916 research outputs found
Why is Tin so soft?
The distribution of isoscalar monopole strength in the neutron-even
112-124Sn-isotopes has been computed using a relativistic
random-phase-approximation approach. The accurately-calibrated model used here
(``FSUGold'') has been successful in reproducing both ground-state observables
as well as collective excitations - including the giant monopole resonance
(GMR) in 90Zr, 144Sm, and 208Pb. Yet this same model significantly
overestimates the GMR energies in the Sn isotopes. It is argued that the
question of ``Why is Tin so soft?'' becomes an important challenge to the field
and one that should be answered without sacrificing the success already
achieved by several theoretical models.Comment: 5 pages, 3 figures to be submitted to PR
Spin-isospin nuclear response using the existing microscopic Skyrme functionals
Our paper aims at providing an answer to the question whether one can
reliably describe the properties of the most important spin-isospin nuclear
excitations, by using the available non-relativistic Skyrme energy functionals.
Our method, which has been introduced in a previous publication devoted to the
Isobaric Analog states, is the self-consistent Quasiparticle Random Phase
Approximation (QRPA). The inclusion of pairing is instrumental for describing a
number of experimentally measured spherical systems which are characterized by
open shells. We discuss the effect of isoscalar and isovector pairing
correlations. Based on the results for the Gamow-Teller resonance in Zr,
in Pb and in few Sn isotopes, we draw definite conclusions on the
performance of different Skyrme parametrizations, and we suggest improvements
for future fits. We also use the spin-dipole resonance as a benchmark of our
statements.Comment: Submitted to Phys. Rev.
The Giant Monopole Resonance in Pb isotopes
The extraction of the nuclear incompressibility from the isoscalar giant
monopole resonance (GMR) measurements is analysed. Both pairing and mutually
enhanced magicity (MEM) effects play a role in the shift of the GMR energy
between the doubly closed shell Pb nucleus and other Pb isotopes.
Pairing effects are microscopically predicted whereas the MEM effect is
phenomenologically evaluated. Accurate measurements of the GMR in open-shell Pb
isotopes are called for.Comment: 4 page
Spectroscopy of neutron-rich nitrogen isotopes with Agata+Paris+Vamos
Excited states of 17N, 18N and 19N were investigated through the measurement of gamma rays, following their population via deep-inelastic reactions induced by an 18O beam (7 MeV/u) on a thick 181Ta target. The experimental setup comprised the AGATA+PARIS detection system, coupled to the VAMOS++ magnetic spectrometer. In the 17N nucleus, the analysis of gamma-ray transitions de-exciting two states around 4-5 MeV clearly pointed to discrepancies with the lifetime values reported in literature. Three new gamma rays were observed in 18N at the energies of 1662.3 (3) keV, 2073.4 (8) keV and 2300.9 (8) keV, and hints for other two new transitions around 1566 keV and 1720 keV were found. In addition, a new transition with energy of 2489.7 (8) keV was observed in 19N.</p
Short-range lifetime measurements for deep-inelastic reaction products:The 19o test case
An experiment, aiming at measuring lifetimes of excited states in neutron- rich C and O isotopes, was performed at the GANIL laboratory with the use of the AGATA segmented HPGe tracking array, coupled to the PARIS scintillator array and to the VAMOS++ magnetic spectrometer. The nuclei of interest were populated in transfer and deep-inelastic processes induced by an 18O beam at 126 MeV (7.0 MeV/u) on a 181Ta target. This paper contains a brief description of a novel implementation of a Monte-Carlo technique, which allowed us to obtain excited states lifetimes in the range from tens to hundreds femtoseconds for a reaction with complex initial velocity distribution, making use of the Doppler-shift attenuation method (DSAM). As a test case, we present here the analysis for two states in 19O: 2371-keV 9=2+ and 2779-keV 7=2+, for which lifetimes of Γ > 400 fs and Γ = 140+50 -40 fs were obtained, respectively, in agreement with literature values. This newly developed approach will be essential for short lifetimes measurements in neutron-rich systems, exploiting intense ISOL-type beams, currently under development.</p
Dipole responses in Nd and Sm isotopes with shape transitions
Photoabsorption cross sections of Nd and Sm isotopes from spherical to
deformed even nuclei are systematically investigated by means of the
quasiparticle-random-phase approximation based on the Hartree-Fock-Bogoliubov
ground states (HFB+QRPA) using the Skyrme energy density functional. The
gradual onset of deformation in the ground states as increasing the neutron
number leads to characteristic features of the shape phase transition. The
calculation well reproduce the isotopic dependence of broadening and emergence
of a double-peak structure in the cross sections without any adjustable
parameter. We also find that the deformation plays a significant role for
low-energy dipole strengths. The strengths are fragmented and considerably
lowered in energy. The summed strength up to 10 MeV is enhanced by a
factor of five or more.Comment: 5 pages including 6 figure
The Dynamical Dipole Mode in Fusion Reactions with Exotic Nuclear Beams
We report the properties of the prompt dipole radiation, produced via a
collective bremsstrahlung mechanism, in fusion reactions with exotic beams. We
show that the gamma yield is sensitive to the density dependence of the
symmetry energy below/around saturation. Moreover we find that the angular
distribution of the emitted photons from such fast collective mode can
represent a sensitive probe of its excitation mechanism and of fusion dynamics
in the entrance channel.Comment: 5 pages, 3 figures, to appear in Phys.Rev.
