66 research outputs found
Impact of Triaxiality on the Emission and Absorption of Neutrons and Gamma Rays in Heavy Nuclei
For many spin-0 target nuclei neutron capture measurements yield information
on level densities at the neutron separation energy. Also the average photon
width has been determined from capture data as well as Maxwellian average cross
sections for the energy range of unresolved resonances. Thus it is challenging
to use this data set for a test of phenomenological prescriptions for the
prediction of radiative processes. An important ingredient for respective
calculations is the photon strength function for which a parameterization was
proposed using a fit to giant dipole resonance shapes on the basis of
theoretically determined ground state deformations including triaxiality.
Deviations from spherical and axial symmetry also influence level densities and
it is suggested to use a combined parameterization for both, level density and
photon strength. The formulae presented give a good description of the data for
low spin capture into 124 nuclei with 72<A<244 and only very few global
parameters have to be adjusted when the predetermined information on ground
state shapes of the nuclei involved is accounted for.Comment: Contribution to the Proceedings of Scientific Workshop on Nuclear
Fission Theory-2, November 2012 at Biarritz, Franc
Importance of nuclear triaxiality for electromagnetic strength, level density and neutron capture cross sections in heavy nuclei
Cross sections for neutron capture in the range of unresolved resonances are
predicted simultaneously to level distances at the neutron threshold for more
than 100 spin-0 target nuclei with A >70. Assuming triaxiality in nearly all
these nuclei a combined parameterization for both, level density and photon
strength is presented. The strength functions used are based on a global fit to
IVGDR shapes by the sum of three Lorentzians adding up to the TRK sum rule and
theory-based predictions for the A-dependence of pole energies and spreading
widths. For the small spins reached by capture level densities are well
described by only one free global parameter; a significant collective
enhancement due to the deviation from axial symmetry is observed. Reliable
predictions for compound nuclear reactions also outside the valley of stability
as expected from the derived global parameterization are important for nuclear
astrophysics and for the transmutation of nuclear waste.Comment: Contribution to the proceedings of the ERINDA workshop held at CERN
in October 2013 with modification
Breaking of axial symmetry in excited heavy nuclei as identified in Giant Dipole Resonance data
A recent theoretical prediction of a breaking of axial symmetry in quasi all
heavy nuclei is confronted to a new critical analysis of photon strength
functions of nuclei in the valley of stability. For the photon strength in the
isovector giant dipole resonance (IVGDR) regime a parameterization of GDR
shapes by the sum of three Lorentzians (TLO) is extrapolated to energies below
and above the IVGDR. The impact of non-GDR modes adding to the low energy slope
of photon strength is discussed including recent data on photon scattering and
other radiative processes. These are shown to be concentrated in energy regions
where various model calculations predict intermediate collective strength; thus
they are obviously separate from the IVGDR tail. The triple Lorentzian (TLO)
ansatz for giant dipole resonances is normalized in accordance to the dipole
sum rule. The nuclear droplet model with surface dissipation accounts well for
positions and widths without local, nuclide specific, parameters. Very few and
only global parameters are needed when a breaking of axial symmetry already in
the valley of stability is admitted and hence a reliable prediction for
electric dipole strength functions also outside of it is expected.Comment: 21 pages, 21 figures, PACS: 26.50.+x, 25.20.Dc,27.60.+j Accepted by
Europ. Phys. Journal
Description of dipole strength in heavy nuclei in conformity with their quadrupole degrees of freedom
In conformity to new findings about the widespread occurrence of triaxiality
arguments are given in favor of a description of the giant dipole resonance in
heavy nuclei by the sum of three Lorentzians. This TLO parameterization allows
a strict use of resonance widths {\Gamma} in accordance to the theoretically
founded power law relation to the resonance energy. No additional variation of
{\Gamma} with the photon energy and no violation of the sum rule are necessary
to obtain a good agreement to nuclear photo-effect, photon scattering and
radiative capture data. Photon strength other than E1 has a small effect, but
the influence of the level density on photon emission probabilities needs
further investigation.Comment: Presented at the 3rd International Workshop on Compound Nuclear
Reactions and Related Topics at Prague, 2011, to be published via the EPJ Web
of Conference
Strength of the =1.842 MeV resonance in the Ca(p,)Sc reaction revisited
The strength of the MeV resonance in the
Ca(p,)Sc reaction is determined with two different
methods: First, by an absolute strength measurement using calcium hydroxide
targets, and second, relative to the well-determined strength of the resonance
triplet at = 4.5 MeV in the Ca(,)Ti
reaction. The present new value of eV is 37%
(equivalent to ) higher than the evaluated literature value. In
addition, the ratio of the strengths of the 1.842 MeV
Ca(p,)Sc and 4.5 MeV
Ca(,)Ti resonances has been determined to be
. The newly corrected strength of the 1.842-MeV resonance can
be used in the future as a normalization point for experiments with calcium
targets.Comment: Submitted to Phys. Rev.
- …