Ts. Venkova

Transition strengths B(E2) in the yrast bands of doubly even nuclei

Abstract Experimental data on absolute E 2 transition strengths in the yrast bands of even-even nuclei are summarized in a table. In diagrams, the moments of inertia J ( I → I − 2) and the E 2 transition moments Q 0 ( I → I − 2) derived for each nucleus are presented versus ( khω ) 2 (ω is the rotational frequency). Data available up to May 1980 have been included.

Evidence for deformation in 113-116Cd isotopes

Abstract: High-spin states in 113-116Cd have been investigated with the fusion-fission reaction 28Si +176Yb at 145 > MeV. The experiment has been performed with the Eurogam2 array. New rotational bands based on two quasi-particle states have been observed for even-even cadmium isotopes. A new level scheme based on the 11/2− isomeric state is proposed for 115Cd and the one of 113Cd has been extended to spin (31/2−). The decoupled bands identified in both odd-A nuclei are interpreted as being built upon a low-Ωh11/2 quasi-neutron configuration. Microscopic Hartree-Fock + BCS calculations confirm the prolate deformation in this mass region especially for the odd cadmium isotopes.

From highly to superdeformed shapes: study of 143Gd

Abstract A superdeformed band has been discovered in 143Gd consisting of 15 transitions. It does not show the band crossing observed in the neighbouring heavier Gd isotopes and it is degenerate with the superdeformed band in 143Eu. In contrast to other degenerate bands at superdeformed shape, the configurations of the bands are quite different here. They result from the exchange of a ν64 with a π[404]9/2 configuration. In addition, a collective band has been observed which adopts a well-deformed triaxial shape at high spins according to calculations. The transition from highly to superdeformed shapes proceeds via triaxial shapes.

Shape changes in 189 Au at high spins

The level scheme of 189Au has been extended to high spins. Decoupled, strongly coupled as well as vibrational bands have been found indicating shape coexistence in this nucleus.

Observation of a (ν72−[514])2 crossing in 180Os

Abstract High-spin states in 180Os have been populated by means of the 150Nd(36S,6n) reaction at 177 MeV and studied with the γ-spectrometers OSIRIS and GASP. Eight known bands have been extended to higher spins and a new strongly coupled band has been discovered. Configuration assignments of the bands have been carried out in the framework of the tilted-axis cranking model utilizing experimental ratios of reduced transition probabilities B( M1 ) B( E2 ) . The investigation of the alignment behaviour of the bands revealed that besides the well-known first band crossing caused by the alignment of an i 13 2 quasineutron pair, a second band crossing occurs in four of the bands. The crossing frequency is ⩾ 0.38 MeV and the alignment gain is fairly small, ranging between 2 and ⩾4 ℏ;. The crossing can consistently be explained as a two-quasineutron (ν 7 2 − [514]) 2 alignment. This crossing has been observed for the first time.

Isomeric states in180Os

Two new isomers have been observed in180Os. A high-K isomer withI, K ≧20 and a half-life ofT1/2=12+4 ns have been established. It deexcites via two transitions into the 18+ level of the yrare band indicating an unusually smallK-hindrance factor. Evidence for an isomer withI, K>16 and a half-life ofT1/2=41±10 ns was found. A half-life of 17±3 ns was measured for the previously known 7− state at 1862 keV. The decay scheme of the previously known 7− isomer at 1928 keV has been extended and a revised version is presented.

Systematic behavior ofB(E2) values in the yrast bands of doubly even nuclei

The experimental information onB(E2) transition rates in the yrast bands of doubly even nuclei (126≦A≦184) is systematized. The strength functionSexp≡B(E2,I→I−2)×E(I→I−2) is found to reveal characteristic behavior significant for structure studies of yrast bands. The energy-weightedB(E2,I→I−2) values (Sexp) and 2ℐ/ℏ2(ℐ: moment of inertia) are plotted versus the rotational frequency squared ℏ2ω2 for each nucleus. In strongly deformed nuclei (N≧90), theSexp curves smoothly increase for low rotational frequencies suggesting that up to spin valuesI≈8 the ratioQ02ℐ is nearly constant (Q0: quadrupole moment). This is not the case in nuclei with a soft core (N≦88). In the relevant discussion, the hydrodynamical model as well as the CAP effect are considered. The results in the backbending region are qualitatively discussed in terms of the two-band crossing model. Evidence is found supporting the prediction of an oscillating behavior of the yrast-yrare interaction.

Lifetimes in E2 bands of 142Gd measured with DSAM at EUROBALL

Lifetimes have been measured for four quadrupole bands in 142Gd with EUROBALL using the Doppler-shift attenuation method. The deduced B(E2) values have been compared with results of Interacting Boson Model calculations.

High-spin structures of five N=82 isotones: 136Xe, 137Cs, 138Ba, 139La, and 140Ce

Five N=82 isotones have been produced in two fusion-fission reactions and their gamma-rays studied with the Euroball array. The high-spin states of 139La have been identified for the first time, while the high-spin yrast and near-to-yrast structures of the four others have been greatly extended. From angular correlation analysis,spin values have been assigned to some states of 136Xe and 137Cs. Several cascades involving gamma-rays of 139La have been found to be delayed, they deexcite an isomeric state with T1/2= 315(35) ns located at 1800-keV excitation energy. The excited states of these five N=82 isotones are expected to be due to various proton excitations involving the three high-j subshells located above the Z=50 shell closure. This is confirmed by the results of shell-model calculations performed in this work. In addition, high-spin states corresponding to the excitation of the neutron core have been unambiguously identified in 136Xe, 137Cs, and 138Ba.