M. Bergström

More than thirty bands in 177Re

Abstract The level scheme of 177Re has been extended extensively, with fragments of more than thirty bands, characterised by a given signature, observed. Alignments at low frequency in one-quasiparticle bands are interpreted as deformation effects and compared with the predictions of cranking calculations. The previously assigned one-quasiparticle π i 13 2 band is shown to be a mixture of three-quasiparticle and vibrational character. The non-observation of the one-quasiparticle π i 13 2 band is explained as being due to crossings of its s-band by competing bands. A number of high-K bands are also reported, including five- and seven-quasiparticle bands. Configuration assignments for these bands are aided by comparing experimental gκ values with “effective” gκ values, calculated with a semi-classical expression taking Coriolis mixing into account. The population of the yrast band was observed to oscillate with every four units of spin.

On the use of spin adiabatic energy surfaces: Applications to the description of rotational bands in 171,172W

Abstract The use of frequency diabatic quasiparticle energy spectra for constructing diabatic and spin adiabatic energy surfaces is described and the relation between such energy surfaces and the more familiar total routhian surfaces is discussed. Applications are made to the tungsten isotopes 171 W and 172 W. It is shown that relatively small deformation changes can have a significant effect on the energy spectrum. Considering such deformation changes leads to a much improved agreement with the experimental data for both of the investigated isotopes.

Interpretation of bands in 163Er within the tilted rotation scheme: Enhanced E1 decay strength

Abstract High-spin data are presented for levels in 163Er and in particular the structure of three high-K rotational bands are discussed and interpreted within the tilted rotation scheme. Large observed B(E1) values are interpreted in terms of coupling to octupole degrees of freedom.

FourtyTwo - a two-dimensional analysis program for γ-γ coincidence matrices

Abstract A program for two-dimensional analysis of γ-γ coincidence matrices is described. The program is tailored to the Digital Equipment VT340 colour graphic terminal and written in VAX Fortran. The program provides viewing of the matrix as well as projections in both energy coordinates, determination of centroids and volumes of peaks, full two-dimensional Gauss fits to a maximum of five peaks in an arbitrarily shaped area of the matrix, etc.

Probing the limits of complete spectroscopy in 164Yb

Abstract High spin states of 164Yb have been populated using the 152Sm(16O,4n) reaction. Analysis has been made using γγ coincidences and, for the first time in a large detector array, angular correlations. The experimental level scheme has been extended to include a total of 15 decay cascades, for some of which the spins have been measured. Extensive cranked shell model calculations have been performed to map the quasiparticle rotational bands in a window, reaching up to 1 MeV above the yrast line, in the spin-energy plane. The experimental data are compared with the calculated bands, and most of the experimental bands are interpreted in terms of quasiparticle excitations. It is also found that the number of experimentally detected bands is in good agreement with the calculated number of bands in the observed spin-energy region.

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.

INTERPRETATION OF BANDS IN ER-163 WITHIN THE TILTED ROTATION SCHEME

Abstract High-spin data are presented for levels in 163 Er and their structure is interpreted within the tilted rotation scheme.

High-spin studies of 164 Lu using EUROBALL

SummaryA new region of superdeformed shapes with a pronounced triaxiality coexisting with normal deformation exists for nuclei withN ~ 94 andZ ~ 71. Calculations with the Ultimate Cranker have revealed that triaxial SD minima with γ ~ ±20° are expected for all symmetry groups (π, α). In an odd-odd nucleus, the possibility exists to exploit the various combinations of proton and neutron orbitals which are expected to sample the triaxial minima differently. A search for such structures was performed as one of the first Euroball experiments using the reaction139La(29Si,4n) with thin self-supporting targets at a beam energy of 145 MeV, provided by the Legnaro XTU tandem accelerator. The preliminary analysis has provided a major extension of the known (normal deformed) level scheme in164Lu, in addition to ~7 new, presumably triaxial, SD bands in164Lu. These bands have dynamical moments of inertia similar to those found in163,165Lu. The new data are compared to expectations from calculations with the code “Ultimate Cranker”.