G.B. Hagemann

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.

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

The identification of the 12+[660] proton orbitals at high spins in rare-earth nuclei

Abstract Decay sequences based on the 1 2 + [660] proton orbital have been identified in 171 Ta and 177 Re based on spin, parity, and large alignment. This decay sequence is observed higher in energy than predicted in cranking calculations based on modified oscillator potentials. Similarly known 1 2 − [541] decay sequences in these and other neighbouring isotopes are observed lower in energy than predicted. A reduction in the strength of the spin-orbit potential for protons is suggested as a solution to these problems.

Triaxial superdeformation in 163 Lu

Abstract High-spin states in 163 Lu have been investigated using the Euroball spectrometer array. The previously known superdeformed band has been extended at low and high energies, and its connection to the normal-deformed states has been established. From its decay the mixing amplitude and interaction strength between superdeformed and normal states are derived. In addition, a new band with a similar dynamic moment of inertia has been found. The experimental results are compared to cranking calculations which suggest that the superdeformed bands in this mass region correspond to shapes with a pronounced triaxiality ( γ ≈±20°).

Configuration-dependent pairing and deformation in 163Er

Abstract High-spin data are given for 163Er. Crossing frequencies of odd-parity rotational bands are discussed. The observed late band crossing of the band built on the strongly oblate orbital [505] 11 2 − cannot be explained by configuration-dependent pairing alone. The need for the inclusion of the deformation-driving effects of these states are discussed. Cranking Hartree-Fock-Bogoliubov calculations including configuration-dependent pairing and deformation reproduce the data very well.

Configuration-dependent transition rates in 157Ho

Abstract The lifetimes of the high-spin states of 157 Ho have been measured using the Doppler shift attenuation method (DSAM). The comparison of the observed shifts following the 124 Sn( 37 Cl, 4n) reaction at a beam energy of 150 MeV, and the 142 Ce( 19 F, 4n) reaction at 85.5 MeV shows that the yrast states above spin 71 2 − are much less collective than the states below. Transition quadrupole moments were obtained for states from spin 45 2 − to 55 2 − h in the yrast band, and from 37 2 − to 53 2 − h in the third lowest negative-parity band. The difference in collectivity agrees with the assignment of these bands to a [523] 7 2 − ( h 11 2 ) and a [541] 1 2 − ( h 9 2 ) quasiproton excitation, respectively. However, it cannot explain the difference in the i 13 2 neutron crossing frequencies of the two bands. The measured Q t values for the [523] 7 2 − configuration are compared to those of other N = 90 isotones.

Double crossing in 174Hf: A deformation jump?

Abstract At I π =22 + , the yrast band of 174 Hf is observed to have a second rotational anomaly, which may be linked with the high-spin extension of the K π =0 + “β-band”. A possible explanation of the anomaly is that the nucleus is more deformed in the excited band than in the yrast band.

Co-existing coupling schemes at high spin in 166Hf

Two types of particles, A and B with their corresponding antiparticles, are defined in a one dimensional cyclic lattice with an odd number of sites. In each step of time evolution, each particle acts as a source for the polarization field of the other type of particle with nonlocal action but with an effect decreasing with the distance: A → · · · B̄BB̄BB̄ · · · ; B → · · ·AĀAĀA · · ·. It is shown that the combined distribution of these particles obeys the time evolution of a free particle as given by quantum mechanics. PACS 03.65.Bz, 03.65.Ca Typeset using REVTEX

Lifetime measurements of the rotational quasi-continuum transitions in 163 Tm

Abstract The lifetime of the unresolved transitions forming the rotational quasi-continuum spectra has been measured in 163 Tm using the Doppler shift attenuation technique. Spectra associated to different values of the coincidence folds, angular anisotropies and the dynamical moment of inertia were also obtained from the present experiment. The Doppler shift has been measured for discrete transitions, for the ridge structures in E γ 1 × E γ 2 spectra and for the rotational E2 bump in one dimensional spectra. The data were compared to the theory using a simulation model which makes use of microscopically calculated levels and transition probabilities. Above 1300 keV the results indicate collectivity with lifetime faster than 10 −14 s, while at lower E γ the results are well described by predictions with B (E2) = 400 WU.

Lifetime measurements of high spin states in Yb nuclei

Abstract Transition rates for high spin discrete line states in 168 Yb have been extracted using a Doppler Shift Attenuation Method and a loss of collectivity has been found at the highest spins. Similar results for other Yb nuclei are reviewed and an explanation, involving a reduction of deformation due to changes of intrinsic structure, is given. It is shown that this loss of collectivity for the yrast and near-yrast bands is in sharp contrast to the recent finding that the transitions contributing to the ridge structure in a γ-γ coincidence matrix retain full collectivity up to the highest spins. Possible reasons for this difference are discussed