W. Korten

Level spin and moments of inertia in superdeformed nuclei near A = 194

Abstract Experimental transition energies in the superdeformed (SD) bands near A = 190 are least-squares fit to rotational model formulae in order to extract level spin. The data set includes 16 SD bands, which show no evidence of either irregular behavior near the bottom of the bands or abrupt angular momentum alignment at low ħw . The 9 transitions lowest in energy in each band are well described by the formulae. The fitted spin of the final state in the γ-ray cascade is within ±0.1 ħ of an integer or half-odd integer for 13 of the bands. The weight of the evidence suggests that meaningful level spins corresponding to these transitions can be inferred. The moment of inertia at ω = 0 is correlated, with J = 88.3(5) ħ 2 / MeV and J = 93.4(5) ħ 2 / MeV for the strong and weak bands, respectively.

Oblate collectivity in sup 197 Pb

Evidence for collective behavior in the high-spin region of lead nuclei is provided by the observation of two collective structures in {sup 197}Pb. One of these bands is interpreted as an oblate collective structure built on oblate proton and possibly neutron states. A second more irregular band is suggested to be most likely a triaxial rotational band. Neutron and proton configurations for these bands are suggested from the results of quasiparticle Routhians and total Routhians surface calculations. A partial level scheme up to {ital I}{approx}69/2 and {ital E}{sub {ital x}}{approx}8 MeV is presented.

Superdeformed bands in195Tl

Two superdeformed (SD) bands have been found and assigned to195Tl on the basis of excitation function and cross bombardment results. The two bands are almost identical in transition energies to those observed in193Tl. They are signature partners with a splitting, presumably due to the proton i13/2 (Ω=5/2) orbital, characteristic of all known SD bands in the thallium isotopes. Their alignments relative to the193Tl bands were found to be zero.

LIFETIMES OF SUPERDEFORMED STATES IN 194PB

Lifetimes of members of the superdeformed band in 194Pb were measured by the Doppler-shift attenuation method. Quadrupole moments around 20 eb that are constant over the whole frequency range were derived. The results rule out large centrifugal stretching effects.

Oblate L=1 bands in 194,196–201Pb and 193Hg

Abstract Reports of recent experiments have included observations of regular and irregular bands in neutron deficient Pb isotopes with A =194, 196–201. The bands are populated strongly in (HI, xn ) reactions. The shared characteristics of the bands include: 1) bandhead energies of a few MeV; 2) high bandhead spin; 3) large alignments; 4) small dynamic moments of inertia, and 5) strong L =1 transitions and weaker L =2 crossover transitions, with B( M 1) B( E 2) ≈ 20 μ 2 e 2 b 2 . Lifetimes of band members in the 198 Pb regular band are B (M1) ≈ 1 W.u. and B (E2) ≈ 12 W.u. (with large errors). These observations are consistent with an interpretation of the regular structures as collective oblate bands with both proton and neutron excitations involved; the closed proton shell at Z =82 is broken, and coupled to v( i 13 2 ) −n excitations. The irregular structures may correspond to triaxial shapes, with similar orbits involved. A similar structure has been also found in 193 Hg.

Superdeformation in lead nuclei

A rotational band, with energy spacings characteristic of superdeformed shapes, has been identified in194Pb.The band, which consists of 12 members, was produced in the bombardment of176Yb with24Mg. This band extends the mass-194 region of superdeformation to higher Z.

Oblate collectivity in 197,198Pb

Abstract Evidence for collective behavior in the high-spin region of neutron deficient lead nuclei is provided by the observation of four collective structures in 197,198 Pb. These bands consist of strong dipole (M1) transitions with a few E2 crossover transitions observed. The transition energies of three of these bands show a rather regular behavior while those of one of them show an irregular behavior. We interpret the regular bands as oblate collective bands built on oblate proton and neutron states, whereas the irregular band might either be built on a state with very small oblate deformation, or be a triaxial rotational band. A lifetime measurement (DSAM) has been done for the regular bands in 198 Pb. Neutron and proton configurations for the bands are suggested from the

Very elongated nuclei near A = 194

Abstract A γ-ray cascade in 191Hg of 12 members with average energy spacing 37 keV and Qt= 18(3)eb was reported by Moore and co-workers in 1989. This was the first report of very elongated nuclei (superdeformation) in this mass region. Since then, some 25 γ-ray cascades have been observed in 11 (slightly neutron deficient) Hg, Pb and Tl nuclei. Our collaboration has investigated 7 of these nuclei. The bands have similar dynamic moments-of-inertia. Some nuclei exhibit multiple bands, and the backbending phenomenon has been observed. Level spins can be obtained from comparison of transition energies to rotational model formulas. Selected bands (in different nuclei) have equal transition energies (within 0.1%). Alignment in integer multiples of has been observed. Properties of these bands will be described.

‘‘Identical’’ superdeformed bands in the mass A=190 region

Several new rotational bands with energy spacings typical for superdeformed shapes have been identified in Tl and Pb nuclei. Their γ‐ray transition energies are often identical or closely related to those of 192Hg or 193Tl. The resulting dynamic moments of inertia are much more similar than experienced in any other case. Spins, although not directly measured in any superdeformed band, can be assigned with the help of a rigid‐rotor expansion of the γ‐ray energies. The alignments obtained as spin differences with respect to the reference band are quantized for rotational frequencies above 0.2 MeV.

Thallium Isotopes: A Set of “Identical” Superdeformed Bands

A total of eight superdeformed (SD) bands have been identified in two thallium isotopes (A = 194–195). Striking similarities were found between the transition energies of these bands and transition energies of the two SD bands in 193T1. This implies similarities between the dynamical moment of inertia of all the known SD bands in the thallium isotopes. The identical moment of inertia can in turn be interpreted as a constant alignment which was found, in all these cases, to be surprisingly close to 0h or lh.