J. Borggreen

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).

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.

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.

A study of the rotational side-bands in 162Dy

Abstract Rotational side-bands in 162Dy have been studied using the 160Gd(α, 2nγ)162Dy reaction. Seven side-bands are observed, with Kπ = 2+, 2−, (0)−, 0+, 5−, 4+ and (6−). Four of these bands have collective structure at low spin: the Kπ = 2+ γ-vibrational band, the Kπ = 0+ β-vibrational band, and the Kπ = 2− and (0)− octupole vibrational bands. Of the remaining bands, the 4+ band is deformation coupled while the 5− and (6−) bands are rotation-aligned. Several bandcrossings are observed in this nucleus. The β and γ-bands are crossed at I = 6 h and 12 h , respectively, by a highly aligned ( i 13 2 )2 S-band; extrapolation of this S-band to higher spin suggests that it crosses the g.s.b. between I = 18 h and 20 h . The 2− octupole band is crossed by the 5− band at I = 9 h and again by the (6−) band at I = 12 h . The latter bandcrossings are discussed in terms of two-quasiparticle plus rotor calculations.

High-spin rotational levels in 178W populated in the 177Hf(α, 3nγ) reaction☆

High-spin rotational levels in 178W were populated in the 177Hf(α, 3nγ) reaction. The yrast sequence of states is identified to spin 16 and is contrasted with similar data for 180W and 182W. Two probable two quasi-neutron bands with Kπ = 6+ and 7− are characterized from derived gK values and decay patterns. The Kπ = 2− octupole band is identified to spin 13, and the higher-spin members of the lowest Kπ = 0+ band are placed. A 35 ns isomer is identified at 3528 keV.

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.

Discrete and continuum Gamma-ray studies of 147Gd and 148Gd

Abstract Discrete and continuum γ-ray studies of the N = 83, 84 nuclei 147 Gd and 148 Gd have been performed using ( 36 S, 5n) and ( 36 S, 4n) reactions on 116 Cd. The level schemes of both nuclei have been extended to excitation energies above 18.5 MeV, with assigned spins up to 83 2 in 147 Gd and up to 46 in 148 Gd. Evidence for a change in character along the yrast line from single particle to collective was found at the highest spins in 148 Gd, but not in 147 Gd. For 148 Gd a study of γγ correlation matrices revealed a ridge-valley structure with a ridge spacing indicative of superdeformation. No evidence for a similar structure in 147 Gd was found.

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.