212 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
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
Self-consistent account for phonon induced corrections to quadrupole moments of odd nuclei. Pole and non-pole diagrams
Recent results of the description of quadrupole moments of odd semi-magic
nuclei are briefly reviewed. They are based on the self-consistent theory of
finite Fermi systems with account for the phonon-particle coupling (PC)
effects. The self-consistent model for describing the PC effects was developed
previously for magnetic moments. Account for the non-pole diagrams is an
important ingredient of this model. In addition to previously reported results
for the odd In and Sb isotopes, which are the proton-odd neighbors of even tin
nuclei, we present new results for odd Bi isotopes, the odd neighbors of even
lead isotopes. In general, account for the PC corrections makes the agreement
with the experimental data significantly better.Comment: 8 pages, 4 figures. Presented at ICNFP1
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
Phonon coupling effects in magnetic moments of magic and semi-magic nuclei
Phonon coupling (PC) corrections to magnetic moments of odd neighbors of
magic and semi-magic nuclei are analyzed within the self-consistent Theory of
Finite Fermi Systems (TFFS) based on the Energy Density Functional by Fayans et
al. The perturbation theory in g_L^2 is used where g_L is the phonon-particle
coupling vertex. A model is developed with separating non-regular PC
contributions, the rest is supposed to be regular and included into the
standard TFFS parameters. An ansatz is proposed to take into account the
so-called tadpole term which ensures the total angular momentum conservation
with g_L^2 accuracy. An approximate method is suggested to take into account
higher order terms in g_L^2. Calculations are carried out for four odd-proton
chains, the odd Tl, Bi, In and Sb ones. Different PC corrections strongly
cancel each other. In the result, the total PC correction to the magnetic
moment in magic nuclei is, as a rule, negligible. In non-magic nuclei
considered it is noticeable and, with only one exception, negative. On average
it is of the order of -(0.1 - 0.5) \mu_N and improves the agreement of the
theory with the data. Simultaneously we calculated the gyromagnetic ratio
g_L^{ph} of all low-lying phonons in 208Pb. For the 3^-_1 state it is rather
close to the Bohr-Mottelson model prediction whereas for other L-phonons, two
5^- and six positive parity states, the difference from the Bohr-Mottelson
values is significant.Comment: 21 pages, 24 figure
Self-consistent calculations within the Green's function method including particle-phonon coupling and the single-particle continuum
The Green's function method in the \emph{Quasiparticle Time Blocking
Approximation} is applied to nuclear excitations in Sn and Pb.
The calculations are performed self-consistently using a Skyrme interaction.
The method combines the conventional RPA with an exact single-particle
continuum treatment and considers in a consistent way the particle-phonon
coupling. We reproduce not only the experimental values of low- and high-lying
collective states but we also obtain fair agreement with the data of
non-collective low-lying states that are strongly influenced by the
particle-phonon coupling.Comment: 6 pages, 9 figures, documentclass{svjour
Microscopic description of the pygmy and giant electric dipole resonances in stable Ca isotopes
The properties of the pygmy (PDR) and giant dipole resonance (GDR)in the
stable , and isotopes have been calculated within
the \emph{Extended Theory of Finite Fermi Systems}(ETFFS). This approach is
based on the random phase approximation (RPA) and includes the single particle
continuum as well as the coupling to low-lying collectives states which are
considered in a consistent microscopic way. For we also include
pairing correlations. We obtain good agreement with the experimental data for
the gross properties of both resonances. It is demonstrated that the recently
measured A-dependence of the strength of the PDR below 10 MeV is well
understood in our model:due to the phonon coupling some of the strength in
is simply shifted beyond 10 MeV. The predicted fragmentation of the
PDR can be investigated in and experiments.
Whereas the isovector dipole strength of the PDR is small in all Ca isotopes,
we find in this region surprisingly strong isoscalar dipole states, in
agreement with an experiment. We conclude that for the
detailed understanding of the structure of excited nuclei e.g. the PDR and GDR
an approach like the present one is absolutely necessary.Comment: 6 figure
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