45 Matching Results

Search Results

Advanced search parameters have been applied.

New yttrium evaluated cross sections and impact on 88-Y(n,2n)87-Y radchem

Description: We evaluate new n+{sup 89}Y radchem cross sections using recent LANSCE/GEANIE measurements and GNASH nuclear model calculations, together with previous measurements at Livermore by Dietrich et al. A quantification of margins and uncertainties (QMU) analysis leads to evaluated cross sections for the (n,2n) population of the {sup 88}Y ground state and m1, m2 isomers, together with uncertainties. Our new results agree with historic radchem database cross sections within a few percent below 15 MeV (with larger differences above 15 MeV) and they therefore provide a validation of the historic Arthur work that is used in LANL simulation codes. Since the (n,2n) cross sections to the {sup 88}Y g.s. and m1, m2 isomers impact the average {sup 88}Y(n,2n){sup 87}Y cross section at leading-order, we determine the new 14.1 MeV average {sup 88}Y(n,2n){sup 87}Y cross section (crucially important for radchem). Our new 14 MeV average {sup 88}Y(n,2n){sup 87}Y cross section is 1107 mb ({+-} 4%) which agrees with the value obtained from the historic Arthur cross section data to 0.7%.
Date: March 4, 2004
Creator: White, M; Kawano, T; Fotiadis, N; Devlin, M; Nelson, R; Garrett, P et al.
Partner: UNT Libraries Government Documents Department

The 239 Pu(n,2n)238Pu cross section: preliminary calculations

Description: The primary motivation for the present work is to provide theoretical values for the ratio of the partial <sup>239</sup>Pu(n,2nx{gamma})<sup>238</sup>Pu to total <sup>239</sup> Pu(n,2n)<sup>238</sup>Pu cross section for several discrete gamma transitions. Results and conclusions of preliminary calculations from threshold to 20 MeV are presented. Calculations are based on theoretical models with parameters obtained from the literature or from our ad hoc systematics. Optical model cross sections and transmission coefficients were determined using the coupled-channels method. The calculations included a preequilibrium component followed by multiple particle and gamma-ray emissions. Fission competition was included at all stages of de-excitation. Suggestions for further verifications and possible improvements are provided.
Date: March 12, 1999
Creator: Chen, H.; Reffo, G.; Ross, M. A. & White, R. M.
Partner: UNT Libraries Government Documents Department

Extended direct-semidirect mechanism and the role of multistep processes in fast-nucleon radiative capture

Description: The authors have recently developed an extension of the direct semidirect (DSD) radiative capture model to unstable final stages and have confirmed its utility in explaining the spectrum of {gamma} rays from capture of polarized 19.6 MeV protons on {sup 89}Y. It was found that the extended DSD model, supplemented by a Hauser Feshbach contribution, successfully explains the observed {gamma} spectra, angular distributions, and analyzing powers, without requiring additional mechanisms, such as precompound or multistep emission, or nucleon-nucleon bremsstrahlung. In this contribution they show that the model also successfully explains data at higher energies (34 MeV incident protons), and that there is no need for additional contributions other than Hauser-Feshbach at this energy as well.
Date: March 1, 1998
Creator: Dietrich, F.S.; Chadwick, M.B. & Kerman, A.K.
Partner: UNT Libraries Government Documents Department

Modeling (p,xn) reactions producing proton-rich nuclides for radioactive ion beam studies

Description: Over the last few years, an intense worldwide interest has grown in the use of radioactive ion beams (RIB) to study the properties of nuclei far from stability. The relatively recent technological developments that have enabled the production of high-quality radioactive beams promise to lead to a renaissance in nuclear structure and nuclear reaction physics, as well as nuclear astrophysics. Group T-2 at Los Alamos has been working with US Nuclear Reaction Data Network Radioactive Ion Beam Task Force to address RIB nuclear data needs. The current focus is nuclear data needed to guide the choice of targets for the production of various radioactive product species using the isotope-separation on line (ISOL) method; future work will include the study of RIB nuclear reaction and structure physics. Laboratories that are undertaking RIB studies with the ISOL method in the US are principally Oak Ridge National Laboratory (ORNL) and Argonne National Laboratory (ANL).
Date: March 1, 1998
Creator: Chadwick, M.B. & Young, P.G.
Partner: UNT Libraries Government Documents Department

