161 research outputs found
Magnetic dipole excitations in nuclei: elementary modes of nucleonic motion
The nucleus is one of the most multi-faceted many-body systems in the
universe. It exhibits a multitude of responses depending on the way one
'probes' it. With increasing technical advancements of beams at the various
accelerators and of detection systems the nucleus has, over and over again,
surprised us by expressing always new ways of 'organized' structures and layers
of complexity. Nuclear magnetism is one of those fascinating faces of the
atomic nucleus we discuss in the present review. We shall not just limit
ourselves to presenting the by now very large data set that has been obtained
in the last two decades using various probes, electromagnetic and hadronic
alike and that presents ample evidence for a low-lying orbital scissors mode
around 3 MeV, albeit fragmented over an energy interval of the order of 1.5
MeV, and higher-lying spin-flip strength in the energy region 5 - 9 MeV in
deformed nuclei, nor to the presently discovered evidence for low-lying
proton-neutron isovector quadrupole excitations in spherical nuclei. To the
contrary, we put the experimental evidence in the perspectives of understanding
the atomic nucleus and its various structures of well-organized modes of motion
and thus enlarge our discussion to more general fermion and bosonic many-body
systems.Comment: 59 pages, 59 figures, accepted for publication in Rev. Mod. Phys
Electric and magnetic dipole modes in high-resolution inelastic proton scattering at
Inelastic proton scattering under extreme forward angles including
and at energies of a few hundred MeV has been established as a new
spectroscopic tool for the study of complete dipole strength distributions in
nuclei. Such data allow an extraction of the electric dipole polarizability
which provides important constraints parameters of the symmetry energy, which
determine the neutron skin thickness and the equation of state (EOS) of
neutron-rich matter. Also new insight into the much-debated nature of the pygmy
dipole resonance (PDR) is obtained. Additionally, the isovector spin-M1
resonance can be studied in heavy nuclei, where only limited experimental
information exists so far. Together with much improved results on the isoscalar
spin-M1 strength distributions in nuclei, these data shed new light on
the phenomenon of quenching of the nuclear spin response. Using dispersion
matching techniques, high energy resolution (
full width at half maximum, FWHM) can be achieved in the experiments. In
spherical-vibrational nuclei considerable fine structure is observed in the
energy region of the isovector giant dipole resonance (IVGDR). A quantitative
analysis of the fine structure with wavelet methods provides information on the
role of different damping mechanisms contributing to the width of the IVGDR.
Furthermore, level densities can be extracted from a fluctuation analysis at
excitation energies well above neutron threshold, a region hardly accessible by
other means. The combination of the gamma strength function (GSF) extracted
from the E1 and M1 strength distributions with the independently derived level
density permits novel tests of the Brink-Axel hypothesis underlying all
calculations of statistical model reaction cross sections in astrophysical
applications in the energy region of the PDR.Comment: 52 pages, 64 figures, review article submitted to Eur. Phys. J.
Gross, intermediate and fine structure of nuclear giant resonances: Evidence for doorway states
We review the phenomenon of fine structure of nuclear giant resonances and
its relation to different resonance decay mechanisms. Wavelet analysis of the
experimental spectra provides quantitative information on the fine structure in
terms of characteristic scales. A comparable analysis of resonance strength
distributions from microscopic approaches incorporating one or several of the
resonance decay mechanisms allows conclusions on the source of the fine
structure. For the isoscalar giant quadrupole resonance (ISGQR), spreading
through the first step of the doorway mechanism, i.e.\ coupling between one
particle-one hole () and two particle-two hole () states is
identified as the relevant mechanism. In heavy nuclei it is dominated by
coupling to low-lying surface vibrations, while in lighter nuclei stochastic
coupling becomes increasingly important. The fine structure observed for the
isovector giant dipole resonance (IVGDR) arises mainly from the fragmentation
of the strength (Landau damping), although some indications for the
relevance of the spreading width are also found.Comment: 15 pages, 16 figures. Submitted to Eur. Phys. J A, special issue
"Giant, pygmy, pairing resonances and related topics
Pair decay width of the Hoyle state and carbon production in stars
The pair decay width of the first excited 0⁺ state in ¹²C (the Hoyle state) is deduced from a novel analysis of the world data on inelastic electron scattering covering a wide momentum transfer range, thereby resolving previous discrepancies. The extracted value Γπ = (62.3 ± 2.0) μeV is independently confirmed by new data at low momentum transfers measured at the S-DALINAC and reduces the uncertainty of the literature values by more than a factor of three. A precise knowledge of Γπ is mandatory for quantitative studies of some key issues in the modeling of supernovae and of asymptotic giant branch stars, the most likely site of the slow-neutron nucleosynthesis process
Gamma strength functions and level densities from high-resolution inelastic proton scattering at very forward angles
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