125 research outputs found
Excitations of the unstable nuclei ^{48}Ni and ^{49}Ni
The isoscalar E1 and E2 resonances in the proton-rich nuclei ^{48,49}Ni and
the {f_{7/2}3^-} multiplet in ^{49}Ni have been calculated taking into account
the single-particle continuum exactly. The analogous calculations for the
mirror nuclei ^{48}Ca and ^{49}Sc are presented. The models used are the
continuum RPA for ^{48}Ni, ^{48}Ca and the Odd RPA for ^{49}Ni, ^{49}Sc, the
latter has been developed recently and describes both single-particle and
collective excitations of an odd nucleus on a common basis. In all four nuclei
we obtained a distinct splitting of the isoscalar E1 resonance into 1 h-bar
omega and 3 h-bar omega peaks at about 11 MeV and 30 MeV, respectively. The
main part of the isoscalar E1 EWSR is exhausted by the 3 h-bar omega
resonances. The 1 h-bar omega resonances exhaust about 35% of this EWSR in
^{48,49}Ni and about 22% in ^{48}Ca and ^{49}Sc. All seven {f_{7/2}3^-}
multiplet members in ^{49}Ni are calculated to be in the (6-8) MeV energy
region and have noticeable escape widths.Comment: 11 pages, 3 Postscript figure
On Cooper Pairing in Finite Fermi Systems
In order to analyse the role of the quasiparticle-phonon interaction in the
origin of nuclear gap, we applied an approach which is similar to the
Eliashberg theory for usual superconductors. We obtained that the averaged
contribution of the quasiparticle-phonon mechanism to the observed value of the
pairing gap for Sn is 26% and the BCS-type mechanism gives 74% . Thus,
pairing is of a mixed nature at least in semi-magic nuclei -- it is due to the
quasiparticle-phonon and BCS mechanisms, the first one being mainly a surface
mechanism and the second one mainly a volume mechanism. The calculations of the
strength distribution for the odd-mass nuclei and have
shown that the quasiparticle-phonon mechanism mainly improves the description
of the observed spectroscopic factors in these nuclei.
For the case of nuclei with pairing in both proton and neutron systems it is
necessary to go beyond the Eliashberg-Migdal approximations and include the
vertex correction graphs in addition to the rainbow ones. The estimations for
spectroscopic factors performed within a three-level model have shown that the
contribution of the vertex correction graphs was rather noticeable.Comment: The 7-th International Spring Seminar on Nuclear Physics, "Challenges
of Nuclear Structure",Maiori, May 27-31, 200
Covariant theory of particle-vibrational coupling and its effect on the single-particle spectrum
The Relativistic Mean Field (RMF) approach describing the motion of
independent particles in effective meson fields is extended by a microscopic
theory of particle vibrational coupling. It leads to an energy dependence of
the relativistic mass operator in the Dyson equation for the single-particle
propagator. This equation is solved in the shell-model of Dirac states. As a
result of the dynamics of particle-vibrational coupling we observe a noticeable
increase of the level density near the Fermi surface. The shifts of the
single-particle levels in the odd nuclei surrounding 208-Pb and the
corresponding distributions of the single-particle strength are discussed and
compared with experimental data.Comment: 27 pages, 8 figure
Self-consistent calculations of the strength function and radiative neutron capture cross section for stable and unstable tin isotopes
The E1 strength function for 15 stable and unstable Sn even-even isotopes
from A=100 till A=176 are calculated using the self-consistent microscopic
theory which, in addition to the standard (Q)RPA approach, takes into account
the single-particle continuum and the phonon coupling. Our analysis shows two
distinct regions for which the integral characteristics of both the giant and
pygmy resonances behave rather differently. For neutron-rich nuclei, starting
from Sn, we obtain a giant E1 resonance which significantly deviates
from the widely-used systematics extrapolated from experimental data in the
-stability valley. We show that the inclusion of the phonon coupling is
necessary for a proper description of the low-energy pygmy resonances and the
corresponding transition densities for
region the influence of phonon coupling is significantly smaller. The radiative
neutron capture cross sections leading to the stable Sn and unstable
Sn and Sn nuclei are calculated with both the (Q)RPA and the
beyond-(Q)RPA strength functions and shown to be sensitive to both the
predicted low-lying strength and the phonon coupling contribution. The
comparison with the widely-used phenomenological Generalized Lorentzian
approach shows considerable differences both for the strength function and the
radiative neutron capture cross section. In particular, for the neutron-rich
Sn, the reaction cross section is found to be increased by a factor
greater than 20. We conclude that the present approach may provide a complete
and coherent description of the -ray strength function for astrophysics