Two-component multistep direct reactions: A microscopic approach

Description: The authors present two principal advances in multistep direct theory: (1) A two-component formulation of multistep direct reactions, where neutron and proton excitations are explicitly accounted for in the evolution of the reaction, for all orders of scattering. While this may at first seem to be a formidable task, especially for multistep processes where the many possible reaction pathways becomes large in a two-component formalism, the authors show that this is not so -- a rather simple generalization of the FKK convolution expression 1 automatically generates these pathways. Such considerations are particularly relevant when simultaneously analyzing both neutron and proton emission spectra, which is always important since these processes represent competing decay channels. (2) A new, and fully microscopic, method for calculating MSD cross sections which does not make use of particle-hole state densities but instead directly calculates cross sections for all possible particle-hole excitations (again including an exact book-keeping of the neutron/proton type of the particle and hole at all stages of the reaction) determined from a simple non-interacting shell model. This is in contrast to all previous numerical approaches which sample only a small number of such states to estimate the DWBA strength, and utilize simple analytical formulae for the partial state density, based on the equidistant spacing model. The new approach has been applied, along with theories for multistep compound, compound, and collective reactions, to analyze experimental emission spectra for a range of targets and energies. The authors show that the theory correctly accounts for double-differential nucleon spectra.
Date: March 1, 1998
Creator: Koning, A.J. & Chadwick, M.B.
Partner: UNT Libraries Government Documents Department

Some guidelines for the evaluation of nuclear data

Description: Modern data evaluation methodology draws upon basic principles from statistics. It differs from earlier ad hoc approaches which are completely subjective (e.g., eye guides to data) or are objective in a limited sense (e.g., combinations of reported data by a simple least-squares procedure without regard to correlations in the data errors or a careful scrutiny of the data included in the evaluation). In addition to utilizing more rigorous mathematical procedures, modern evaluation methodology involves taking great care to insure that the data which are being evaluated are equivalent to what has been assumed in the evaluation model and that the values are consistent with respect to the use of standards and other fundamental physical parameters. This short memorandum cannot substitute for more comprehensive treatments o the subject such as can be found in the listed references. The intent here is to provide an overview of the topic and to impress upon the reader that the evaluation of data of any sort is not a straightforward enterprise. Certainly evaluations cannot be carried out automatically with computer codes without considerable intervention on the part of the evaluator.
Date: March 20, 1996
Creator: Smith, D.L.
Partner: UNT Libraries Government Documents Department

Dynamics of density fluctuations in a non-Markovian Boltzmann- Langevin model

Description: In the course of the past few years, the nuclear Boltzmann-Langevin (BL)model has emerged as a promising microscopic model for nuclear dynamics at intermediate energies. The BL model goes beyond the much employed Boltzmann-Uehling-Uhlenbeck (BUU) model, and hence it provides a basis for describing dynamics of density fluctuations and addressing processes exhibiting spontaneous symmetry breaking and catastrophic transformations in nuclear collisions, such as induced fission and multifragmentation. In these standard models, the collision term is treated in a Markovian approximation by assuming that two-body collisions are local in both space and time, in accordance with Boltzmann`s original treatment. This simplification is usually justified by the fact that the duration of a two-body collision is short on the time scale characteristic of the macroscopic evolution of the system. As a result, transport properties of the collective motion has then a classical character. However, when the system possesses fast collective modes with characteristic energies that are not small in comparision with the temperature, then the quantum-statistical effects are important and the standard Markovian treatment is inadequate. In this case, it is necessary to improve the one-body transport model by including the memory effect due to the finite duration of two-body collisions. First we briefly describe the non-Markovian extension of the BL model by including the finite memory time associated with two-body collisions. Then, using this non-Markovian model in a linear response framework, we investigate the effect of the memory time on the agitation of unstable modes in nuclear matter in the spinodal zone, and calculate the collisional relaxation rates of nuclear collective vibrations.
Date: March 1, 1996
Creator: Ayik, S.
Partner: UNT Libraries Government Documents Department

