218 research outputs found
Pairing and deformation effects in nuclear excitation spectra
We investigate effects of pairing and of quadrupole deformation on two sorts
of nuclear excitations,-vibrational states and dipole
resonances (isovector dipole, pygmy, compression, toroidal). The analysis is
performed within the quasiparticle random-phase approximation (QRPA) based on
the Skyrme energy functional using the Skyrme parametrization SLy6. Particular
attention is paid to i) the role of the particle-particle (pp) channel in the
residual interaction of QRPA, ii) comparison of volume pairing (VP) and surface
pairing (SP), iii) peculiarities of deformation splitting in the various
resonances. We find that the impact of the pp-channel on the considered
excitations is negligible. This conclusion applies also to any other excitation
except for the states. Furthermore, the difference between VP and
SP is found small (with exception of peak height in the toroidal mode). In the
low-energy isovector dipole (pygmy) and isoscalar toroidal modes, the branch
is shown to dominate over one in the range of
excitation energy 8--10 MeV. The effect becomes impressive for the
toroidal resonance whose low-energy part is concentrated in a high peak of
almost pure nature. This peculiarity may be used as a fingerprint
of the toroidal mode in future experiments. The interplay between pygmy,
toroidal and compression resonances is discussed, the interpretation of the
observed isoscalar giant dipole resonance is partly revised.Comment: 12 pages, 12 figure
Individual low-energy E1 toroidal and compression states in light nuclei: deformation effect, spectroscopy and interpretation
The existence of individual low-energy E1 toroidal and compression states (TS
and CS) in Mg was predicted recently in the framework of quasiparticle
random-phase-approximation (QRPA) model with Skyrme forces. It was shown that
the strong axial deformation of Mg is crucial to downshift the toroidal
strength to the low-energy region and thus make the TS the lowest E1(K=1)
dipole state. In this study, we explain this result by simple mean-field
arguments. Comparing TS in two strongly axial nuclei, Mg and Ne,
we show that the lowest TS is not not a universal phenomenon but rather a
peculiarity of Mg. The spectroscopy of TS and CS is analyzed and some
additional interpretation of these states is suggested.Comment: 6 pages, 8 figures, 2 tables. Submitted to proceedings of
International Conference on Nuclear Structure and Related Topics - NSRT18
(Bulgaria, Bourgas, June 3-9, 2018). The paper was resubmitted to enlarge the
reference lis
An estimate of the branching ratio for Z -> nu nubar gamma gamma
The effective interaction for two neutrino, two photon coupling is used to
find an approximate width for the decay of the Z boson into the nu nubar gamma
gamma final state.Comment: Three pages, 2 figure
Systematics of toroidal dipole modes in Ca, Ni, Zr, and Sn isotopes
We analyze the relation between isoscalar toroidal modes and so-called pygmy
dipole resonance (PDR) which both appear in the same region of low-energy
dipole excitations. To this end, we use a theoretical description within the
fully self-consistent Skyrme quasiparticle random-phase approximation (QRPA).
Test cases are spherical nuclei Ca, Ni, Zr, and
Sn which cover four different elements and for each element at
least two isotopes with different neutron excess, one small and another large.
The structure of the modes is investigated in terms of strength functions,
transition densities (TD) and transition currents (TC). For all considered
nuclei, we see that, independently on whether PDR strength exists or not, the
flow pattern in the lower part of the "PDR energy region" is basically the
isoscalar vortical toroidal motion with a minor irrotational fraction. A
one-to-one correspondence between calculated TD and TC can be established. The
toroidal flow appears already in the uncoupled two-quasiparticle (2qp)
excitations and becomes definitively strong for the QRPA modes. Altogether, we
find that low-lying dipole strength often denoted as isoscalar PDR is actually
an oversimplified imitation of the basically toroidal motion in nuclei with a
sufficient neutron excess.Comment: 12 pages, 14 figure
Deformation effects in Giant Monopole Resonance
The isoscalar giant monopole resonance (GMR) in Samarium isotopes (from
spherical Sm to deformed Sm) is investigated within the
Skyrme random-phase-approximation (RPA) for a variety of Skyrme forces. The
exact RPA and its separable version (SRPA) are used for spherical and deformed
nuclei, respectively. The quadrupole deformation is shown to yield two effects:
the GMR broadens and attains a two-peak structure due to the coupling with the
quadrupole giant resonance.Comment: 6 pages, 4 figures, proceedings of 11th Intern. Spring Seminar on
Nuclear Physics (Ischia, Italy, May 12-16, 2014
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