applications. In particular, such calculations are highly recommended for a
reliable estimate of the electromagnetic properties of exotic nuclei
Magnetic moments of odd-odd spherical nuclei
Magnetic moments of more than one hundred odd-odd spherical nuclei in ground
and excited states are calculated within the self-consistent TFFS based on the
EDF method by Fayans {\it et al}. We limit ourselves to nuclei with a neutron
and a proton particle (hole) added to the magic or semimagic core. A simple
model of no interaction between the odd nucleons is used. In most the cases we
analyzed, a good agreement with the experimental data is obtained. Several
cases are considered where this simple model does not work and it is necessary
to go beyond. The unknown values of magnetic moments of many unstable odd and
odd-odd nuclei are predicted including sixty values for excited odd-odd nuclei.Comment: 10 page
M1 Resonances in Unstable Magic Nuclei
Within a microscopic approach which takes into account RPA configurations,
the single-particle continuum and more complex
configurations isoscalar and isovector M1 excitations for the unstable nuclei
Ni and Sn are calculated. For comparison, the
experimentally known M1 excitations in Ca and Pb have also been
calculated. In the latter nuclei good agreement in the centroid energy, the
total transition strength and the resonance width is obtained. With the same
parameters we predict the magnetic excitations for the unstable nuclei. The
strength is sufficiently concentrated to be measurable in radioactive beam
experiments. New features are found for the very neutron rich nucleus Ni
and the neutron deficient nucleus Sn.Comment: 17 pages (LATEX), 12 figures (available from the authors),
KFA-IKP(TH)-1993-0
On the mechanisms of superfluidity in atomic nuclei
A system of equations is obtained for the Cooper gap in nuclei. The system
takes two mechanisms of superfluidity into account in an approximation
quadratic in the phonon- production amplitude : a Bardeen- Cooper- Schrieffer
(BCS) type mechanism and a quasiparticle- phonon mechanism. These equations are
solved for 120 Sn in a realistic approximation. If the simple procedures
proposed are used to determine the new particle- particle interaction and to
estimate the average effect, then the contribution of the quasiparticle- phonon
mechanism to the observed width of the pairing gap is 26% and the BCS-type
contribution is 74%. This means that at least in semimagic nuclei pairing is of
a mixed nature - it is due to the two indicated mechanisms, the first being
mainly a surface mechanism and the second mainly a volume mechanism.Comment: 6 page
Self-consistent calculations of radiative nuclear reaction characteristics for 56
The photon strength functions (PSF), neutron capture cross sections and average radiative widths of neutron resonances for three double-magic nuclei 56Ni, 132Sn and 208Pb have been calculated within the self-consistent version of the microscopic theory. Our approach includes phonon coupling (PC) effects in addition to the standard QRPA approach. With our microscopic PSFs, calculations of radiative nuclear reaction characteristics have been performed using the EMPIRE 3.1 nuclear reaction code. Three nuclear level density (NLD) models have been used: the phenomenological so-called GSM, phenomenological Enhanced GSM (EGSM) and microscopical combinatorial HFB model. For all the considered characteristics, we found a noticeable contribution of the PC effects and a significant disagreement between the results obtained with the GSM and the other two NLD models. The results confirm the necessity of using consistent microscopic approaches for calculations of radiative nuclear characteristics in double-magic nuclei
Covariant response theory beyond RPA and its application
The covariant particle-vibration coupling model within the time blocking
approximation is employed to supplement the Relativistic Random Phase
Approximation (RRPA) with coupling to collective vibrations. The Bethe-Salpeter
equation in the particle-hole channel with an energy dependent residual
particle-hole (p-h) interaction is formulated and solved in the shell-model
Dirac basis as well as in the momentum space. The same set of the coupling
constants generates the Dirac-Hartree single-particle spectrum, the static part
of the residual p-h interaction and the particle-phonon coupling amplitudes.
This approach is applied to quantitative description of damping phenomenon in
even-even spherical nuclei with closed shells Pb and Sn. Since
the phonon coupling enriches the RRPA spectrum with a multitude of
phphonon states a noticeable fragmentation of giant monopole and
dipole resonances is obtained in the examined nuclei. The results are compared
with experimental data and with results of the non-relativistic approach.Comment: 12 pages, 4 figures, Proceedings of the NSRT06 Conferenc
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