Isomeric states in180Os

Two new isomers have been observed in¹⁸⁰Os. A high-K isomer withI, K ≧20 and a half-life ofT _1/2=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 ofT _1/2=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.     

Properties of side bands observed in180Os

Bands in ¹⁸⁰ Os have been studied with in-beam spectroscopy methods. Four side bands were established. Configurations have been assigned to them considering the features of similar bands in neighbouring nuclei. New aspects result from an interpretation in the framework of the cranked shell model.     

Triaxial superdeformation in Lu

High-spin states in ¹⁶³Lu 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°).     

Triaxial superdeformation in 163Lu

High-spin states in ¹⁶³Lu 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°).     

First evidence for triaxial superdeformation in 161Lu and 162Lu

High-spin states in ¹⁶¹Lu and ¹⁶²Lu have been investigated using the GASP γ-ray spectrometer array. Excited states in these nuclei have been populated through the ¹⁰⁰Mo(⁶⁵Cu, xn) reaction at a beam energy of 260 MeV. Four presumably triaxial superdeformed bands, three in ¹⁶²Lu and one in ¹⁶¹Lu, have been observed. This is the first evidence for triaxial superdeformation in the two isotopes. Received: 18 October 2002 / Accepted: 21 November 2002 / Published online: 4 February 2003 RID="a" ID="a"e-mail: bringel@iskp.uni-bonn.de RID="b" ID="b"Present address: Wright Nuclear Structure Laboratory, Yale University, New Haven, CT 06520, USA. RID="c" ID="c"Present address: Physik Department, Technische Universit?t München, James Franck Str., D-85748 Garching, Germany. Communicated by D. Schwalm     

Possible existence of triaxial superdeformation in 172,174,176W

Two-dimensional total routhian surface (TRS) calculations are carried out to determine the triaxial superdeformation (TSD) of the even-even nucleus 174W, and the result indicates that TSD state exists with deformation parameters ε2=0.42 and γ=34.7°. In the same way, the total routhian surfaces for the nuclei 172,176W are also calculated. It shows that the neutron shell correction energy plays a key role in the formation of TSD nuclei 172,174,176W, while the high j intruder orbitals and rotational energy are also crucial in the formation mechanism.     

Clustering aspects of nuclei with octupole and superdeformation

Extreme nuclear shapes are considered from the clustering point of view. General aspects of light nuclei which possess very exotic, elongated shapes (“α-strings”) are given. The relation between superdeformation and di-molecules is considered, and it is shown that nuclear di-molecules have the same magic numbers and orbits filled as the ellipsoidal superdeformations for light nuclei, while for heavier nuclei they are expected in different mass-regions. The structure of possible octupole-unstable 3∶1 nuclear shapes (“hyperdeformations”) is discussed. Indications and consequences of a four-fold symmetry of the nuclear hamiltonian are briefly discussed.     

Possible existence of triaxial superdeformation in ^172,174,176W

Two-dimensional total routhian surface （TRS） calculations are carried out to determine the triaxial superdeformation （TSD） of the even-even nucleus 174W, and the result indicates that TSD state exists with deformation parameters ε2=0.42 andγ=34.7°. In the same way, the total routhian surfaces for the nuclei ^172,176W are also calculated. It shows that the neutron shell correction energy plays a key role in the formation of TSD nuclei ^172,174,176W, while the high j intruder orbitals and rotational energy are also crucial in the formation mechanism.     

Search for solar-neutrino related M1 transitions in 6Li using 180° electron scattering

The ⁶Li analog of the 11.5 MeV, 0⁺ state in ⁶Be proposed to explain the low flux of high-energy solar neutrinos has been searched for and not observed. An upper limit of 3 eV is placed on the M1 ground-state width of any such narrow 0⁺ state.     

The rotational quasi-continuum in specific configurations: high-K states and superdeformation

The analysis of the fluctuations of the counts of unresolved γ-γ energy spectra can extend our knowledge of the rotational motion at high angular momentum and excitation energy above the yrast band. Detailed studies of the rotational quasi-continuum built on specific intrinsic nuclear configurations are here discussed. First, the validity of the K-selection rules in the quasi-continuum region is investigated in the case of the normal deformed nucleus ¹⁶³Er. The γ cascades feeding into low-K and high-K bands are analyzed studying variance and covariance of the spectrum fluctuations. Low-K bands are found to be fed by a much larger effective number of cascades than high-K bands, while the covariance between pairs of gated spectra shows that the cascades feeding into low-K bands are different from those feeding into high-K bands. These findings suggest a persistence of K-selection rules in the region of excitation energy and angular momentum probed by the rotational decay of ¹⁶³Er. As a second case, the existence of a rotational quasi-continuum in the super deformed well of ¹⁴³Eu is discussed. An intense bump of rotationally correlated transitions is observed to develop as function of fold in the high spin region of gated spectra. Fold distribution, angular anisotropies, moment of inertia and life time show that the bump consists of fully damped super deformed transitions. This gives experimental evidence of damped rotational motion up to several MeV excitation energy above the super deformed yrast line.     

Search for the (n th, α) reaction on180Hf,181Ta and209Bi

The (n _th,α) reaction on¹⁸⁰Hf,¹⁸¹Ta and²⁰⁹Bi was studied at the thermal neutron guide of the Grenoble high flux reactor. Upper limits ofσ _α ≦13μb, ≦μb and ≦0.3μb, respectively, were obtained for the a branches to the ground states in contrast to the values of ≈100μb to ≈500 mb reported in the literature. Possible reasons for these discrepancies are discussed. Furthermore, the present upper limits of the cross sections are compared with the values calculated from the statistical model.