Observation of strongly deformed shapes in 154-152Dy nuclei at medium temperatures

The gamma-decay of the giant dipole resonance (GDR) built on excited states of 154-152Dy nuclei is studied. The selection of GDR decay from high spin states leading to specific exit channels was made possible by triggering on high spin isomers. The deduced energy splitting of the GDR implies large deformations (\beta\ congruent-to 0.4-0.5). The resonance widths of the components are comparable to those of the GDR built on the ground state indicating small shape fluctuations

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

Isomer-selected decay of the GDR in 156Dy

The strength distribution of high-energy gamma-rays emitted in the statistical decay of the 156Dy compound nucleus, with E* = 92.5 MeV, has been measured inclusively and for selected decay pathways in which isomers in 149Dy, 151Dy and 152Dy were populated. For both spectra a deformation \beta\ = 0.3 is deduced and the centroid energies agree with those found from ground-state systematics. The high-energy gamma-ray spectrum triggered on isomeric decay shows an increase of the width of the GDR in comparison with the inclusive spectrum

STUDY OF HIGH-SPIN STATES IN THE NUCLEUS EU-149

In-beam studies of high-spin states in Eu-149 are reported. The level scheme extends up to an excitation energy of 7.1 MeV and a spin of 55/2HBAR. This nucleus is weakly deformed and most of the high-spin structure is interpreted through its multi-particle-hole nature. Octupole-phonon vibrations coupled to multi-particle-hole states are observed and discussed. In contrast with the Z less-than-or-equal-to 62 isotones, there is no conclusive evidence in Eu-149 for octupole deformation induced by rotation

ANGULAR-MOMENTUM AND TEMPERATURE-DEPENDENCE OF THE GDR IN DY NUCLEI

The statistical gamma-ray decay of the GDR built on excited states in Dy-151-156 has been investigated as a function of angular momentum and temperature. The selection on angular momentum has been made by using the total gamma-ray energy and gamma-ray multiplicity. The temperature dependence in the range T almost-equal-to 1.0 MeV to T almost-equal-to 2.0 MeV has been measured by taking data for two reactions, i.e. Cd-114(40Ar,xn-gamma)Dy-154-x* and Cd-116(40Ar,xn-gamma)Dy-156-x* at beam energies of 180 MeV and 200 MeV, respectively, populating the same final nuclei. A systematic shift of the strength to lower energies has been observed at increasing angular momentum, which can be attributed to a change of the nuclear shape from prolate to oblate