Where we stand on structure dependence of ISGMR in the Zr-Mo region: Implications on K_\infty
Isoscalar giant resonances, being the archetypal forms of collective nuclear
behavior, have been studied extensively for decades with the goal of
constraining bulk nuclear properties of the equation of state, as well as for
modeling dynamical behaviors within stellar environments. An important such
mode is the isoscalar electric giant monopole resonance (ISGMR) that can be
understood as a radially symmetric density vibration within the saturated
nuclear volume. The field has a few key open questions, which have been
proposed and remain unresolved. One of the more provocative questions is the
extra high-energy strength in the region, which manifested in
large percentages of the sum rule in Zr and Mo above the
main ISGMR peak. The purpose of this article is to introduce these questions
within the context of experimental investigations into the phenomena in the
zirconium and molybdenum isotopic chains, and to address, via a discussion of
previously published and preliminary results, the implications of recent
experimental efforts on extraction of the nuclear incompressibility from this
data.Comment: 9 pages, 7 figures, invited to be submitted to a special issue of
EPJA honoring Prof. P. F. Bortigno
Microscopic calculations of double and triple Giant Resonance excitation in heavy ion collisions
We perform microscopic calculations of the inelastic cross sections for the
double and triple excitation of giant resonances induced by heavy ion probes
within a semicalssical coupled channels formalism. The channels are defined as
eigenstates of a bosonic quartic Hamiltonian constructed in terms of collective
RPA phonons. Therefore, they are superpositions of several multiphonon states,
also with different numbers of phonons and the spectrum is anharmonic. The
inclusion of (n+1) phonon configurations affects the states whose main
component is a n-phonon one and leads to an appreacible lowering of their
energies. We check the effects of such further anharmonicities on the previous
published results for the cross section for the double excitation of Giant
Resonances. We find that the only effect is a shift of the peaks towards lower
energies, the double GR cross section being not modified by the explicity
inclusion of the three-phonon channels in the dynamical calculations. The
latters give an important contribution to the cross section in the triple GR
energy region which however is still smaller than the experimental available
data. The inclusion of four phonon configurations in the structure calculations
does not modify the results.Comment: Revtex4, to be published in PR
Microscopic Study of the Isoscalar Giant Monopole Resonance in Cd, Sn and Pb Isotopes
The isoscalar giant monopole resonance (ISGMR) in Cd, Sn and Pb isotopes has
been studied within the self-consistent Skyrme Hartree-Fock+BCS and
quasi-particle random phase approximation (QRPA). Three Skyrme parameter sets
are used in the calculations, i.e., SLy5, SkM* and SkP, since they are
characterized by different values of the compression modulus in symmetric
nuclear matter, namely K=230, 217, and 202 MeV, respectively. We also
investigate the effect of different types of pairing forces on the ISGMR in Cd,
Sn and Pb isotopes. The calculated peak energies and the strength distributions
of ISGMR are compared with available experimental data. We find that SkP fails
completely to describe the ISGMR strength distribution for all isotopes due to
its low value of the nuclear matter incompressibility, namely K=202 MeV. On the
other hand, the SLy5 parameter set, supplemented by an appropriate pairing
interaction, gives a reasonable description of the ISGMR in Cd and Pb isotopes.
A better description of ISGMR in Sn isotopes is achieved by the SkM*
interaction, that has a somewhat softer value of the nuclear incompressibility.Comment: Submitted to Phys. Rev.
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