C. Schück

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.

Superdeformed bands in 150Gd and 151Tb: Evidence for the influence of high-N intruder states at large deformations

Abstract Rotational bands, characteristic of a superdeformed prolate shape ( e ≈0.6) and extending to above spin 60 h , have been observed in both 150 Gd and 151 Tb. The magnitudes of the moments of inertia I (2) were found to vary with frequency and the variation greatly exceeded that seen in 148,149 Gd and 151,152 Dy. The differences in the I (2) s are attributed to the occupation of particular high- N orbitals. Moreover, contrary to the previous examples the bands in both 150 Gd and 151 Tb de-excited at a much higher rotational frequency of h ω≈0.4 MeV and this may indicate that the pair gap extends to higher frequencies in 150 Gd and 151 Tb.

High-spin excitations of 187, 188Hg

Abstract High-spin states in 187, 188 Hg have been investigated via the 163, 164 Dy ( 28 Si , x n ) reactions by in-beam γ-ray spectroscopic techniques using the “Chateau de Cristal” 4π-multidetector array. Level schemes have been established. Spins and parities of the states have been deduced from transition multipolarities obtained from measurements of the angular distribution and linear polarization of the γ-rays. The shape coexistence of both prolate and oblate band structures has been observed for the first time in 187 Hg. In 188 Hg negative-parity bands are well developed. The band structure is interpreted within the frame of the Strutinsky method with the deformed Woods-Saxon average field. Pairing behaviour with increasing spin and general features of shape coexistence in mercury nuclei are illustrated.

The recoil shadow anisotropy method

Abstract The Recoil Shadow Anisotropy Method (RSAM) is a new experimental method for identifying isomers in the nanosecond range and measuring their half-lives. This method can be applied to experiments performed with thin targets and γ-ray multidetector arrays including collimated composite detectors and does not require any additional device. It uses the shadow effect imposed by the collimators on the different elements of composite detectors for γ-rays emitted by recoiling nuclei. RSAM was developed for the clover detectors of the Eurogam-2 array and tested using several data sets obtained with this array. A number of known isomers with half-lives lying between 0.9 and 18 ns in 194Hg, 191Au, 148Gd, 149Gd, 193Pb and 194Pb have been successfully re-measured, proving the ability of RSAM for lifetime measurements.

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-K oblate structures in 194Pb

Two regular dipole gamma -ray cascades have been observed in 194Pb. A comprehensive level scheme related to these structures has been built and first members of rotational bands built on K=8 and K=11 two-proton oblate states have been identified. From the excitation energies, spin values, behaviours of the moments of inertia and alignments, and from the comparison with the isotone 192Hg, multiparticle configurations can be attributed to three high-spin structures of 194Pb built respectively on the 4966 keV, 4334 keV and 6198 keV levels: ( pi 9/2(505) pi 13/2(606))(vi13/2)2, ( pi 9/2(505) pi 13/2(606))(vf5/2vi13/2), ( pi 9/2(505) pi 13/2(606)) (vf5/2(vi13/2)3). Within this interpretation, the regular dipole bands correspond to the presence of two or three i13/2 decoupled neutrons.

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.