374 research outputs found
Investigation of the fundamental constants stability based on the reactor Oklo burn-up analysis
The burn-up for SC56-1472 sample of the natural Oklo reactor zone 3 was
calculated using the modern Monte Carlo codes. We reconstructed the neutron
spectrum in the core by means of the isotope ratios: Sm/Sm and
Lu/Lu. These ratios unambiguously determine the spectrum index
and core temperature. The effective neutron absorption cross section of
Sm calculated using this spectrum was compared with experimental one.
The disagreement between these two values allows to limit a possible shift of
the low laying resonance of Sm even more . Then, these limits were
converted to the limits for the change of the fine structure constant .
We found that for the rate of change the inequality is fulfilled, which is of the next
higher order than our previous limit.Comment: 16 pages, 7 figure
What Do s- and p-Wave Neutron Average Radiative Widths Reveal
A first observation of two resonance-like structures at mass numbers 92 and 112 in the average capture widths of the p-wave neutron resonances relative to the s-wave component is interpreted in terms of a spin-orbit splitting of the 3p single-particle state into P{sub 3/2} and P{sub 1/2} components at the neutron separation energy. A third structure at about A = 124, which is not correlated with the 3p-wave neutron strength function, is possibly due to the Pygmy Dipole Resonance. Five significant results emerge from this investigation: (i) The strength of the spin-orbit potential of the optical-model is determined as 5.7 {+-} 0.5 MeV, (ii) Non-statistical effects dominate the p-wave neutron-capture in the mass region A = 85 - 130, (iii) The background magnitude of the p-wave average capture-width relative to that of the s-wave is determined as 0.50 {+-} 0.05, which is accounted for quantitatively in tenns of the generalized Fermi liquid model of Mughabghab and Dunford, (iv) The p-wave resonances arc partially decoupled from the giant-dipole resonance (GDR), and (v) Gamma-ray transitions, enhanced over the predictions of the GDR, are observed in the {sup 90}Zr - {sup 98}Mo and Sn-Ba regions
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Nuclear model tests with neutron resonance data
Previous determinations of the level density parameters, a, as derived from analyses of resonance parameter data, were carried out on the basis of the Gilbert-Cameron theoretical relation for the spin dispersion parameter, {sigma}. However, other different theoretical expressions for {sigma} have been derived previously on the basis of various nuclear models, such as the Fermi gas model, the unified Bohr-Mottelson model with and without inclusion of nucleon pairing correlations, and the Hartree-Fock model. The different expressions for {sigma} would impact the derivation of the a parameter. Because of these considerations, a new examination of the level density parameter, devoid of a reliance on a theoretical expression for {sigma}, is of great importance in various computations in the fields of reactor physics, astrophysics, and spallation neutrons. Here, the spin dispersion parameter of the level density formula, {sigma}, as determined from the spin dependent level spacings of neutron resonances, is compared with theoretical expressions and found to be in agreement with the results of Ericson. On the basis of the present results, the level density parameters, a, for a few nuclides were then derived and found to be lower than previous determinations by about 20%. The impact of the present findings on the LAHET calculations of neutron yields from thick targets of W is discussed
Test of nuclear level density inputs for Hauser-Feshbach model calculations
The energy spectra of neutrons, protons, and alpha-particles have been
measured from the d+59Co and 3He+58Fe reactions leading to the same compound
nucleus, 61$Ni. The experimental cross sections have been compared to
Hauser-Feshbach model calculations using different input level density models.
None of them have been found to agree with experiment. It manifests the serious
problem with available level density parameterizations especially those based
on neutron resonance spacings and density of discrete levels. New level
densities and corresponding Fermi-gas parameters have been obtained for
reaction product nuclei such as 60Ni,60Co, and 57Fe
Evidence for the pair-breaking process in 116,117Sn
The nuclear level densities of 116,117Sn below the neutron separation energy
have been determined experimentally from the (3He,alpha gamma) and (3He,3He
gamma') reactions, respectively. The level densities show a characteristic
exponential increase and a difference in magnitude due to the odd-even effect
of the nuclear systems. In addition, the level densities display pronounced
step-like structures that are interpreted as signatures of subsequent breaking
of nucleon pairs.Comment: 7 pages, 5 figures, accepted for publication in Phys. Rev. C, 22
December 200
Enhanced radiative strength in the quasi-continuum of 117Sn
Radiative strength functions of 117Sn has been measured below the neutron
separation energy using the (3He,3He'gamma) reactions. An increase in the slope
of the strength functions around E_gamma= 4.5 MeV indicates the onset of a
resonance-like structure, giving a significant enhancement of the radiative
strength function compared to standard models in the energy region 4.5 <=
E_gamma <= 8.0 MeV. For the first time, the functional form of this
resonance-like structure has been measured in an odd tin nucleus below neutron
threshold in the quasi-continuum region.Comment: 4 pages, 3 figure
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Neutron Cross Section Uncertainties in the Thermal and Resonance Regions
In the 'Atlas of Neutron Resonances', special care was expended to ensure that the resonance parameter information reproduces the various measured thermal cross sections, as well as the infinite dilute resonance integrals for Z = 1-100. In contrast, the uncertainties of the recommended quantities do not match those generated from the uncertainties of the resonance parameters. To address this problem, the present study was initiated to achieve consistency for 15 actinides and 21 structural and coolant moderator materials. This is realized by assigning uncertainties to the parameters of the negative-energy resonances and changing, if necessary, significantly the uncertainties of the low-lying positive-energy resonances. The influence of correlations between parameters on the derived uncertainties is examined and discussed
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Nuclear Reaction and Structure Databases of the National Nuclear Data Center
We discuss nuclear data resources of the National Nuclear Data Center (NNDC) of relevance to nuclear astrophysics applications. These resources include databases, tools and powerful web service at www.nndc.bnl.gov. Our objective is to provide an overview of nuclear databases, related products and demonstrate nuclear astrophysics potential of the ENDF/B-VII beta2 library. A detailed discussion on the Maxwellian neutron capture cross sections obtained from the ENDF/B-VII beta2 library is presented
The damping width of giant dipole resonances of cold and hot nuclei: a macroscopic model
A phenomenological macroscopic model of the Giant Dipole Resonance (GDR)
damping width of cold- and hot-nuclei with ground-state spherical and
near-spherical shapes is developed. The model is based on a generalized Fermi
Liquid model which takes into account the nuclear surface dynamics. The
temperature dependence of the GDR damping width is accounted for in terms of
surface- and volume-components. Parameter-free expressions for the damping
width and the effective deformation are obtained. The model is validated with
GDR measurements of the following nuclides, K, Ca, Sc,
Cu, Sn,Eu, Hg, and Pb, and is
compared with the predictions of other models.Comment: 10 pages, 5 figure
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