Coulomb reacceleration as a clock for nuclear reactions -- II

Description: Reacceleration effects in the Coulomb breakup of nuclei are modeled with the two-dimensional time-dependent Schroedinger equation, extending a previous one-dimensional study. The present model better describes the individual contributions of longitudinal and transverse forces to the breakup and reacceleration. Reacceleration effects are found to preserve a strong memory of the pre-breakup phase of the reaction, as was concluded with the one-dimensional model.
Date: March 1, 1994
Creator: Bertulani, C. A. & Bertsch, George F.
Partner: UNT Libraries Government Documents Department

Nuclear structure calculations for astrophysical applications

Description: Here we present calculated results on such diverse properties as nuclear energy levels, ground-state masses and shapes, {beta}-decay properties and fission-barrier heights. Our approach to these calculations is to use a unified theoretical framework within which the above properties can all be studied. The results are obtained in the macroscopic-microscopic approach in which a microscopic nuclear-structure single-particle model with extensions is combined with a macroscopic model, such as the liquid drop model. In this model the total potential energy of the nucleus may be calculated as a function of shape. The maxima and minima in this function correspond to such features as the ground state, fission saddle points and shape-isomeric states. Various transition rate matrix elements are determined from wave-functions calculated in the single-particle model with pairing and other relevant residual interactions taken into account.
Date: March 13, 1992
Creator: Moeller, P.; Kratz, K. L. & Nix, J. R.
Partner: UNT Libraries Government Documents Department

Nuclear structure calculations for astrophysical applications

Description: Here we present calculated results on such diverse properties as nuclear energy levels, ground-state masses and shapes, {beta}-decay properties and fission-barrier heights. Our approach to these calculations is to use a unified theoretical framework within which the above properties can all be studied. The results are obtained in the macroscopic-microscopic approach in which a microscopic nuclear-structure single-particle model with extensions is combined with a macroscopic model, such as the liquid drop model. In this model the total potential energy of the nucleus may be calculated as a function of shape. The maxima and minima in this function correspond to such features as the ground state, fission saddle points and shape-isomeric states. Various transition rate matrix elements are determined from wave-functions calculated in the single-particle model with pairing and other relevant residual interactions taken into account.
Date: March 13, 1992
Creator: Moeller, P.; Kratz, K.L. (Mainz Univ. (Germany). Inst. fuer Kernchemie) & Nix, J.R. (Los Alamos National Lab., NM (United States))
Partner: UNT Libraries Government Documents Department

Systematic effects in CALOR simulation code to model experimental configurations

Description: CALOR89 code system is being used to simulate test beam results and the design parameters of several calorimeter configurations. It has been bench-marked against the ZEUS, D{theta} and HELIOS data. This study identifies the systematic effects in CALOR simulation to model the experimental configurations. Five major systematic effects are identified. These are the choice of high energy nuclear collision model, material composition, scintillator saturation, shower integration time, and the shower containment. Quantitative estimates of these systematic effects are presented. 23 refs., 6 figs., 7 tabs.
Date: March 27, 1991
Creator: Job, P.K.; Proudfoot, J. (Argonne National Lab., IL (USA)); Handler, T. (Tennessee Univ., Knoxville, TN (USA). Dept. of Physics and Astronomy) & Gabriel, T.A. (Oak Ridge National Lab., TN (USA))
Partner: UNT Libraries Government Documents Department

