78 research outputs found
Pygmy dipole resonance in exotic nuclei
The evolution of the PDR strength with the neutron excess is investigated in
Sn isotopic and N=82 isotonic chains with regard to its possible connection
with the neutron skin thickness. For this purpose a recently proposed method
incorporating both HFB and multi-phonon QPM theory is applied. Analysis of the
corresponding neutron and proton dipole transition densities is presented.Comment: International Workshop on Nuclear Physics 28th Course - Radioactive
Beams, Nuclear Dynamics and Astrophysics, Ettore Majorana Center for
Scientific Cultur
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
Probing the Nuclear Neutron Skin by Low-Energy Dipole Modes
Dipole excitations below the neutron threshold in neutron rich Sn isotopes
are studied theoretically in the Quasiparticle-Phonon Model with HFB single
particle input. Of special interest are the low-lying two-phonon 1- states and
the Pygmy Dipole Resonance (PDR). The evolution of low-energy dipole
excitations with neutron excess is investigated over the Sn isotopic chain
including the experimentally unknown regions close to 132Sn. A dependence of
the PDR strengths and centroid energies on the neutron skin thickness is found.
Despite significant multi-phonon contributions to mean energies and transition
strengths, the PDR states retain their one-phonon character. The fragmentation
pattern is reduced with increasing neutron excess towards the N=82 shell
closure which will be of advantage for future experimental work.Comment: 4 pages, 2 figure
Pygmy Dipol Resonances as a Manifestation of the Structure of the Neutron-Rich Nuclei
Dipole excitations in neutron-rich nuclei below the neutron threshold are
investigated. The method is based on Hartree-Fock-Bogoliubov (HFB) and
Quasiparticle-Phonon Model (QPM) theory. Of our special interest are the
properties of the low-lying 1- Pygmy Resonance and the two-phonon
quadrupole-octupole 1- states in Sn-isotopes including exploratory
investigations for the experimentally unknown mass regions. In particular we
investigate the evolution of the dipole strength function with the neutron
excess. The use of HFB mean-field potentials and s.p. energies is found to
provide a reliable extrapolation into the region off stability.Comment: 8 pages, 3 figures, Proceedings of the International Conference on
Collective Motion in Nuclei Under Extreme Conditions (COMEX1), Paris, France,
10-13 June 200
Pygmy dipole strength close to particle-separation energies - the case of the Mo isotopes
The distribution of electromagnetic dipole strength in 92, 98, 100 Mo has
been investigated by photon scattering using bremsstrahlung from the new ELBE
facility. The experimental data for well separated nuclear resonances indicate
a transition from a regular to a chaotic behaviour above 4 MeV of excitation
energy. As the strength distributions follow a Porter-Thomas distribution much
of the dipole strength is found in weak and in unresolved resonances appearing
as fluctuating cross section. An analysis of this quasi-continuum - here
applied to nuclear resonance fluorescence in a novel way - delivers dipole
strength functions, which are combining smoothly to those obtained from
(g,n)-data. Enhancements at 6.5 MeV and at ~9 MeV are linked to the pygmy
dipole resonances postulated to occur in heavy nuclei.Comment: 6 pages, 5 figures, proceedings Nuclear Physics in Astrophysics II,
May 16-20, Debrecen, Hungary. The original publication is available at
www.eurphysj.or
Giant resonances in exotic spherical nuclei within the RPA approach with the Gogny force
Theoretical results for giant resonances in the three doubly magic exotic
nuclei Ni, Sn and Sn are obtained from Hartree-Fock (HF)
plus Random Phase Approximation (RPA) calculations using the D1S
parametrization of the Gogny two-body effective interaction. Special attention
is paid to full consistency between the HF field and the RPA particle-hole
residual interaction. The results for the exotic nuclei, on average, appear
similar to those of stable ones, especially for quadrupole and octupole states.
More exotic systems have to be studied in order to confirm such a trend. The
low energy of the monopole resonance in Ni suggests that the compression
modulus in this neutron rich nucleus is lower than the one of stable ones.Comment: 16 pages, 10 figure
Nuclear vorticity and the low-energy nuclear response - Towards the neutron drip line
The transition density and current provide valuable insight into the nature
of nuclear vibrations. Nuclear vorticity is a quantity related to the
transverse transition current. In this work, we study the evolution of the
strength distribution, related to density fluctuations, and the vorticity
strength distribution, as the neutron drip line is approached. Our results on
the isoscalar, natural-parity multipole response of Ni isotopes, obtained by
using a self-consistent Skyrme-Hartree-Fock + Continuum RPA model, indicate
that, close to the drip line, the low-energy response is dominated by L>1
vortical transitions.Comment: 8 pages, incl. 4 figures; to appear in Phys.Lett.
Kabul Times (November 9, 1967, vol. 6, no. 184)
Recent experimental findings and theoretical approaches to the electric dipole (El) strength distribution below the particle emission threshold at shell closures are revisited. Results from photon scattering experiments are discussed and compared to predictions within the quasiparticle-phonon nuclear model. An analysis of the fine structure of the El strength is presented. Recent studies of the E1 response of light exotic nuclei are also discussed
Low-lying GT(+) strength in Co-64 studied via the Ni-64(d,He-2)Co-64 reaction
The Ni-64(d,He-2)Co-64 reaction was studied at the AGOR cyclotron of KVI, Groningen, with the Big-Bite Spectrometer and the EuroSuperNova detector using a 171-MeV deuteron beam. An energy resolution of about 110 keV was achieved. In addition to the J(pi) = 1(+) ground state, several other 1(+) states could be identified in Co-64 and the strengths of the corresponding Gamow-Teller transitions were determined. The obtained strength distribution was compared with theoretical predictions and former (n,p) experimental results and displayed a good agreement. Due to the good energy resolution, detailed spectroscopic information was obtained, which supplements the data base needed for network calculations for supernova scenarios
Photon data shed new light upon the GDR spreading width in heavy nuclei
A global study of the electric dipole strength in and below the isovector
giant dipole resonance (GDR) is presented for mass numbers A>80. It relies on
the recently established remarkably good match between data for the nuclear
photo effect to novel photon scattering data covering the region below the
neutron emission threshold as well as by average resonance neutron capture
(ARC). From the wide energy coverage of these data the correlation of the GDR
spreading width with energy can be studied with remarkable accuracy. A clear
sensitivity to details of the nuclear shape, i.e. the beta- and
gamma-deformations, is demonstrated. Based hereon a new parameterization of the
energy dependence of the nuclear electric-dipole strength is proposed which -
with only two new parameters - allows to describe the dipole strength in all
heavy nuclei with A>80. Although it differs significantly from previous
parameterizations it holds for spherical, transitional, triaxial and well
deformed nuclei. The GDR spreading width depends in a regular way on the
respective resonance energy, but it is independent of the photon energy.Comment: accepted by Phys. Lett. B after minor modification
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