Annual Report 2012 - Latvijas Universitātes Cietvielu fizikas institūts

France
1. Institute Laue-Langevin, Grenoble, France ( Dr. W. Urban, Dr. M. Jentchel).
Canada
1. Memorial University of Newfoundland, Newfoundland (Dr.A.Aleksejevs)
2. Department of Physics, Acadia University, Wolfville, NS (Dr.S.Barkanova)
Czech Republik
1. Nuclear Research Institute, Ŕež (Dr. J.Honzatko, Dr. I.Tomandl).
Denmark
Riso National Laboratory, Roskilde, (Dr. S. Nielsen)
Main results
STUDY OF EFFECTS DUE TO NUCLEAR SHAPE CHANGE
IN A~190 REGION NUCLEI
T.Krasta, M.Balodis, J.Bērzinš, Ļ.Simonova, V. Bondarenko
In the nuclear mass number A~190 region, one observes transition from the strong
axially-symmetric prolate shape, characteristic to nuclei in the middle of deformation
region, to the spherical shape at Z=82 and N=126 particle shell closures. Weakening of
nuclear deformation results in the γ instability of nuclear shape, which notably affects
nuclear properties, especially its low-energy decay scheme. In the case of non-axial
deformation, intrinsic nuclear excitations lose their reflection symmetry along the
nuclear core symmetry axis, and valence particle orbital momentum projection Ω, just
like the total nuclear momentum projection K, are not good quantum numbers any more.
As a result: a) for each nuclear spin I value, one has 2*I+1 states with projections K
ranging from -I, -I+1,..., resulting in a considerably higher level density even at
relatively low energy values; b) increased fragmentation of valence particle basis states
manifests itself in the enhancement of single-particle transitions in comparison with
collective intra-band transitions; c) one observes “inverted bands” - level sequences
with different internal structure but connected with intense cascade γ-transitions.
Because of extreme level density, the development of nuclear level schemes in the
A~190 region requires high precision data about γ-transition energies and intensities.
Such data have been obtained via measurements of single and coincidence γ-spectra
following thermal neutron capture reaction with enriched targets in the high-flux reactor
of ILL (Grenoble, France). The single γ-spectra of odd-odd nuclei 186,188 Re have been
measured in the energy range from 100 to 2000 keV employing the high precision
crystal-diffraction spectrometer GAMS5. The energy and angular γγ-coincidence
measurements for 188 Re in the energy range from 100 to 1200 keV have been performed
in ILL at the PF1b polarized cold neutron guide using the multi-detector experimental
set-up. These data, together with the results of our earlier high-low and low-low energy
γγ-coincidence measurements [1] performed in Řež (Czech Republic), allowed to
develop the level scheme of 188 Re nucleus up to 1.5 MeV energy. This level scheme
includes more than 190 levels. The development of the level scheme for the neighboring
odd-odd nucleus 186 Re is in progress.
The proposed model-independent 188 Re level scheme is strongly supported by γγcoincidences.
However, its model interpretation poses considerable difficulties. The
Nilsson particle-plus-rotor model calculations have shown that, while one can reproduce
most of the 188 Re low-lying levels assuming axially-symmetric core deformation with
ε=0.18, and ε 4 =0.05, then the experimentally observed level density above 400 keV is
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