Proceedings of a symposium on the occasion of the 40th anniversary of the nuclear shell model

Description: This report contains papers on the following topics: excitation of 1p-1h stretched states with the (p,n) reaction as a test of shell-model calculations; on Z=64 shell closure and some high spin states of {sup 149}Gd and {sup 159}Ho; saturating interactions in {sup 4}He with density dependence; are short-range correlations visible in very large-basis shell-model calculations ; recent and future applications of the shell model in the continuum; shell model truncation schemes for rotational nuclei; the particle-hole interaction and high-spin states near A-16; magnetic moment of doubly closed shell +1 nucleon nucleus {sup 41}Sc(I{sup {pi}}=7/2{sup {minus}}); the new magic nucleus {sup 96}Zr; comparing several boson mappings with the shell model; high spin band structures in {sup 165}Lu; optical potential with two-nucleon correlations; generalized valley approximation applied to a schematic model of the monopole excitation; pair approximation in the nuclear shell model; and many-particle, many-hole deformed states.
Date: March 1, 1990
Creator: Lee, T. S. H. & Wiringa, R. B. (eds.)
Partner: UNT Libraries Government Documents Department

Quasifragments: hot nuclei embedded in a nucleon vapor

Description: In nuclear collisions at intermediate energies, metastable complex nuclear fragments are produced abundantly during the primary stage of the reaction. Therefore, statistical models for nuclear disassembly must incorporate highly excited unstable fragment states in the phase space considered. In our previous treatments of nuclear disassembly, a particular unstable fragment state was included in the final phase space provided its (estimated) half-life exceeded the time characterizing the breakup process. A similar prescription was also employed in the recent exact microcanonical model of nuclear disassembly. Although such life-time arguments are intuitively appealing in the context of a disassembling source, their relevance is less clear for the treatment of static problems, e.g. excited infinite nuclear matter at subsaturation densities. It is therefore desirable to seek a better foundation for the description of high excited nuclear states, applicable to both static and dynamical scenarios. This is also practically important for the implementation of event generators developed to provide samples of multi-fragment final states of medium-energy nuclear collisions. Furthermore, microscopic dynamical simulations also encounter the problem when seeking to give a realistic description of the final nuclear fragments.
Date: March 1, 1988
Creator: Fai, G. & Randrup, J.
Partner: UNT Libraries Government Documents Department

Topics in nuclear chromodynamics: Color transparency and hadronization in the nucleus

Description: The nucleus plays two complimentary roles in quantum chromodynamics: (1) A nuclear target can be used as a control medium or background field to modify or probe quark and gluon subprocesses. Some novel examples are color transparency, the predicted transparency of the nucleus to hadrons participating in high momentum transfer exclusive reactions, and formation zone phenomena, the absence of hard, collinear, target-induced radiation by a quark or gluon interacting in a high momentum transfer inclusive reaction if its energy is large compared to a scale proportional to the length of the target. (Soft radiation and elastic initial state interactions in the nucleus still occur.) Coalescence with co-moving spectators is discussed as a mechanism which can lead to increased open charm hadroproduction, but which also suppresses forward charmonium production (relative to lepton pairs) in heavy ion collisions. Also discussed are some novel features of nuclear diffractive amplitudes--high energy hadronic or electromagnetic reactions which leave the entire nucleus intact and give nonadditive contributions to the nuclear structure function at low /kappa cur//sub Bj/. (2) Conversely, the nucleus can be studied as a QCD structure. At short distances, nuclear wave functions and nuclear interactions necessarily involve hidden color, degrees of freedom orthogonal to the channels described by the usual nucleon or isobar degrees of freedom. At asymptotic momentum transfer, the deuteron form factor and distribution amplitude are rigorously calculable. One can also derive new types of testable scaling laws for exclusive nuclear amplitudes in terms of the reduced amplitude formalism.
Date: March 1, 1988
Creator: Brodsky, S.J.
Partner: UNT Libraries Government Documents Department

