J. Pedersen

Decay of high-spin isomers in Os nuclei by barrier penetration

Abstract A decay scheme for the 130±20 ns high-spin isomer in 182 Os has been established. The excitation energy of the isomer is 7049±1 keV and it has I π = 25 (+) . A 2.4% decay directly to the yrast 24 + level at 5988 keV is observed. In 184 Os a 20±5 ns isomer is observed at 2366±1 keV excitation energy with I π = 10 + . Again, direct transitions into the yrast 8 + and 10 + levels are observed. Contrary to previous speculations, there is no compelling evidence for stable triaxial shapes in the structure of the levels through which the isomers decay. The abnormally short half-lives observed, as well as the unusual decay patterns, are best understood in terms of a γ-soft nuclear potential. Motion in the γ-direction allows the isomer to decay via barrier penetration from an axially symmetric prolate shape with the angular momentum along the nuclear symmetry axis (deformation aligned state) via oblate shapes to another prolate shape with the angular momentum perpendicular to the nuclear symmetry axis (rotation-aligned state).

Nuclear structure effects in the feeding of yrast states of Gd, Dy and Er nuclei

Abstract The population of the yrast and near-yrast levels in Gd, Dy and Er nuclei has been investigated experimentally in (heavy ion, xn) reactions through high-resolution γ-ray intensity measurements. A clear difference between non-rotor (N ⩽ 86) and rotor nuclei (N > 86) is evident from the data. For the non-rotor nuclei we define the spin value Isat below which the yrast population saturates. One finds that Isat is independent of the bombarding energy when the latter becomes sufficiently high. We discuss the feeding pattern in relation to the single-particle structure of the yrast and near-yrast levels and in relation to the effect of shape changes, including the possibility of superdeformed shapes at high spin.

Triaxiality in 166Er

Abstract The nucleus 166 Er has been Coulomb excited using beams of 16 O, 32 S and 58 Ni. An almost complete set of reduced E2 matrix elements for the ground- and γ-bands up to spin 14 + and 12 + , respectively, has been measured. A total of 44 E2 matrix elements have been determined in a model-independent way, including the static quadrupole moments up to spin 10 + of the ground band and spin 8 + of the γ-band. The data are compared with the predictions of geometric collective models and with the IBA-1 model. Overall, the experimental data are well reproduced by both the asymmetric rotor model and the rotation-vibration model. The data slightly favours the latter model and suggest that the collective structure of 166 Er can be understood in terms of a varying dynamic triaxiality from γ ≈ 18° for the ground state to γ ≈ 10° for the higher spin states of the ground band and for the γ-band. The data also suggest that the Coriolis interaction is strong already at spin 8 + in both the ground band and the γ-band.

Coulomb excitation of 172Yb

Abstract The nucleus 172 Yb has been Coulomb excited using beams of 16 O, 32 S and 58 Ni. The transitional and diagonal E2 matrix elements of the ground band have been measured up to spin 14 + and 12 + , respectively, as well as the transitional E2 matrix elements between the lowest-lying levels of both the K π = 0 + band at 1043 keV and the K π = 2 + band at 1466 keV and the ground band. The data are compared with the predictions of simple geometric collective models and with the IBA-1 model. The K π = 2 + band is likely to be the γ-band, and the experimental data are well reproduced by both the asymmetric rotor model with an asymmetry parameter of γ ⋍ 5° , and by the rotation-vibration model. The K π = 0 + excited band is found to be strongly deformed, but the excitation of the band is non-collective, which cannot be reproduced by any of the models.

Single particle radiation between high spin states in 147Gd

Abstract Transitions above the T 1 2 = 550 ns , 8.59 MeV isomer in 147 Gd have been studied using the ( 30 Si, 5n) reaction. Results from γγ coincidence, angular distribution and recoil distance measurements are combined to establish a level scheme up to 16.9 MeV and I ∼ 79 2 . Single particle configurations are assigned on the basis of the deformed independent particle model. The single particle nature of the highest spin states and the apparent lack of collectivity is discussed.

Intrinsic Excitations as Yrast Structures in 179W above I ~ 35/2 hbar

Gamma cascades through high-spin configurations with high seniority are found to feed the t1/2 = 710 ns isomer in 179W. The abnormally short half life for the isomeric state itself, which decays into the K = 7/2 ground-state rotational 31/2 member, violating the K-selection rule twelve times, is explained as a result of an accidental degeneracy in energy between the Kπ = 35/2- isomeric state and the 35/2- groundband rotational state. Above the isomer, the yrast structure is dominated by deformation aligned states.

Non-Collective Properties of the Continuum Gamma Rays Feeding Two High-Spin Isomers in 152Er

The decay and feeding of two yrast isomer (I ~ 26 and I ~ 36 respectively) in 152Er is measured. The isomeric delays are used to separate the spectrum of γ-rays from the highest angular momentum states from the spectrum due to the decay of low spin states. Strong dipole as well as quadrupole radiation dominates the yrast cascades which feed into the I ~ 36 isomer, and the correlation spectrum for the feeding transitions does not indicate any collective structure for spins below 50. It is tentatively concluded that the near magic nucleus 152Er remains spherical or oblate up to this high spin value.

Nuclear shape transitions and some properties of aligned-particle configurations at high spin

Two topics are addressed in this paper. First, we discuss the variation of shapes with spin and neutron number for nuclei in the N approx. = 88 transitional region. Second, we present comments on the feeding times of very high spin single-particle yrast states.