H. Hübel

Single step links of the superdeformed band in Pb-194 : A measure of the absolute excitation energy, spin and parity of the superdeformed states

Abstract The EUROGAM array has been used to investigate the decay out of the yrast superdeformed (SD) band in 194 Pb. Six single step decays from the lowest observed SD states to low-lying states at normal deformation (ND) have been identified. From this observation, the excitation energy of the SD band in 194 Pb is established at 4877 ± 1.5 keV for the 6 + SD state. The most probable spins and parities of all members of the SD band are also deduced assuming that the SD states have even spin and positive parity.

Triaxial Superdeformed Bands in

Abstract In a search for exotic structures in odd-odd 164 Lu, performed as one of the first Euroball experiments eight new, presumably triaxial, superdeformed bands were found. For the first time, evidence is presented for superdeformation in an odd-odd Lu isotope for which theory predicts large triaxiality. The data are compared to expectations from calculations with the code “Ultimate Cranker”. Two of the bands are connected to normal-deformed structures and the E1 strength of the decay-out appears to be enhanced over single particle expectations, possibly due to octupole correlations.

^164

Abstract High-spin states in 163 Lu have been investigated using the Euroball spectrometer array. The previously known superdeformed band has been extended at low and high energies, and its connection to the normal-deformed states has been established. From its decay the mixing amplitude and interaction strength between superdeformed and normal states are derived. In addition, a new band with a similar dynamic moment of inertia has been found. The experimental results are compared to cranking calculations which suggest that the superdeformed bands in this mass region correspond to shapes with a pronounced triaxiality ( γ ≈±20°).

Lu and Enhanced E1 Decay-Out Strength

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.

Triaxial superdeformation in 163 Lu

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.

Lifetimes of the superdeformed band in 192Hg

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.

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

Abstract High-spin states in 199Pb were populated in the 186W(18O, 5n) reaction at a beam energy of 94 MeV. The analysis of the γ-ray coincidences revealed a new oblate rotational band of strong dipole transitions which shows a pronounced backbending. The connection between this band and the normal spherical states is established. Thus, spin and excitation energy of an oblate collective band in the Pb isotopes are determined for the first time. The backbending is interpreted as a crossing of bands with different numbers of aligned i 13 2 neutrons.

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

Abstract The superdeformed nucleus 193 Pb has been studied following a heavy-ion fusion-evaporation reaction with the Gammasphere spectrometer array. Three new superdeformed bands, two of them probably signature partners, have been observed, bringing the total number of superdeformed bands known in this nucleus to nine. An estimate of the g -factor associated with one of the previously known pairs of strongly coupled excited bands has been made, confirming their configuration. The properties of the bands are compared with the results of cranked shell model calculations; good agreement is found and configurations are assigned to each band. The two new signature partner bands are suggested to be based on the two signatures of the [512]5/2 neutron orbital. The third new band is probably based on the 7 3 intruder orbital. In addition, the high-energy region of the spectrum in coincidence with the yrast superdeformed bands has been carefully studied. The current data provide no evidence for the previously reported transitions linking the superdeformed and normal-deformed states with energies around 2.2 – 2.4 MeV.

First observation of a crossing of oblate dipole bands in the A=200 region

Abstract Regular bands of enhanced magnetic dipole transitions have been observed in nearly spherical nuclei in the Pb region. The recent experimental results for the Pb isotopes are reviewed. The bands are formed by rotation of a large magnetic dipole around the nuclear spin. Comparison between experimental results and tilted-axis cranking calculations shows good agreement.

Superdeformation in

The nuclei 198-200Bi were populated via the 186W(19F,xn)198-200Bi reaction at beam energies of 115 MeV and 105 MeV. Another experiment, aimed at investigating the high-spin-level structure of 203,204Bi, used the 198Pt(11B,xn) reaction at a beam energy of 74 MeV. Five new Delta I=1 rotational structures, consisting of stretched magnetic dipole transitions, have been observed. One of these bands is assigned to 198Bi, one to 199Bi, two to 200Bi, and one to 203Bi. The behaviour of the dynamic moments of inertia of these oblate bands is compared with other bands in neighbouring Pb and Bi nuclei.