A fresh look at Bose-Einstein correlations

Description: Recent experimental data on Bose-Einstein (BE) correlations between identical bosons are reviewed, and new results concerning the interpretation of the BE enhancement are discussed. In particular, it is emphasized that the classical interpretation of the correlation function in terms of the space-time distribution of particle production points cannot be directly applied to particle production in high energy reactions.
Date: March 1, 1987
Creator: Hofmann, W.
Partner: UNT Libraries Government Documents Department

Complex fragment emission from hot compound nuclei

Description: The experimental evidence for compound nucleus emission of complex fragments at low energies is used to interpret the emission of the same fragments at higher energies. The resulting experimental picture is that of highly excited compound nuclei formed in incomplete fusion processes which decay statistically. In particular, complex fragments appear to be produced mostly through compound nucleus decay. In the appendix a geometric-kinematic theory for incomplete fusion and the associated momentum transfer is outlined. 10 refs., 19 figs.
Date: March 1, 1986
Creator: Moretto, L.G.
Partner: UNT Libraries Government Documents Department

Fusion-fission in the /sup 16/O + /sup 40,44/Ca and /sup 32/S + /sup 24/Mg reactions

Description: In measurements of fully-damped yields for the /sup 16/O + /sup 40,44Ca and /sup 32/S + /sup 24/Mg reactions, evidence for a fusion-fission reaction mechanism were found. The experiments were performed at the Argonne tandem-linac booster facility. Reaction products with masses 22 < A < 36 for the /sup 16/O + /sup 40,44Ca reactions and 10 < A < 28 for the /sup 32/S + /sup 24/Mg reaction were detected and identified using the time-of-flight technique. The /sup 16/O + /sup 40,44Ca reactions were measured at several center-of-mass energies from 49.5 to 62.5 MeV, while the /sup 32/S + /sup 24/Mg data were obtained at a single energy of E/sub cm/ = 60 MeV. The resulting total kinetic energies (calculated assuming two-body kinematics), angular distributions, and mass distributions of the fragments are found to be consistent with a fusion-fission reaction mechanism. An alternative explanation of the yields in terms of a deep-inelastic scattering mechanism is ruled out by the asymmetric nature of the mass distributions: the maximum cross section for the /sup 32/S + /sup 224/Mg reaction is detected in the mass 12 channel whereas the maximum cross sections for deep-inelastic scattering would be expected to occur near the target-projectile mass partition. For the /sup 32/S + /sup 4/Mg reaction at E/sub cm/ = 60 MeV, the fission cross sections is found to be approximately 6% of the evaporation-residue cross section. These results indicate that at higher energies fission competes with light particle evaporation in compound nuclear decay - even in these light systems. 8 refs.
Date: March 1, 1986
Creator: Sanders, S.J.; Back, B.B.; Betts, R.R.; Dichter, B.K.; Kaufman, S.; Kovar, D.G. et al.
Partner: UNT Libraries Government Documents Department

Friction in nuclear dynamics

Description: The problem of dissipation in nuclear dynamics is related to the breaking down of nuclear symmetries and the transition from ordered to chaotic nucleonic motions. In the two extreme idealizations of the perfectly Ordered Regime and the fully Chaotic Regime, the nucleus should behave as an elastic solid or an overdamped fluid, respectively. In the intermediate regime a complicated visco-elastic behaviour is expected. The discussion is illustrated by a simple estimate of the frequency of the giant quadrupole resonance in the Ordered Regime and by applications of the wall and window dissipation formulae in the Chaotic Regime. 51 refs.
Date: March 1, 1985
Creator: Swiatecki, W.J.
Partner: UNT Libraries Government Documents Department

