K.B. Beard

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°).

A superdeformed band in 151Dy

Abstract A rotational band of 19 (possibly 20) transitions extending to spin ∼ 131 2 h has been observed in 151Dy with an average dynamic moment of inertia I (2) = 79 h 2 MeV −1 . This band is identified as a superdeformed band in 151Dy. The value of I (2) agrees with cranked Strutinsky calculations. Similarities as well as striking differences with the superdeformed bands of neighboring nuclei are observed.

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.

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.

Population of superdeformed bands, the competition with fission, and the barrier between normal and superdeformed states

In this paper we report on the entry points leading to superdeformed (SD) as well as normal bands. We find that, compared to normal bands, the entry spins for the SD bands are about 9 {h bar} higher, and the entry excitation energy 1--3 MeV colder. We also conclude that population of the SD bands represents successful competition against fission. SD bands in both the Dy and Hg regions are considered.

Shape and Structural Changes in Nuclei at High Spin and Temperature

Structural changes both along and above the yrast line have been observed in experiments performed with the Argonne-Notre Dame BGO Gamma Facility at ATLAS. Transitions from prolate to oblate shapes have been detected in transitional Dy nuclei. Above the yrast line, quasicontinuum E2 transitions reveal clear signatures of structural changes which may be related to phase transitions predicted by theory. Superdeformed hands have been discovered in some of these nuclei, as well as in a new region of superdeformation in 191Hg. The initial population of the superdeformed bands appears to be close to the secondary minimum and may explain why the SD bands, which lie several MeV above the yrast line, do not rapidly tunnel to the normal states.

Observation of superdeformation in Hg 191

The first observation of superdeformation in the A {approx equal} 190 mass region is reported. A rotational band of 12 transitions with an average energy spacing of 37 keV, an average moment of inertia of 110 {Dirac h}{sup 2} MeV{sup {minus}1}, and an average quadrupole moment of 18 {plus minus} 3 eb has been observed in {sup 191}Hg. These results are in excellent agreement with a calculation that predicts an ellipsoidal axis ratio of 1.65:1 for the superdeformed shape in this nucleus. Evidence for another discrete superdeformed band and superdeformed structures in the quasi-continuum was also found in the data. 19 refs., 6 figs.