25 research outputs found
Pygmy dipole resonance in 208Pb
Scattering of protons of several hundred MeV is a promising new spectroscopic
tool for the study of electric dipole strength in nuclei. A case study of 208Pb
shows that at very forward angles J^pi = 1- states are strongly populated via
Coulomb excitation. A separation from nuclear excitation of other modes is
achieved by a multipole decomposition analysis of the experimental cross
sections based on theoretical angular distributions calculated within the
quasiparticle-phonon model. The B(E1) transition strength distribution is
extracted for excitation energies up to 9 MeV, i.e., in the region of the
so-called pygmy dipole resonance (PDR). The Coulomb-nuclear interference shows
sensitivity to the underlying structure of the E1 transitions, which allows for
the first time an experimental extraction of the electromagnetic transition
strength and the energy centroid of the PDR.Comment: submitted to Phys. Rev.
Complete electric dipole response and the neutron skin in 208Pb
A benchmark experiment on 208Pb shows that polarized proton inelastic
scattering at very forward angles including 0{\deg} is a powerful tool for
high-resolution studies of electric dipole (E1) and spin magnetic dipole (M1)
modes in nuclei over a broad excitation energy range to test up-to-date nuclear
models. The extracted E1 polarizability leads to a neutron skin thickness
r_skin = 0.156+0.025-0.021 fm in 208Pb derived within a mean-field model [Phys.
Rev. C 81, 051303 (2010)], thereby constraining the symmetry energy and its
density dependence, relevant to the description of neutron stars.Comment: 5 pages, 5 figures, revised mansucrip
Exclusive electroproduction of K+ Lambda and K+ Sigma^0 final states at Q^2 = 0.030-0.055 (GeV/c)^2
Cross section measurements of the exclusive p(e,e'K+)Lambda,Sigma^0
electroproduction reactions have been performed at the Mainz Microtron MAMI in
the A1 spectrometer facility using for the first time the Kaos spectrometer for
kaon detection. These processes were studied in a kinematical region not
covered by any previous experiment. The nucleon was probed in its third
resonance region with virtual photons of low four-momenta, Q^2= 0.030-0.055
(GeV/c)^2. The MAMI data indicate a smooth transition in Q^2 from
photoproduction to electroproduction cross sections. Comparison with
predictions of effective Lagrangian models based on the isobar approach reveal
that strong longitudinal couplings of the virtual photon to the N* resonances
can be excluded from these models.Comment: 16 pages, 7 figure
Event reconstruction methods for the HypHI Phase 0 experiment at GSI
The first experiment of the HypHI project, Phase 0, at GSI-Darmstadt performs the feasibility study of precise hypernuclear spectroscopy with heavy ion induced reactions by using the collision of (6)Li at 2 A GeV on a (12)C target. In preparation of the Phase 0 experiment, simulation studies have been performed with a dedicated event reconstruction procedure. Details of this procedure will be discussed in this paper as well as the physics motivation of the Phase 0 and the experimental setup used during simulations. Expected results of the experiment will also be discussed. (C) 2010 Elsevier B.V. All rights reserved
Low-energy dipole strength in Sn 112 , 120
The 112,120Sn(gamma,gamma') reactions below the neutron separation energies
have been studied at the superconducting Darmstadt electron linear accelerator
S-DALINAC for different endpoint energies of the incident bremsstrahlung
spectrum. Dipole strength distributions are extracted for 112Sn up to 9.5 MeV
and for 120Sn up to 9.1 MeV. A concentration of dipole excitations is observed
between 5 and 8 MeV in both nuclei. Missing strength due to unobserved decays
to excited states is estimated in a statistical model. A fluctuation analysis
is applied to the photon scattering spectra to extract the amount of the
unresolved strength hidden in background due to fragmentation. The strength
distributions are discussed within different model approaches such as the
quasiparticle-phonon model and the relativistic time blocking approximation
allowing for an inclusion of complex configurations beyond the initial
particle-hole states. While a satisfactory description of the fragmentation can
be achieved for sufficently large model spaces, the predicted centroids and
total electric dipole strengths for stable tin isotopes strongly depend on the
assumptions about the underlying mean field.Comment: 14 pages, 12 figures, revised version, accepted for publication in
Phys. Rev.