High energy nuclear structures

Description: In conventional nuclear physics the nucleus is described as a non-relativistic many-body system, which is governed by the Schroedinger equation. Nucleons interact in this framework via static two-body potentials, mesonic degrees of freedom are neglected. An alternative description of nuclear physics in terms of a relativistic field theory has been developed by Walecka. The model Lagrangian containing baryons, sigma-mesons and ..omega..-mesons was subsequently extended to include also ..pi..-mesons and rho-mesons. An essential feature of such a nuclear Lagrangian is its renormalizability. In addition to the description of known nuclear structure the field theoretical approach may reveal entirely new nuclear phenomena, based on the explicit treatment of mesonic degrees of freedom. The existence of such abnormal nuclear states was proposed by Lee and Wick employing the sigma-model Lagrangian. There the non-linearity of the meson field equations allows for soliton solutions in the presence of nucleons, in particular the sigma-field may exhibit a kink. Different types of soliton solutions occur in gauge theories with hidden symmetries. In the phenomenological Lagrangian the rho-meson is described by a non-abelian gauge field, that acquires its mass spontaneously due to the non-vanishing vacuum expectation value of a Higgs field. A general ansatz for soliton solutions of such a gauge theory was given by Dashen et al. A specific solution and its possible implications for nuclear physics like anomalous nuclear states were discussed by Boguta.
Date: March 9, 1984
Creator: Boguta, J. & Kunz, J.
Partner: UNT Libraries Government Documents Department

Incomplete and complete fusion in intermediate energy heavy ion reactions

Description: The yields, angular distributions and differential range spectra have been measured for individual target residues from the interaction of 8.5 MeV/A /sup 16/O, 19 MeV/A /sup 16/O, 35 MeV/A /sup 12/C and 86 MeV/A /sup 12/C with /sup 154/Sm. From the measured data, fragment isobaric yields and velocity spectra were deduced. The results are compared to the sum rule model of Wilczyski et al. and the nuclear firestreak model. 18 references.
Date: March 1, 1984
Creator: Aleklett, K.; Loveland, W.; Sugihara, T.T.; Morrissey, D.J.; Wenxin, L.; Kot, W. et al.
Partner: UNT Libraries Government Documents Department

Nuclear mass formula with a neutron skin degree of freedom and finite-range model for the surface energy

Description: We study the possibility of extending the model used by Moeller and Nix in 1980 to calculate nuclear masses and fission barriers for nuclei throughout the periodic system, to describe compressibility effects and the existence of a neutron skin. 9 references. (WHK)
Date: March 1, 1984
Creator: Moeller, P. & Myers, W.D.
Partner: UNT Libraries Government Documents Department

Target nucleus in relativistic nuclear collisions

Description: Both the fireball model and hydrodynamical model predict, in a central collision, a fast energy dissipation in the early diving stage of the projectile into the target nucleus. In the final state both models show total incident energy largely dispersed over the whole target nucleus. In a quantitative comparison with the data for the Intranuclear Cascade Calculations, however, the 90/sup 0/ double differential cross section seems to be flatter than the calculated one. If the slope is reflecting the temperature in the system then the data are having apparently a higher temperature than the cascade would predict. This report suggests and discusses a picture of the reaction mechanism where the light projectile (e.g. Ne) gets stopped very early in the large target nucleus (e.g. Au or U) forming a small fireball at approximately half the beam rapidity, which decays inside the target nucleus, heating it up and causing thee whole system to expand. The expansion cools the system and big clusters can condense out if the total energy and thus entropy in the system is not too high to prevent it. Such a qualitative picture of a reaction mechanism emerges when we consider the information obtained about relativistic nuclear collisions from measurements of the remnants of a large target nucleus struck by a smaller projectile (Ne + Au) and relate it to the complementary information from earlier measurements of fast light reaction products. (WHK)
Date: March 1, 1982
Creator: Gutbrod, H.H.; Warwick, A.I. & Wieman, H.
Partner: UNT Libraries Government Documents Department