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Consistent Data Assimilation of Structural Isotopes: 23Na and 56Fe

Description: A new approach is proposed, the consistent data assimilation, that allows to link the integral data experiment results to basic nuclear parameters employed by evaluators to generate ENDF/B point energy files in order to improve them. Practical examples are provided for the structural materials 23Na and 56Fe. The sodium neutron propagation experiments, EURACOS and JANUS-8, are used to improve via modifications of 23Na nuclear parameters (like scattering radius, resonance parameters, Optical model parameters, Statistical Hauser-Feshbach model parameters, and Preequilibrium Exciton model parameters) the agreement of calculation versus experiments for a series of measured reaction rate detectors slopes. For the 56Fe case the EURACOS and ZPR3 assembly 54 are used. Results have shown inconsistencies in the set of nuclear parameters used so that further investigation is needed. Future work involves comparison of results against a more traditional multigroup adjustments, and extension to other isotope of interest in the reactor community.
Date: September 1, 2010
Creator: Palmiotti, Giuseppe
Partner: UNT Libraries Government Documents Department

Excitations and Optical Properties of Phenylene Based Polymers

Description: A complex picture of phenylene-based polymers is developed which unifies features of band and molecular exciton models. It incorporates major experimental finding in direct, and photoinduced optical absorption, stimulated photoemission and photoconductivity. The authors give new assignments for the most disputed features and identify new ones as edge states. The authors confirm a low binding energy for the principle emitting exciton and show that it dominates also in the fundamental absorption. Contradictions in the current modeling state-of-art are displayed and discussed.
Date: July 1, 1998
Creator: Kirova, N.; Brazovskii, S.; Bishop, A.R.; McBranch, D. & Klimov, V.
Partner: UNT Libraries Government Documents Department

Use of Covariance Matrices in a Consistent (Multiscale) Data Assimilation for Improvement of Basic Nuclear Parameters in Nuclear Reactor Applications: From Meters to Femtometers

Description: A new approach is proposed, the consistent data assimilation, that allows to link the integral data experiment results to basic nuclear parameters employed by evaluators to generate ENDF/B point energy files in order to improve them. A practical example is provided where the sodium neutron propagation experiments, EURACOS and JANUS-8, are used to improve via modifications of 23Na nuclear parameters (like scattering radius, resonance parameters, Optical model parameters, Statistical Hauser-Feshbach model parameters, and Preequilibrium Exciton model parameters) the agreement of calculation versus experiments for a series of measured reaction rate detectors slopes. Future work involves comparison of results against a more traditional multigroup adjustments, and extension to other isotope of interest in the reactor community as 56Fe, actinides, and fission products.
Date: April 1, 2010
Creator: Palmiotti, G.; Salvatores, M.; Hiruta, H.; Herman, M.; Oblozinsky, P. & Pigni, M.
Partner: UNT Libraries Government Documents Department

Electronic and optical response of functionalized Ru(II) complexes: joint theoretical and experimental study

Description: New photovoltaic and photocatalysis applications have been recently proposed based on the hybrid Ru(II)-bipyridine-complex/semiconductor quantum dot systems. In order to attach the Ru(II) complex to the surface of a semiconductor, a linking bridge -- a carboxyl group -- needs to be added to one or two of the 2,2'-bipyridine (bpy) ligands. Such changes in the ligand structure affect electronic and optical properties and, consequently, the charge transfer reactivity of Ru(II)-systems. In this study, we analyze the effects brought by functionalization of bipyridine ligands with the methyl, carboxyl, and carboxilate groups on the electronic structure and optical response of the [Ru(bpy){sub 3}]{sup 2+} complex. First principle calculations based on density functional theory (DFT) and time dependent DFT (TDDFT) are used to simulate the ground and excited-state properties, respectively, of functionalized Ru-complexes in the gas phase and acetonitrile solution. In addition, an effective Frenkel exciton model is used to explain the optical activity and splitting patterns of the low-energy excited states in all molecules. All theoretical results nicely complement and allow for detailed interpretation of experimental absorption spectra of Ru-complexes that have been done in parallel with our theoretical investigations. We found that the carboxyl group breaks the degeneracy of two low-energy optically bright excited states and red-shifts the absorption spectrum, while leaves ionization and affinity energies of complexes almost unchanged. Experimental studies show that deprotonation of the carboxyl group in the Ru-complexes results in a slight blue shift and decrease of oscillator strengths of the low energy absorption peaks. Comparison of experimental and theoretical linear response spectra of deprotonated complexes demonstrate strong agreement if the theoretical calculations are performed with the addition of a dielectric continuum model. A polar solvent is found to play an important role in calculations of optical spectra: it stabilizes the energy of states localized on the ...
Date: January 1, 2008
Creator: Kilina, Svetlana; Tretiak, Sergei; Sykora, Milan; Albert, Victor; Badaeva, Ekaterina & Koposov, Alexey
Partner: UNT Libraries Government Documents Department

PRECO-D2: program for calculating preequilibrium and direct reaction double differential cross sections

Description: The code PRECO-D2 uses the exciton model for preequilibrium nuclear reactions to describe the emission of particles with mass numbers of 1 to 4 from an equilibrating composite nucleus. A distinction is made between open and closed configurations in this system and between the multi-step direct (MSD) and multi-step compound (MSC) components of the preequilibrium cross section. Additional MSD components are calculated semi-empirically to account for direct nucleon transfer reactions and direct knockout processes involving cluster degrees of freedom. Evaporation from the equilibrated composite nucleus is included in the full MSC cross section. Output of energy differential and double differential cross sections is provided for the first particle emitted from the composite system. Multiple particle emission is not considered. This report describes the reaction models used in writing PRECO-D2 and explains the organization and utilization of the code. 21 refs.
Date: February 1, 1985
Creator: Kalbach, C.
Partner: UNT Libraries Government Documents Department

