REINA() DE TRABALHO SOBRE FiSICA NUCLEAR NO BRASIL

16 Estruturn Nuclear - XX
either as true or chance. For each master gate three
parameters, the energy of both detectors and the coincidence
tag, were recorded with a CAMAC input register,
assisted by a MBD-11 microprocessor, connected
to a PDP-11/84 computer. The measurements
were made on 15 direct and 4 skip cascades, for the
first time. Based on the angular correlation results
it wns possible to determine the multipole mixing ratios.
for 10 transitions: 6(402) = —0.07±0.07, 6(456)=-
0.06±0.07, 6(472) = — 0.7911 6(571)=0.75±0.05,
5(575) = 1.79!1:11, 6(695) = 0.40111 6(809)
0.57111, 6(980)=-0.270.02, 6(1232) = 1.48±0.23 and
6(1956) = 0.661 .1'7. The present results show that
a large number of 7 transitions have considerable Mi
admixtures which are difficult to explain in terms of
simple vibrational model. More refined models which
take in to account the interaction between collective
and quasi-particle effects may be necessary to explain
the level structure in this nucleus.
TESTING THE DYSON EXPANSION
WITHIN THE QUON ALGEBRA
S. S. AVANCINI, J. R. MARINELLI, D. P. MENEZES,
M. M. WATANABE DE MORASS
Dept(' de Fisica - UFSC
N. YOSIIINAGA
Saitama University - Japan
In the literature fermion pairs are replaced by bosons in
many known physical situations [1,2]. This is normally
performed with the help of boson mappings, that link
the fermionic Hilbert space to another Hilbert space
constructed with bosons. Of course boson mapping techniques
are only useful when the Pauli Principle effects
are somehow minimized.
In this work, the quon algebra [3], which interpolates
between the Bose and Fermi algebras and depends on a
free paramenter q, is used to generate a deformed Dyson
boson expansion of the quadrupole operator, which
is written in terms of three deformed bosons (s, d and
g bosons). Then we obtain a quadrupole-quadrupole
harniltonian, for a single j-shell, in terms of this deformed
bosonic operator. The hamiltonian is diagonalized
and its eigenvalue,s are compared with the ones obtained
from the fermionic quadrupole-quadrupole hamiltonian.
The deformation parameter helps in achieving
the correct energy levels, what cannot he encountered
in practice with the usual non-deforrned Dyson expansion.
We use a kinematical criterion for fixing the q
parameters by invoking the Pauli principle and compare
the results to a best fitting procedure.
[1] S.T. Beliaev and V.G. Zelevinsky , Nucl. Phys. 39
(1962) 582 ; E.R. Marshalek , Nucl. Phys. A161
(1971)401, Nucl. Phys. A224 (1974) 221
[2] F.J. Dyson , Phys. Rev. 102 0956) 1217
[31 O.W. Greenberg , Physical Review 1) 43 (1991)
4111
Dipole excitations of low energy in deformed
nucleus studied by Nuclear Resonance
Fluorescence
ANGELA EMILIA DE ALMEIDA PINTO
Institute de Fisica - usP
ULRICH KNEISSL, HEINZ-HERMANN Prrz, .)URGEN
MARGRAF, HERMAN MASER, JORGEN BESSERER,
DANIELA BELIC, ALEXANDER NORD, THOMAS
ECKART, BERND KRISCHOK, OLIVER BECK,
ALEXANDER WOLPERT
Institut fiir Strahlenphysik der Universitiit Stuttgart
Low lying dipole excitations in heavy nuclei are of actual
interest in modern nuclear structure physics. Low
energy photon scaterring off bound states, nuclear resonance
fluorescence (NRF"), represents a higly selective
and sensitive tool to investigate low lying dipole excitations
in heavy nuclei. The discovery of a new class
of enhanced magnetic dipole excitations in heavy deformed
nuclei in high resolution electron scattering experiments
by Richter and coworkers in 1984 initiated
a large number of both experimental and theoretical
work( 1,2). Numerous electron and photon scattering
experiments were performed to study the systematics
and fragmentation of this low lying, predominantly orbital
mode often referred to as "Scissors Mode"N. On
the other hand, the recent NBF experiments also provided
evidence for enhanced, low lying electric dipole
excitations in deformed nuclei. The structure of the
corresponding j"= states is discussed in terms of
different collective excitation modes or two-phonon excitations.
For odd mass nuclei by far less spectroscopy data are
available. Recently the Stuttgart group succeeded to
observe the first time the 'Scissors Mode" in an odd deformed
Nucleus ( 163 1)y)(4). Similar results were found
for 151 Dy, however, in ' 57Gd the dipole strength is completely
fragmented into more than 80 transitions( 5).
This different behavior of diverse odd deformed nuclei
of the Rare Earth region is completely non understood.
The purpose of these work is further investigation of
the dipole strength distributions in odd-mass nuclei
( 153Eu). The En nuclei are odd proton isotopes in between
the well studied, deformed Gd isotopes and the
already investigated Sm isotopes showing a transition
from spherical to deformed nuclear shapes. NRF experiments
in ' 53Eu were performed at the bremsstrahlung
facility installed at the Stuttgart Dynarnitron accelerator,
to contribute more experimental data on the dipole