415 research outputs found
Spectroscopy of the heaviest nuclei (theory)
Recent progress in the applications of covariant density functional theory
(CDFT) to the description of the spectroscopy of the heaviest nuclei is
reviewed. The analysis of quasiparticle spectra in actinides and the heaviest A
~ 250 nuclei provides a measure of the accuracy of the description of
single-particle energies in CDFT and an additional constraint for the choice of
effective interactions for the description of superheavy nuclei. The response
of these nuclei to the rotation is rather well described by cranked
relativistic Hartree+Bogoliubov theory and it serves as a supplementary tool in
configuration assignment in odd-mass nuclei. A systematic analysis of the
fission barriers with allowance for triaxial deformation shows that covariant
density functional theory is able to describe fission barriers on a level of
accuracy comparable with the best phenomenological macroscopic+microscopic
approaches.Comment: 10 pages, 7 figures, invited talk of A.V. Afanasjev at the
International Nuclear Physics Conference (INPC 2010), Vancouver, Canada, July
4-9, 2010, to be published in Journal of Physics G: Conference Series (JPCS
Relativistic RPA plus phonon-coupling analysis of pygmy dipole resonances
The relativistic random-phase approximation (RRPA) plus phonon-coupling (PC)
model is applied in the analysis of E1 strength distributions in Pb and
Sn, for which data on pygmy dipole resonances (PDR) have recently been
reported. The covariant response theory is fully consistent: the effective
nuclear interaction NL3 is used both to determine the spectrum of
single-nucleon Dirac states, and as the residual interaction which determines
the collective phonon states in the relativistic RPA. It is shown that the
picture of the PDR as a resonant oscillation of the neutron skin against the
isospin saturated proton-neutron core, and with the corresponding RRPA state
characterized by a coherent superposition of many neutron particle-hole
configurations, remains essentially unchanged when particle-vibration coupling
is included. The effect of two-phonon admixtures is a weak fragmentation and a
small shift of PDR states to lower excitation energy. Even though the PDR
calculated in the extended model space of phonon configurations
contains sizeable two-phonon admixtures, it basically retains a one-phonon
character and its dynamics is not modified by the coupling to low-lying surface
vibrations.Comment: 17 pages, 3 figures, 4 table
Relativistic quasiparticle time blocking approximation. II. Pygmy dipole resonance in neutron-rich nuclei
Theoretical studies of low-lying dipole strength in even-even spherical
nuclei within the relativistic quasiparticle time blocking approximation
(RQTBA) are presented. The RQTBA developed recently as an extension of the
self-consistent relativistic quasiparticle random phase approximation (RQRPA)
enables one to investigate effects of coupling of two-quasiparticle excitations
to collective vibrations within a fully consistent calculation scheme based on
covariant energy density functional theory. Dipole spectra of even-even
Sn -- Sn and Ni -- Ni isotopes calculated within
both RQRPA and RQTBA show two well separated collective structures: the
higher-lying giant dipole resonance (GDR) and the lower-lying pygmy dipole
resonance (PDR) which can be identified by a different behavior of the
transition densities of states in these regions.Comment: 28 pages, 13 figure
Observed and Physical Properties of Core-Collapse Supernovae
I use photometry and spectroscopy data for 24 Type II plateau supernovae to
examine their observed and physical properties. This dataset shows that these
objects encompass a wide range of ~5 mag in their plateau luminosities, their
expansion velocities vary by x5, and the nickel masses produced in these
explosions go from 0.0016 to 0.26 Mo. From a subset of 16 objects I find that
the explosion energies vary between 0.6x and 5.5x10^51 ergs, the ejected masses
encompass the range 14-56 Mo, and the progenitors' radii go from 80 to 600 Ro.
Despite this great diversity several regularities emerge, which reveal that
there is a continuum in the properties of these objects from the faint,
low-energy, nickel-poor SNe 1997D and 1999br, to the bright, high-energy,
nickel-rich SN 1992am. This study provides evidence that more massive
progenitors produce more energetic explosions, thus suggesting that the outcome
of the core collapse is somewhat determined by the envelope mass. I find also
that supernovae with greater energies produce more nickel. Similar
relationships appear to hold for Type Ib/c supernovae, which suggests that both
Type II and Type Ib/c supernovae share the same core physics. When the whole
sample of core collapse objects is considered, there is a continous
distribution of energies below 8x10^51 ergs. Far above in energy scale and
nickel production lies the extreme hypernova 1998bw, the only supernova firmly
associated to a GRB.Comment: 25 pages, 7 figures, accepted for Part 1 of Astrophysical Journa
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
Low-lying dipole response in the Relativistic Quasiparticle Time Blocking Approximation and its influence on neutron capture cross sections
We have computed dipole strength distributions for nickel and tin isotopes
within the Relativistic Quasiparticle Time Blocking approximation (RQTBA).
These calculations provide a good description of data, including the
neutron-rich tin isotopes Sn. The resulting dipole strengths have
been implemented in Hauser-Feshbach calculations of astrophysical neutron
capture rates relevant for r-process nucleosynthesis studies. The RQTBA
calculations show the presence of enhanced dipole strength at energies around
the neutron threshold for neutron rich nuclei. The computed neutron capture
rates are sensitive to the fine structure of the low lying dipole strength,
which emphasizes the importance of a reliable knowledge of this excitation
mode.Comment: 15 pages, 4 figures, Accepted in Nucl. Phys.
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