A.J.M. Plompen

Collective and quasiparticle structures in 192Pb

Abstract The structure of 192Pb has been investigated using the 173Yb(24Mg, 5n) reaction at Eb = 132 MeV. The level scheme has been extended up to an excitation energy of 7 MeV and a spin of 23 ħ. Two collective bands of levels linked by M1 transitions and four groups of states of non-collective character have been observed. Based on comparison with experimental data on odd-A Tl isotopes and with cranked shell-model calculations assuming an oblate shape, the collective Ml bands are interpreted as having an underlying proton 2-quasiparticle configuration coupled to a pair of rotation-aligned i 13 2 neutrons. The four groups of non-collective level-sequences are understood as 4-quasiparticle excitations associated with aligned neutron 2-quasiparticle states that involve a mixture of neutron and proton degrees of freedom.

GDR DISSIPATION AND NUCLEAR SHAPE IN HOT FAST-ROTATING DY NUCLEI

Abstract The statistical γ-ray decay of the GDR built on excited states in Dy nuclei has been investigated for selected domains of angular momentum up to about 70ħ and temperatures in the range 1–2 MeV. The GDR strength distribution extracted from the data indicate large average nuclear deformations ( β ∼ 0.35) at high angular momentum and average temperatures T ⩾ 1.5 MeV. The experimental observation is supported by results from calculations in which thermal shape fluctuations are taken into account around an oblate equilibrium deformation β eq . Although this equilibrium deformation increases with angular momentum, the calculations show rather large and constant average deformations 〈 β ∼0.35.

VISCOSITY, FISSION TIME SCALE AND DEFORMATION OF 156DY

In the fusion-fission reaction Ar-40 + Cd-116 --> Dy-156*, fission, at E-b = 216 MeV and 238 MeV, gamma-rays were measured in coincidence with fission fragments. The interpretation of the gamma-ray spectra is done with the help of a modified version of the statistical-model code CASCADE. The spectra can be reproduced with nuclear viscosities in the range 0.01 <gamma <4. The extracted fission time scale is of the order of 10(-19) s. The CASCADE analysis seems to favor a prolate deformation with beta approximate to 0.45

GDR γ-ray decay in 156Dy∗ from regions selected on temperature and angular momentum

The strength distribution of the GDR built on highly excited states in a restricted temperature domain in Dy-156 and Dy-155 nuclei has been deduced by subtraction of gamma-ray spectra obtained for the decay of Dy-154* and Dy-156* from regions selected on angular momentum. The resulting difference spectra have been analyzed within the statistical model. The results show a large deformation (beta similar to 0.51 +/- 0.29 and 0.35 +/- 0.14) for the angular-momentum regions with [J] similar to 32 (

NONCOLLECTIVE DEGREES OF FREEDOM IN PB-192

) over bar h at T approximate to 1.8 +/- 0.2 MeV and [J] similar to 46 (

Viscosity and fission time scale of Dy-156 at high excitation energies and large angular momenta

) over bar h at T approximate to 1.7 +/- 0.2 MeV respectively, in satisfactory agreement with calculations performed in the framework of Landau theory of shape transitions and statistical fluctuations. The deduced centroid energies are in agreement with the systematics of the GDR built on the ground state. The width of the GDR shows a systematic increase with increasing temperature.

Reply to ``Comment on `Lack of evidence for a superdeformed band in^{192}' ''

Abstract Two positive-parity and two negative-parity groups of states of non-collective character with excitation energies ranging from 3 to 7 MeV and spins as high as 21 ħ have been established in 192 Pb. The main features of each group are described phenomenologically as four-quasiparticle configurations, arising from coupling of the 2 + 1 , 4 + 1 and 6 + 1 states to a specific aligned neutron two-quasiparticle configuration. Supporting evidence for this interpretation and the importance of proton excitations in this singly closed-shell nucleus is obtained from the predictions of the quasiparticle multi-step shell model (QMSM). In addition, a four-quasiparticle structure of predominant neutron character is proposed on the basis of comparisons with the experimental data on 194 Pb and with the results of the QMSM calculations.

Lack of evidence for a superdeformed band in^{192}Pb

The reaction Ar-40 + Cd-116 --> Dy-156* --> fission as Studied at beam energies E(b) = 216 MeV and E(b) = 238 MeV, wherein gamma-rays were measured in coincidence with fission fragments. From these spectra the nuclear viscosity gamma and fission time scale were deduced by comparing to statistical model calculations yielding viscosities in the range 0.01 <gamma <4 and fission time scales of the order of 10(-19) s.