast cascade which is followed up to its 24+ member. The ground state band has thus been identified to its 18+ state, while the crossing Stockholm band becomes the yrast band from its 16+ member. The S-band is established from its 12+ state. The observed level energies and transition rates are well reproduced in calculations within the particle rotor model.

Near-yrast spectroscopy of 165 − 167Yb

Abstract The lowest (π, α) = (+, +), (+, −) and (−, +) rotational sequences in 165, 167 Yb as well as the lowest (−, −) sequence in 165 Yb have been established to high spins. The single-quasi-neutron portions of these sequences are used to construct two- and three-quasineutron states which are compared to the corresponding configurations in 165, 166, 167 Yb. Such a comparison indicates that the residual interactions between two quasineutrons are nearly independent of h ω and configuration (for those studied), are attractive and are equal to about 300 keV. For three-quasineutron configurations an h ω dependence is observed in the residual interaction, which can be interpreted as a reduction in alignment for the three-quasineutron configurations, a result of reduced pairing. Difficulties in reproducing the signature-dependent energy-splitting and band-crossing frequencies in the positive-parity decay sequence are not completely understood. The signature dependence of the interband-intraband branching ratios as well as of the interband M1 E2 mixing ratios provide an experimental justification of the signature dependence of B (M1) transition rates recently suggested by Hamamoto.

More than thirty bands in 177Re

Abstract The level scheme of 177Re has been extended extensively, with fragments of more than thirty bands, characterised by a given signature, observed. Alignments at low frequency in one-quasiparticle bands are interpreted as deformation effects and compared with the predictions of cranking calculations. The previously assigned one-quasiparticle π i 13 2 band is shown to be a mixture of three-quasiparticle and vibrational character. The non-observation of the one-quasiparticle π i 13 2 band is explained as being due to crossings of its s-band by competing bands. A number of high-K bands are also reported, including five- and seven-quasiparticle bands. Configuration assignments for these bands are aided by comparing experimental gκ values with “effective” gκ values, calculated with a semi-classical expression taking Coriolis mixing into account. The population of the yrast band was observed to oscillate with every four units of spin.

One- and two-phonon wobbling excitations in triaxial 165Lu

High-spin states in Lu-165 have been investigated by in-beam gamma-ray coincidence spectroscopy using the EUROBALL spectrometer array. Two new excited rotational bands have been discovered with features similar to a previously known triaxial superdeformed band in that nucleus. Comparison of the decay pattern of these bands, in particular the unusually large E2 transition strength from the first excited to the yrast superdeformed band, to theoretical calculations shows that they belong to a family of wobbling excitations with phonon numbers n(w) = 0, 1 and 2. These results, together with evidence for nuclear wobbling in the neighbouring isotopes Lu-163 and Lu-167, firmly establish this mode of excitation in the A = 165 mass region. The observation of wobbling is a unique signature of stable nuclear triaxiality

Superdeformed triaxial bands in Lu-163,165

An experimental investigation of the nucleus 165Lu, using the reactions 138Ba(31P,4n) 165Lu and 150Sm(19F,4n) 165Lu at beam energies of E = 155 and 95 MeV, respectively, has been performed. Among other additions to the existing level scheme, a new band, with transition energies almost identical to a strongly deformed (β2 0.42) πi13/2[660 1/2+] band recently discovered in 163Lu has been established. A theoretical analysis of the structure of the two Lu isotopes, 165Lu and 163Lu is carried out by detailed calculations of total potential energy surfaces for specific configurations. By a diabatic treatment of crossings specific proton configurations as πi13/2[660 1/2+] are identified throughout the deformation space and as a function of spin. It is found as a general feature that well deformed local minima of considerable nonaxial symmetry coexist with a normal deformed global minimum. The depth of these local minima depend on configuration. The structure of the different global and local minima found in these surfaces are analysed and discussed in terms of occupation of available basis configurations and their orientation relative to the rotation axis. The strongly deformed minima are found to belong to a group of superdeformed triaxial structures, expected to appear at low energies for certain favourable combinations of proton and neutron numbers.

Production of charged pions in intermediate-energy heavy-ion collisions

Abstract Cross sections for the production of π + and π − have been measured over a wide range of angles for 12 C + 7 Li, 12 C + 12 C and 12 C + 208 Pb collisions at 60 A , 75 A and 85 A MeV. Energy distributions are less steep and absolute yields of pions larger than expected from a straightforward Fermi gas nucleon-nucleon scattering prescription. The apparent velocity of the system in which the π + emission is symmetric, is closer to the nucleus-nucleus c.m. velocity than to the mean speed system. The Coulomb-corrected π − π + ratio is close to unity for 12 C + 12 C. The corresponding ratio for 12 C + 208 Pb is, however, much larger than expected from the neutron excess in 208 Pb only.