R. M. Clark

First observation of a collective dipole rotational band in the A∼200 mass region

Abstract The nucleus 198 Pb was populated via the 186 W( 17 O,5n) 198 Pb reaction at beam energies of 92 and 98 MeV. A highly regular rotational ΔI =1 band has been found. Angular correlation measurements confirm the transitions to be dipoles. Assuming M1 multipolarity for these transitions the experimental data are interpreted in terms of a π([505] 9 2 − ⊗[606] 13 2 + ) K π =11 − ⊗ν( i 13 2 ) 2 configuration of oblate shape. This is the first such collective oblate structure identified in the A ∼200 mass region.

Collective oblate dipole rotational bands in 198Pb

Abstract The nucleus 198 Pb was populated via the 186 W( 17 O, 5n) 198 Pb reaction at beam energies of 92 and 98 MeV. Five collective rotational cascades of ΔI = 1 transitions have been found. Four are highly regular, one much more irregular. The structures are incorporated into a level scheme which extends up to approximately spin 32ħ and an excitation energy of about 10 MeV. Angular correlation measurements confirm the dipole character of the interband transitions. Their M1 multipolarity is inferred, and from this supposition the experimental data are interpreted in terms of oblate high- K two quasiproton configurations coupled to aligned neutron excitations. This interpretation is extended to include other ΔI = 1 oblate structures observed in 194–201 Pb. It is shown that the pattern of observed moments of inertia can be understood in the simple unpaired picture involving neutron i 13 2 excitations. The identical bands observed are interpreted in terms of the normal-parity weakly-coupled singlet orbital.

Intruder bands in 108Sn

Abstract The nucleus 108Sn has been populated via the 54Fe(58Ni, 4p) reaction channel at a beam energy of 243 MeV. The high-spin structure is dominated by three ΔI = 2 rotational sequences. These bands can be interpreted in terms of particle-hole excitations involving the proton g 7 2 , g 9 2 and h 11 2 orbitals and also aligned neutrons from the bottom of the h 11 2 shell. Lifet measurements have also been performed using the Doppler-shift attenuation method. These data have enabled quadrupole moments to be deduced for the two strongest bands. The results yield Q0 = 2.6±0.4 e·b for the positive-parity band and 3.4 ± 0.6 fse·b for one of the proposed negative-parity bands. These values yield quadrupole deformations of β2 = 0.20 and 0.26, respectively, for the two bands. The results obtained are discussed in terms of Woods-Saxon and total routhian surface calculations.

Single particle excitations and properties of multiple band terminations near spin 50ħ in 158Er

The competition between collective rotation and single-particle behaviour at high spin in 158Er has been investigated with the Eurogam spectrometer. Band terminating states have been observed in three structures at Iπ = 46+, 48− and 49− and specific single-particle configurations are assigned by comparison with cranked Nilsson-Strutinsky calculations. These special states are found to be related by very simple single particle excitations. These data indicate that an oblate mean field (ϵ ∼ −0.14) is established for a nuclear core plus 12 aligned valence nucleons which is stable against single-particle rearrangements. In the (π, α) = (+, 0) states the collective and weakly collective terminating structures are observed to coexist between 30+ and 46+.

Lifetimes of the superdeformed band in 192Hg

Lifetimes of states in the superdeformed band in 192Hg have been measured using the Doppler-shift attenuation and the recoil-distance methods for the high-spin and the low-spin regions, respectively. Line shapes and decay curves were obtained from high-fold γ-ray coincidence spectra measured with the EUROGAM array. The transition quadrupole moments, derived from the lifetimes, are constant with an average value of Qt = 18.6 b down to the bottom of the band where it decays to the normal states. This suggests that the superdeformed potential-energy minimum is still present at that point and that the decay proceeds by tunneling.

M1 transitions between superdeformed states in 195Tl: the fingerprint of the i132 proton intruder orbital

Abstract Dipole transitions linking the superdeformed Yrast signature-partner bands in 195Tl have been observed using the EUROGAM 1 spectrometer. From the measurements of the photon decay branching ratios, taken together with the average quadrupole moment measured in neighbouring superdeformed nuclei, we find that the two signature partner superdeformed bands involve the excitation of the 81 st proton into the i 13 2 Ω= 5 2 intruder orbital

The yrast superdeformed band in 194Pb. Differences with 192Hg

The yrast superdeformed (SD) structure of 194Pb has been studied using the EUROGAM γ-ray spectrometer. New results have been obtained at low as well as at high spin values. At low frequency, the SD band has been extended to the lowest-spin SD state ever observed in the A ∼ 190 region. The energies of 194Pb and 192Hg SD bands are identical for ω < 0.25 MeV. However, they diverge for higher frequency, which rules out the idea that these two SD bands are identical. Moreover, the SD band of 194Pb is populated at lower spin values than that of 192Hg and de-excites towards normally-deformed (ND) states at a spin value lower than that of 192Hg. This suggests that the barrier separating the SD and ND minima in the potential energy surface is higher in 194Pb than in 192Hg.

Two proton high-spin excitations and dipole bands in192Hg

The192Hg nucleus was populated in the160Gd(36S, 4n) reaction at a beam energy of E=159 MeV. Emittedγ-rays were detected with the EUROGAM array comprising 43 Compton-suppressed large volume Ge detectors. The level scheme of192Hg has been extended up to an excitation energy of E=10.4 MeV and spin I=34 ħ. Two new structures, made of competing ΔI=1 and ΔI=2 transitions have been observed and their connexions with the low-lying levels established. Their lowest levels are located at 6.304 MeV and 6.879 MeV excitation energy. The experimental results are compared with mean-field HF+BCS calculations. It is proposed that the new structures originate from deformation-aligned quasi-proton excitations π(i13/2 * h9/2)K=11 andπ (h9/2)K=82, coupled to rotation-aligned quasi-neutron ν(i13/2)n and quasi-proton π(h11/2)2 excitations.

Observation and lifetime of the first excited superdeformed band in Hg-192

An excited superdeformed (SD) band has been observed in 192Hg with the UROGAM spectrometer. It is interpreted as a two quasi-neutron excitation in the 192Hg SD nucleus. Nuclear level lifetimeshave been measured using the Dopple-shift attenuation method. An average transition quadrupole moment of 19.5 ± 1.5 b has been obtained which is similar to the value measured in the yeast SD band. These results indicate that the deformation in the 192Hg SD nucleus is stable despite changes in the occupancy of high-N-orbitals.

Neutron orbitals above the N = 74 shell gap at large deformation: spectroscopy in the superdeformed minimum of 133Ce

Abstract High-spin states in 133 Ce were populated via the 116 Cd( 22 Ne,5n) reaction at 120 MeV. Analysis of these data has revealed three new superdeformed bands in 133 Ce, two of which have transitions which are identical to those observed in the yrast bands of 132 Ce and 136 Nd. These bands have been interpreted as signature partners of the [530] 1 2 − orbital coupled to the 132 Ce yrast superdeformed core. The third band is believed to be built upon a π5 4 ν6 3 configuration at high rotational frequency and to show the effects resulting from an alignment of a pair of f 7 2 neutrons at ℏgω ⋍ 0.7 MeV. It is tentatively suggested that the sharp rise in the dynamic moment of inertia of this band at low rotational frequency may result from an admixture of the π5 4 ν6 1 configuration.