M.W. Drigert

Excited superdeformed bands in 191Hg

Abstract Two weakly populated rotational bands have been observed in 191 Hg with properties (energy spacings, moments of inertia and lifetimes) very similar to those of the previously reported superdeformed band. Based on cranked Woods-Saxon calculations, these structures are interpreted as the first excited bands in the superdeformed minimum of 191 Hg. Comparisons between the data and the calculations highlight the role of specific orbitals at large deformations.

Multiple band structures in 191Hg

Abstract The level structure of the nucleus 191Hg has been considerably extended from previous studies by using the 160Gd(36S, 5n) reaction in conjunction with an array of Compton-suppressed germanium detectors. A series of 13 different level sequences has been established in addition to three superdeformed bands. A majority of the band structures can be understood in cranked shell model calculations assuming an oblate collective nuclear shape. There is some evidence for the onset of triaxiality. Finally, two bands of single-particle character have been found. They are interpreted as being associated with a prolate non-collective shape (γ = −120°).

Superdeformed bands in 189,190Hg

Abstract Experiments using the 160 Gd ( 34 S , x n ) reaction at 159 and 162 MeV have revealed two γ-ray cascades which have properties similar to those of the previously reported superdeformed bands in the heavier Hg isotopes. These sequences have been identified as superdeformed structures in 189Hg and 190Hg. For the band in 190Hg, an average quadrupole moment Q0 = 18 ± 3 e · b was obtained from a DSAM measurement. The data are interpreted within the cranked Woods-Saxon formalism and the rise in the dynamical moment of inertia for both bands is linked to the presence of pair correlations and quasiparticle alignments. For the 189–194Hg superdeformed bands, the variations in the observed population intensity and in the rotational frequencies at which decay out of the bands occur are found to be consistent with the calculated change in the well depth of the superdeformed minimum with mass.

Proton excitations in the superdeformed well of 193Tl

Abstract Two superdeformed bands of 13 transitions each have been found in 193 Tl with the 160 Gd( 37 Cl, 4n) reaction. The dynamic moments of inertia for the two bands are found to rise with rotational frequency, as for all observed superdeformed bands in other nuclei in this region. The two bands can be interpreted as signature partners which exhibit some signature splitting for rotational frequencies above 0.2 MeV. The data are interpreted with cranked Woods-Saxon calculations and illustrate the role of the proton i 3 2 (Ω = 5 2 ) intruder orbital.

Feeding and decay of superdeformed states

Abstract The mechanisms for feeding and decay of superdeformed (SD) bands are examined. Data connected with both processes in 192 Hg are compared with model calculations. The calculations successfully reproduce the data, suggesting that the mechanisms for both processes are understood. Constraints on the energy of the SD band energies and on the well-depths at low and high spins have been obtained. At the point of decay around spin 10, we suggest that the SD band 1s 3.3–4.3 MeV above the normal yrast line and that the well depths at spin 10 and 40 are 0.5–1.3 and 3.5–4.5 MeV, respectively.

Shape-driving effects in 193Tl from the spectroscopy of yrast and near-yrast states

Abstract The nucleus 193Tl has been populated with the 160Gd(37Cl,4n) reaction and its level scheme has been extended up to spin 41 2 h by using an array of 12 Compton-suppressed Ge detectors. Near-oblate band structures arising from a 9 − 2 state and markedly different, single-particle-like, structures above a 13 + 2 state have been established. The two structures are consistent with total routhian surface calculations indicating the existence of two minima in the total routhian surface of the nucleus which survive separately up to the observed spins. Details of the rotational-like part are discussed in terms of the cranked shell model, while the single-particle-like features of the nucleus are compared with non-collective structures in neighboring nuclei, in particular 191Hg.

Nucleon alignment in 191Hg. A competing mechanism at moderate spins

Abstract Detailed spectroscopic studies of the discrete γ -rays feeding and deexciting a 41 2 − , 4.6 MeV level in 191 Hg are reported. The resulting decay scheme indicates single-particle nature for the states above the 41 2 − level. At moderate spin, the corresponding particle alignment mode competes favorably with collective oblate rotation. It is suggested that this sequence is associated with a non-collective prolate shape ( ϵ =0.1–0.15, γ ≅−120°).

Superdeformation in the mercury nuclei

We shall first summarize the present experimental situation concerning {sup 192}Hg, the nucleus regarded as the analog of {sup 152}Dy for this superdeformation (SD) region in that gaps are calculated to occur at large deformation for Z = 80 and N = 112. Proton and neutron excitations out of the {sup 192}Hg core will then be reviewed with particular emphasis on {sup 191}Hg and {sup 193}Tl. The presentation will conclude with a brief discussion on limits of the SD region for neutron deficient Hg nuclei. 26 refs., 10 figs.

Search for entrance channel dependence in the population of superdeformed bands in 191Hg

The population intensity of some SD bands in the mass 150 region were observed to depend on the mass symmetry of the entrance channel in the fusion reaction. The authors raised the possibility that the population of SD bands had a memory of the entrance channel. To check this interesting possibility, we made measurements of the population intensities of superdeformed (SD) bands in the {sup 160}Gd({sup 36}S,5n){sup 191}Hg and {sup 130}Te({sup 64}Ni,3n){sup 191}Hg reactions. To ensure that any observed effect was not due to a simple angular momentum difference in the entrance channels, we also measured the average entry points and spin distributions of normal and SD states in {sup 191}Hg in the two reactions. The entry points and spin distributions for {sup 191}Hg are the same and, indeed, so are the SD intensities in the two reactions. Hence, no entrance-channel effect is observed in the population of the SD band in {sup 191}Hg, in contrast with data for SD bands in the mass 150 regions. We suggest that the effect observed previously in the mass 150 region is due to an angular momentum effect. A letter reporting our results was submitted for publication.

Characterization of the superdeformed band in189Hg

Much improved data on the superdeformed band in189Hg have been obtained in the study of the160Gd(34S, 5n) reaction at 165 MeV. Energies of ten intraband transitions have now been determined and the decay into the yrast line has been delineated. A discussion of probable spins for the189Hg superdeformed band raises questions about proposed procedures for assigning spins to superdeformed states.