24 research outputs found
Angular and energy dependence of cross sections for orbital 1 excitations
The main features of the cross sections of low-lying orbital
excitations with in heavy deformed nuclei are studied in RPA on
the example of Gd. The dependence of the DWBA E2 and M1 cross sections
on the scattering angle and incident
electron energy MeV is analyzed in PWBA. The cross section is
larger for M1 than for E2 transitions at any angle if MeV. The
longitudinal (Coulomb) C2 excitation dominates the E2 response for . Only transverse M1 and E2 excitations compete for
and the former one is dominant for fm.
The M1 response is almost purely orbital up to fm even in
backward scattering. Qualitative PWBA estimates based on the -dependence of
the form factors alone are not able to predict some important features of the
cross sections stemming from the strong magnetic and orbital
character of the studied 1 excitations. The expectation for M1 over E2
dominance in backward scattering should not be extended to higher momentum
transfers and incident energies.Comment: Latex, 28 pages, 12 postscript figures included using uufile
Competing electric and magnetic excitations in backward electron scattering from heavy deformed nuclei
Important contributions to the cross sections of
low-lying orbital excitations are found in heavy deformed nuclei, arising
from the small energy separation between the two excitations with and 1, respectively. They are studied microscopically in QRPA using
DWBA. The accompanying response is negligible at small momentum transfer
but contributes substantially to the cross sections measured at for fm ( MeV)
and leads to a very good agreement with experiment. The electric response is of
longitudinal type for but becomes almost purely
transverse for larger backward angles. The transverse response
remains comparable with the response for fm
( MeV) and even dominant for MeV. This happens even at
large backward angles , where the dominance is
limited to the lower region.Comment: RevTeX, 19 pages, 8 figures included Accepted for publication in Phys
Rev
High-energy scissors mode
All the orbital M1 excitations, at both low and high energies, obtained from
a rotationally invariant QRPA, represent the fragmented scissors mode. The
high-energy M1 strength is almost purely orbital and resides in the region of
the isovector giant quadrupole resonance. In heavy deformed nuclei the
high-energy scissors mode is strongly fragmented between 17 and 25 MeV (with
uncertainties arising from the poor knowledge of the isovector potential). The
coherent scissors motion is hindered by the fragmentation and for single transitions in this region. The cross
sections for excitations above 17 MeV are one order of magnitude larger for E2
than for M1 excitations even at backward angles.Comment: 20 pages in RevTEX, 5 figures (uuencoded,put with 'figures') accepted
for publication in Phys.Rev.
The effects of deformation and pairing correlations on nuclear charge form factor
A set of moderately deformed shell nuclei is employed for testing the
reliability of the nuclear ground state wave functions which are obtained in
the context of a BCS approach and offer a simultaneous consideration of
deformation and pairing correlations effects. In this method, the mean field is
assumed to be an axially symmetric Woods-Saxon potential and the effective
two-body interaction is a monopole pairing force. As quantities of main
interest we have chosen the nuclear form factors, the occupancies of the active
(surface) orbits and the Fermi sea depletion, which provide quite good tests
for microscopic descriptions of nuclei within many body theories. For our
comparisons with results emerging from other similar methods, an axially
deformed harmonic oscillator field is also utilized.Comment: 20 pages, 12 figures, 2 table
Self-Consistent Velocity Dependent Effective Interactions
The theory of self-consistent effective interactions in nuclei is extended
for a system with a velocity dependent mean potential. By means of the field
coupling method, we present a general prescription to derive effective
interactions which are consistent with the mean potential. For a deformed
system with the conventional pairing field, the velocity dependent effective
interactions are derived as the multipole pairing interactions in
doubly-stretched coordinates. They are applied to the microscopic analysis of
the giant dipole resonances (GDR's) of , the first excited
states of Sn isotopes and the first excited states of Mo isotopes.
It is clarified that the interactions play crucial roles in describing the
splitting and structure of GDR peaks, in restoring the energy weighted sum
rule, and in reducing the values of .Comment: 35 pages, RevTeX, 7 figures (available upon request), to appear in
Phys.Rev.
Spin M1 excitations in deformed nuclei from self-consistent Hartree-Fock plus random-phase approximation
We present a method to study spin magnetic dipole excitations in deformed nuclei within the quasiparticle random phase approximation based on self-consistent Hartree-Fock mean fields and residual interactions derived from the same effective two-body force. We perform a comprehensive study covering different Skyrme forces and various mass regions, and discussing the role of the mean field and of the residual interaction. An overall agreement with experimental data is obtained with the SG2 force. We study the systematics and the deformation dependence of the spin M1 strength distributions of K-pi=1(+) excitations. It is found for the first time that the summed spin MI strength obeys a quadratic dependence on deformation in the two isotope chains studied, Nd-142,Nd-146,Nd-148,Nd-150 and Sm-144,Sm-148,Sm-150,Sm-152,Sm-154.The work is supported by the EC program ‘‘Human Capital and Mobility’’ under Contracts No. CHRX-CT 93-0323 and CHRX-CT 94-
0562. P.S. and E.M.G. are indebted to DGICYT ~Spain! for partial financial support under Contract No. 92/0021-C02-01.Peer reviewe