G. Sletten

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).

The wobbling mode in 167Lu

Abstract High spin states in 167 Lu were populated through the 123 Sb( 48 Ca, xn ) reaction at 203xa0MeV. Four, presumably triaxial, strongly deformed (TSD) bands have been found in this nucleus. Several transitions linking an excited TSD band (TSD2) to the lowest (yrast) TSD band (TSD1) were observed. The electromagnetic properties of the connecting transitions have been investigated. Evidence for the assignment of TSD2 as a wobbling mode built on TSD1 is presented. This assignment is based on comparisons of the experimental data to theoretical calculations. The wobbling mode of excitation is an unambiguous signal of a stable triaxial deformation associated with these bands.

Co-existence of spherical and oblate configurations in 147Gd

Abstract The decay of the 550 ns high-spin isomer in 147 Gd has been investigated by pulsed beam γ-ray and conversion electron spectroscopy. A detailed level scheme has been deduced and interpreted on the basis of shell-model calculations generalized to non-spherical shapes. At spins I ≦ 41 2 the yrast levels can be understood as shell-model states in a spherical potential whereas for I > 49 2 the yrast sequence is reproduced assuming an oblate-deformed shape. In the intermediate region it can be surmised that both structures are present.

Anomalous four and five quasiparticle isomer decays in 174Hf and 175Hf

Abstract High-spin isomeric decays in 174 Hf and 175 Hf are found to populate a large number of different quasiparticle structures. Many weak γ-ray transitions are revealed that violate the usual K -selection rules. Analysis of transition hindrance factors shows the importance of K -mixing in the populated levels. The effects of both chance near-degeneracies and Coriolis coupling are separately identified. The role of high- K components in Fermi-aligned “tilted” bands is emphasized. The 4-quasiparticle excitation energies are compared with the sum of the constituent 2-quasiparticle energies.

Study of high-spin states in 181, 182Os

Abstract High-spin states in the nuclei 181, 182Os have been populated in the 150 Nd( 36 S ,x n ) reactions and studied with the ESSA30 array. The nucleus 181Os has also been studied at the NBI tandem accelerator using the 167Er(18O,4n) reaction. The previously known bands in both nuclei have been extended to higher spins and two new side bands have been found in 181Os. In the latter nucleus the ground state has been established to have I π = 1 2 − . The extraction of the ratios of reduced transition probabilities B(M1) B(E2) from branching and E2 M1 mixing ratios permitted configuration assignments for most of the bands in both nuclei. The analysis has been carried out within the semiclassical vector model for M1 radiation. The positive-parity yrare sequences in 182Os and the band based on the I π = K π = 23 2 − state in 181Os have been interpreted as t-bands arising from a rotation about a tilted axis. The alignment behaviour and the crossing frequencies are for most of the bands consistent with predictions of the cranked shell model.

Rotational bands in 170Yb observed following (α,xn) reactions

Abstract At least seven side bands, with Kπ = (0+), (1−), (2+), (3−), 4−, 6−, and 7−, have been identified in 170Yb up to high spin, using γ-ray techniques following (α, 2n) and (α, 4n) reactions. Backbending in the positive-parity yrast band is confirmed, and candidates for low-spin members of the S-band are found. The negative-parity yrast band has behaviour characteristic of both strong coupling and decoupling. These properties, together with those in the other negative-parity side bands, are interpreted as arising from Coriolis effects on the quasiparticle configurations, and the experimental results are compared with a two-quasipartiele-plus-rotor calculation. Evidence of ΔK = 2 mixing is found in the K = (3) side band. The relationship between the moments of inertia of the positiveparity and negative-parity yrast bands in the N = 100 isotones is discussed in the framework of a simple empirical model, giving insight into the presence or absence of backbending. The negativeparity yrast bands in the N = 100 isotones are compared. Their significant differences are qualitatively reprodued in two-quasiparticle-plus-rotor calculations.

Nucleon alignment to very high spins in 147Gd: Rapid “rotation” of a fermion system

Abstract Detailed spectroscopic studies of the discrete γ-rays feeding the 550 ns isomer at 8.590 MeV in 147Gd are reported. The resulting decay scheme indicates single-particle nature up to the highest states with I ∼ 40 h and ∼ 17 MeV of excitation energy, with no evidence of collectivity near the yrast line. Comparison with deformed shell-model calculations makes it possible to suggest configuration assignments to many of the levels.

Gamma-spectroscopy within the island of high-spin isomers nearN=82

A NaI sum-spectrometer combined with Ge-counters has been used to characterize the members of the island of high spin isomers nearN=82. On the basis of half lives, totalγ-decay energies and discreteγ-lines, assignments of 22 isomers are given or confirmed. The isomers are localized to the region 82≦N≦86 andZ≦68, and the excitation energies vary from 3 MeV to 12.2 MeV. An empirical relation between spin and excitation energy is presented and on this basis isomeric spin values up to (33±2)ħ are deduced. The isomers are thought to be due to strong alignment of 2 to 8 shell-model particles in a spherical or possibly weakly oblate potential.

High-K structures and triaxiality in 186Os

Abstract The high-spin structure of the stable nucleus 186Os has been studied, using a radioactive 14C beam. New rotational bands built on multi-quasiparticle states with Kπ = 5−, 7−, 9−, 10+ and 15+ are observed and the bandhead energies are compared to predictions of blocked BCS calculations. The first crossing of the ground-state band at I = 14 h , involving a high-K t-band structure, is observed and explained using a two-band mixing model. The structure at higher angular momentum is dominated by intrinsic states, that exhibit a dramatic loss of isomerism. Potential-energy-surface calculations, with Lipkin-Nogami pairing, show these configurations to be triaxial, accounting for the breakdown in K conservation. The relation between the K projection and the total angular momentum is investigated for these non-axial states.

Lifetimes of shears bands in 199Pb

Abstract Lifetimes of states within the two strongest shears bands in 199 Pb have been measured using Doppler-shift techniques. The deduced B (M1) values confirm the proposed proton-particle neutron-hole structure of these bands. They show a decrease with increasing spin as expected within the tilted-axis cranking model. The B (E2) values confirm the low quadrupole collectivity and small deformation predicted by the calculations. The results give further support to the interpretation of the dipole bands in terms of the shears mechanism.