226 research outputs found
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
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
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
Determination of gamma-ray widths in N using nuclear resonance fluorescence
The stable nucleus N is the mirror of O, the bottleneck in the
hydrogen burning CNO cycle. Most of the N level widths below the proton
emission threshold are known from just one nuclear resonance fluorescence (NRF)
measurement, with limited precision in some cases. A recent experiment with the
AGATA demonstrator array determined level lifetimes using the Doppler Shift
Attenuation Method (DSAM) in O. As a reference and for testing the
method, level lifetimes in N have also been determined in the same
experiment. The latest compilation of N level properties dates back to
1991. The limited precision in some cases in the compilation calls for a new
measurement in order to enable a comparison to the AGATA demonstrator data. The
widths of several N levels have been studied with the NRF method. The
solid nitrogen compounds enriched in N have been irradiated with
bremsstrahlung. The -rays following the deexcitation of the excited
nuclear levels were detected with four HPGe detectors. Integrated
photon-scattering cross sections of ten levels below the proton emission
threshold have been measured. Partial gamma-ray widths of ground-state
transitions were deduced and compared to the literature. The photon scattering
cross sections of two levels above the proton emission threshold, but still
below other particle emission energies have also been measured, and proton
resonance strengths and proton widths were deduced. Gamma and proton widths
consistent with the literature values were obtained, but with greatly improved
precision.Comment: Final published version, minor grammar changes, 10 pages, 4 figures,
8 tables; An addendum is published where the last section is revised: T.
Sz\"ucs and P. Mohr, Phys. Rev. C 92, 044328 (2015) [arXiv:1510.04956
Calibration sources for the LEGEND-200 experiment
In the search for a monochromatic peak as the signature of neutrinoless double beta decay an excellent energy resolution and an ultra-low background around the Q-value of the decay are essential. The LEGEND-200 experiment performs such a search with high-purity germanium detectors enriched in 76Ge immersed in liquid argon. To determine and monitor the stability of the energy scale and resolution of the germanium diodes, custom-made, low-neutron emission 228Th sources are regularly deployed in the vicinity of the crystals. Here we describe the production process of the 17 sources available for installation in the experiment, the measurements of their alpha- and gamma-activities, as well as the determination of the neutron emission rates with a low-background LiI(Eu) detector operated deep underground. With a flux of (4.27±0.60stat±0.92syst)×10−4 n / (kBq⋅s), approximately one order of magnitude below that of commercial sources, the neutron-induced background rate, mainly from the activation of 76Ge, is negligible compared to other background sources in LEGEND-200
Neutron capture cross section measurements for 197Au from 3.5 to 84 keV at GELINA
Cross section measurements have been performed at the time-of-flight facility GELINA to determine the average capture cross section for 197Au in the energy region between 3.5 keV and 84 keV. Prompt gamma-rays, originating from neutron induced capture events, were detected by two C6D6 liquid scintillators. The sample was placed at about 13 m distance from the neutron source. The total energy detection principle in combination with the pulse height weighting technique was applied. The energy dependence of the neutron flux was measured with a double Frisch-gridded ionization chamber based on the 10B(n,alpha)reaction. The data have been normalized to the well-isolated and saturated 197Au resonance at 4.9 eV. Special care was taken to reduce bias effects due to the weighting function, normalization, dead time and background corrections. The total uncertainty due to normalization, neutron flux and weighting function is 1.0%. An additional uncertainty of 0.5% results from the correction for self-shielding and multiple interaction events. Fluctuations due to resonance structures have been studied by complementary measurements at a 30 m flight path station. The results reported in this work deviate systematically by more than 5% from the cross section that is recommended as a reference for astrophysical applications. They are about 2% lower compared to an evaluation of the 197Au(n,gamma) cross section, which was based on a least squares fit of experimental data available in the literature prior to this work. The average capture cross section as a function of neutron energy has been parameterized in terms of average resonance parameters. Maxwellian average cross sections at different temperatures have been calculated.JRC.D.4-Standards for Nuclear Safety, Security and Safeguard
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