Pairing interaction effects in exciton level densities

Description: Recent progress in pairing corrections for exciton state-density formulas used in pre-compound nuclear reaction theories is reviewed. These correction factors are, strictly speaking, dependent on the nuclear excitation energy U and the exciton number n. A simple formula for (U,n)-dependent pairing corrections has been derived, based on the BCS pairing equations for constant single-particle spacing, for the exciton state-density formula for one kind of Fermion. It has been shown that the constant-pairing-energy correction used in standard state-density formulas, such U{sub 0} in Gilbert and Cameron, is a limiting case of the general (U,n)-dependent results. Spin cutoff factors with pairing effects were also obtained using the same theory and parameterized into an explicit (U,n)-dependent function, thereby defining a simple exciton level-density formula for applications in quantum mechanical precompound theories. Preliminary results from extending such simple pairing-interaction representations to level-density formulas for two kinds of Fermions are summarized. The results show that the ratios in the exciton level densities in the one-Fermion and two-Fermion approaches vary with both U and n, thus likely leading to differences in calculated compound to precompound ratios. However, the differences in the spin cutoff factors in the two cases are found to be rather small. 12 refs., 3 figs.
Date: January 1, 1989
Creator: Fu, C.Y.
Partner: UNT Libraries Government Documents Department

Comparison of 14 MeV isomer production of {sup 178m2}Hf and {sup 179m2}Hf using Feshbach-Kerman-Koonin and exciton preequilibrium models

Description: The {sup 178m2}Hf(16+) isomeric state has a 31-yr half life and could pose serious radioactive problems in nuclear fusion reactors if its production in 14 MeV neutron-induced reactions is significant. We present statistical/preequilibrium model calculations for the production of this isomer in the {sup 179}Hf(n, 2n) {sup 178m2}Hf reaction, as well as the 25-days 12.5{sup {minus}} isomer in the {sup 179}Hf(n,n{prime}){sup 179m2}Hf reaction, using two different preequilibrium models: the exciton model and the Feshbach-Kerman-Koonin (FKK) theory. Our calculations which use the exciton model agree well with measurements, but those with the FKK theory underestimate measurements. Our calculations axe the first to probe angular momentum transfer effects in the FKK theory and suggest that, as it is presently applied, high spin-transfer reactions are underestimated. We suggest modifications to the FKK statistical averaging procedure which may result in an improved agreement with experiment.
Date: November 1, 1993
Creator: Chadwick, M. B. & Young, P. G.
Partner: UNT Libraries Government Documents Department

Application of multistep compound and multistep direct models for data evaluation

Description: We have implemented the quantum mechanical multistep compound (MSC) and multistep direct (MSD) theories of Feshbach, Kerman, and Koonin (FKK) for the calculation of nucleon-induced multistep reactions. Our code system, FKK-GNASH, uses the FKK theory for primary preequilibrium emission and describes subsequent equilibrium emission using the Hauser-Feshbach code GNASH. The MSC contribution yields emitted particles with angular distributions symmetric about ninety degrees, whereas the MSD contribution, calculated by averaging DWBA cross sections for particle-hole excitations in the continuum, results in forward-peaked preequilibrium emission. The original picture of the evolution of the reaction, as described by FKK, is modified to allow transitions from the MSD to MSC chain. This modification is consistent with semiclassical preequilibrium descriptions, and allows a good description of preequilibrium spectra and angular distributions for a range of different reactions. As an example of our methodology, we calculate {sup 93}Nb(n,n') reactions at 14, 20 and 26 MeV, and {sup 93}Nb(n,p) at 14 MeV, comparing our results with experimental data. Use of the FKK theory has the advantage that preequilibrium calculations are based on rigorous physical principles, and (unlike semiclassical approaches) allow a good description of angular distributions. We argue that our implementation of the FKK theory maximizes the level of predictability, which is needed for data evaluations.
Date: January 1, 1992
Creator: Chadwick, M.B. & Young, P.G.
Partner: UNT Libraries Government Documents Department

Consistent nuclear model for compound and precompound reactions with conservation of angular momentum. [14. 6 MeV]

Description: The exciton model is modified such that it automatically reduces to the usual evaporation formula after equilibrium has been reached. The result is further modified to conserve angular momentum in a form compatible with the Hauser-Feshbach formula. This approach allows a consistent description of intermediate excitations from which tertiary reaction cross sections can be calculated for transitions to discrete residual levels with known spins and parities. Level densities used for the compound component of reaction cross sections are derived from direct summation of the particle-hole state densities used for the precompound component.
Date: January 1, 1979
Creator: Fu, C.Y.
Partner: UNT Libraries Government Documents Department

Multiple preequilibrium decay processes

Description: Several treatments of multiple preequilibrium decay are reviewed with emphasis on the exciton and hybrid models. We show the expected behavior of this decay mode as a function of incident nucleon energy. The algorithms used in the hybrid model treatment are reviewed, and comparisons are made between predictions of the hybrid model and a broad range of experimental results. 24 refs., 20 figs.
Date: November 1, 1987
Creator: Blann, M.
Partner: UNT Libraries Government Documents Department