George Dracoulis

Energy traps in atomic nuclei

A small proportion of atomic nuclei can form highly excited metastable states, or isomers. Of particular interest is a class of isomers found in deformed axially symmetric nuclei; these isomers are among the longest-lived and have the potential to reach the highest energies. By probing their properties, insights into nuclear structure have been gained. The possibility of stimulated isomer decay may ultimately lead to new forms of energy storage and γ-ray lasers.

Shape coexistence in very neutron-deficient Pt isotopes

Yrast bands in 176Pt(N=98) and 178Pt have been identified. The level scheme of 176Pt changes from a quasi-vibrational pattern to that of a well deformed rotor, at very low spins, a behaviour similar to that attributed to shape coexistence in the light Hg isotopes. Analysis of The 176-188Pt yrast bands supports a shape coexistence interpretation, the excitation energy of the intruder state behaving as predicted by Wood, (1981).

Lifetimes of high spin rotational states

Abstract Lifetimes of 8 + , 10 + and 12 + states in fourteen rotational and quasi-rotational doubly even nuclei have been measured with a Doppler broadened lineshape technique. In addition, the lifetime of the 14 + state in 164 Dy has been determined. The B (E2) values have been extracted and compared with rotational model predictions. All the 8 + → 6 + and 10 + → 8 + transitions and most of the 12 + → 10 + transitions are well described by the model. However, the 12 + → 10 + transitions in 162 Dy, 164 Er and 170 Yb are found to be retarded significantly. It is Suggested that the hindrances may be related to the known backbending properties of two of these nuclei.

High-spin states in 222Th

Abstract Levels to spin 16 + in the ground band and 17 − in the octupole band in 222 Th have been identified using the reaction 208 Pb( 18 O, 4n) 222 Th at 93 MeV. To suppress intense γ-ray background from fission a residue detector was built and operated in coincidence with the Ge(Li) and NaI(Tl) detectors. The apparent moment of inertia in the ground-state rotational band rises very rapidly with increasing spin, however, no indications of backbending were observed. The octupole band at low spin has an aligned angular momentum J ≈ 3 h relative to the ground-state band. Strong cross-band E1 transitions competed with collective E2 transitions within each band. Analysis showed that the ratios B( E 1) B( E 2) were within experimental uncertainty independent of both the spin and parity of the parent state. The average γ-ray multiplicity per cascade was measured for 222 Th. The results were in reasonable agreement with the computer code ORNL-ALICE.

High-spin yrast and non-yrast bands in 176Os, 178Os and 180Os

Abstract High-spin yrast and non-yrast states have been identified in 176 Os, 178 Os and 180 Os using ( 16 O , x n) reactions, and γ-ray techniques. Band crossing anomalies are observed in each of the positive-parity yrast bands. The magnitude of these anomalies decreases with decreasing neutron number, an effect attributed to the change in the moment of inertia of the ground state rotational bands. A 23 ns isomer, predominantly K π = 7 − , is identified at 1930 keV in 180 Os. The configuration of this isomer is discussed on the basis of the properties of its rotational band. Negative parity, odd and even spin, sideband sequences are observed in each isotope. Their relationship to rotation-aligned octupole and 2-quasiparticle bands is discussed from their excitation energies, band spacings, and decay properties. Detailed calculations for Coriolis mixed bands are carried out for the likely 2-quasiproton and 2-quasineutron configurations. An anomaly observed at spin 17 in the odd-spin negative-parity sequence in 180 Os is attributed to a band crossing with a fourquasiparticle configuration.

Exotic Isomers in Deformed Atomic Nuclei

Excited states of atomic nuclei can have long half lives, due to the angular-momentum couplings of unpaired nucleons. Such isomeric states provide opportunities for exploring novel nuclear physics, astrophysics and physics at the atomic/nuclear interface. This review focuses on the properties of isomers in deformed nuclei, and emphasises the importance of axial symmetry in preserving the integrity of the K quantum number. A region of neutron-rich nuclei around 188Hf (Z=72, N=116) is predicted to have exceptional isomer properties, and experimental advances are now opening up this region to detailed investigation.

K-forbidden transitions from multi-quasiparticle states

Abstract Four-quasiparticle, K π = 12 + states in 172 Hf and 178 W, with half-lives less than 2 ns, are found to decay to their respective K π = 0 + ground-state bands, in competition with transitions to intermediate- K , two-quasiparticle structures. All decay transitions are weakly hindered. When taken together with other K -isomer decay rates, an overall trend of decreasing hindrance with increasing excitation energy is evident. Estimates based on density-of-states considerations approximately reproduce the trend of the data, but also indicate the need to include other K -mixing effects.

Sidebands and high-spin states in 182Os

Abstract High-spin rotational states in 182Os have been studied using singles and coincidence γ-ray techniques and the 170Er(16O, 4n) reaction. The yrast sequence of states has been extended to spin 24+ and 7 sidebands identified. These are the γ-vibrational and K = 3− octupole bands and five two-quasiparticle bands including the K = 8−(τ = 1.1 ms) isomer band. The octupole vibrational band is rotation aligned and may also mix with two-quasiparticle configurations involving an i 13 2 neutron or an h 9 2 proton orbit. The bands with K = 5(−) and 8− also involve an i 13 2 neutron and have high apparent moments of inertia. As well as the ( i 13 2 ) 2 s-band, two other rotation aligned positive-parity bands are observed and they are suggested to be the yrare ( i 13 2 ) 2 neutron bands. The bands are characterized in terms of their most probable Nilsson configurations. The level scheme of 181Os was reinvestigated and the i 13 2 neutron band extended to spin 49 2 + .

High-spin states in 172W, 174W and 176W studied by (16O, xn) reactions: decoupled quasiparticle bands and backbending

High-spin states in 172W, 174W and 176W have been studied using gamma -ray techniques following (16O, xn) reactions on enriched Dy targets. The ground-state band has been shown to backbend in 174W and 175W, but not in 172W. Side-bands are populated in each nucleus, with preferential population of an odd-spin sequence in each case. The decay properties of these bands, which have negative parity, suggest that they are due to the partial decoupling of a pair of quasi-particles by the Coriolis force. The properties of the bands are used to estimate in an empirical way the properties of a rotation-aligned (i132/)2 neutron band in order to predict the systematics of backbending due to bandcrossing.

High-spin and single-particle structure in 173W and 175W

The level structures in 173W and 175W have been studied using (16O,4n) reactions. Rotational bands based on three different single-particle configurations are observed to high spin in each nucleus. From a comparison of their contrasting behaviour with the ground-state rotational bands in 172W and 174W, it is concluded that the decoupling of a pair of i13/2 neutrons causes the backbending in 174W. A single-quasiparticle Coriolis mixing calculation is able to reproduce the observed band structure up to high spin.