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Experimental Searches for Exotic Alpha-Cluster Configurations

Description: Exotic and unusual shapes have recently been suggested to occur in light nuclei composed of alpha particle sub-units. Typically, these occur at very high excitation energy, where the nucleus is unbound with respect to decay into many charged particles. As a result these structures have been generally inaccessible to experiments conducted using conventional nuclear physics techniques. A new generation of detector devices now permits the simultaneous detection of many charged particles emerging from such an event with high efficiency, combined with excellent spatial and energy resolution. Such measurements for the first time allow the author to make detailed experimental tests of theoretical predictions of extremely deformed alpha-cluster nuclei. He applies these techniques to the study of extremely deformed configurations predicted to exist at high excitation energy in the nucleus {sup 24}Mg.
Date: December 31, 1994
Creator: Wuosmaa, A. H.
Partner: UNT Libraries Government Documents Department

Additional degrees of freedom in skyrmion motion

Description: The authors consider the quantization of chiral solitons with baryon number B > 1. Classical solitons are obtained within the framework of a variational approach. From the form of the soliton solution it can be seen that besides the group of symmetry describing transformations of the configuration as a whole there are additional symmetries corresponding to internal transformations. Taking into account the additional degrees of freedom leads to some sort of spin alignment for light nuclei and gives constraints on their spectra.
Date: September 1, 1994
Creator: Musatov, I.; Nikolaev, V. & Tkachev, O.
Partner: UNT Libraries Government Documents Department

The dynamics of fragment formation

Description: We demonstrate that in the Quantum Molecular Dynamics model, dynamical correlations can result in the production rate for final state nucleon clusters (and hence composite fragments) being higher than would be expected if statistics and the available phase space were dominant in determining composite formation. An intranuclear cascade or a Boltzmann-Uehling-Uhlenbeck model, combined with a statistical approach in the late stage of the collision to determine composites, provides an equivalent description only under limited conditions of centrality and beam energy. We use data on participant fragment production in Au + Au collisions in the Bevalac`s BOS time projection chamber to map out the parameter space where statistical clustering provides a good description. In particular, we investigate momentum-space densities of fragments up to {sup 4}He as a function of fragment transverse momentum, azimuth relative to the reaction plane, rapidity, multiplicity and beam energy.
Date: September 1, 1994
Creator: Keane, D. & Collaboration, EOS
Partner: UNT Libraries Government Documents Department

An accurate nucleon-nucleon potential with charge-independence breaking

Description: The authors present a new high-quality nucleon-nucleon potential with explicit charge dependence and charge asymmetry, which they designate Argonne {upsilon}{sub 18}. The model has a charge-independent part with fourteen operator components that is an updated version of the Argonne {upsilon}{sub 14} potential. Three additional charge-dependent and one charge-asymmetric operators are added, along with a complete electromagnetic interaction. The potential has been fit directly to the Nijmegen pp and np scattering data base, low-energy nn scattering parameters, and deuteron binding energy. With 40 adjustable parameters it gives a {chi}{sup 2} per datum of 1.09 for 4,301 pp and np data in the range 0--350 MeV.
Date: August 1, 1994
Creator: Wiringa, R. B.; Stoks, V. G. J. & Schiavilla, R.
Partner: UNT Libraries Government Documents Department

Testing the IBM plus broken pairs model at high spin: An experimental viewpoint

Description: Extensive measurements and calculations have been made on neutron deficient A = 80--90 nuclei. The measurements focused on extracting electromagnetic matrix elements between states of angular momentum J > 12h, which provided stringent tests of the IBM plus broken pairs model in the four quasi-particle regime. Starting from a detailed analysis of {sup 86}Zr, the model was extended to both isotones and isotopes in the region. The model is especially powerful for interpreting transitional nuclei where the competition between vibrational and single particle degrees of freedom determine the structure of states and their decays. It appears entirely appropriate for analysis of the new features revealed by the ``third generation`` gamma arrays.
Date: August 1, 1994
Creator: Lister, C. J.; Chowdhury, P. & Vretenar, D.
Partner: UNT Libraries Government Documents Department

Correlations of collective observables and the phonon structure of nuclei

Description: A ``horizontal`` view of nuclear structures is described in which various observables are correlated over broad mass ranges. This approach leads to a number of remarkable correlations, to new understanding of the evolution of structure, to a challenge to microscopic theories, and to new signatures of structure that will be especially useful with radioactive beam experiments. In particular, this and other evidence suggests a nearly universal and pervasive role of phonon and multi-phonon excitations in nuclei.
Date: July 1, 1994
Creator: Casten, R. F. & Zamfir, N. V.
Partner: UNT Libraries Government Documents Department

Lead 207, 208 (n, xn gamma) reactions for neutron energies up to 200 MeV

Description: High-resolution {gamma}-ray spectra from the interaction of neutrons in the energy range from 3 to 200 MeV with {sup 207,208}Pb were measured with the white neutron source at the WNR facility at Los Alamos National Laboratory. From these data, excitation functions for prominent {gamma} transitions in {sup 200,202,204,206,207,208}Pb were derived from threshold to 200 MeV incident neutron energy. These {gamma}-production cross sections represent formation cross sections for excited states of the residual nuclei. The results are compared with the predictions of nuclear reaction calculations based on the exciton model for precompound emission, the Hauser-Feshbach theory for compound nuclear decay, and coupled channels calculations to account for direct excitation of collective levels. Good agreement was obtained over the entire energy range covered in the experiment with reasonable model parameters. The results demonstrate that multiple preequilibrium emission has to be taken into account above about 40 MeV, and that the level density model of Ignatyuk should be used instead of the Gilbert-Cameron and back-shifted Fermi-gas models if excitation energies exceed about 30 MeV.
Date: July 1, 1994
Creator: Pavlik, A.; Vonach, H.; Chadwick, M. B.; Haight, R. C.; Nelson, R. O.; Wender, S. A. et al.
Partner: UNT Libraries Government Documents Department

Thick target spallation product yields from 800 MeV protons on tungsten

Description: A number of newly-conceived accelerator based technologies will employ medium-energy particles stopping in thick targets to produce large numbers of neutrons. It is important to quantify the residual radionuclides in the target because one must understand what nuclei and decay gammas are produced in order to design adequate shielding, to estimate ultimate waste disposal problems, and to predict possible effects of accidental dispersion during operation. Because stopping-length targets are considered, radionuclide production must be known as a function of energy. Moreover, secondary particle production, mostly neutrons, implies a need to be able to calculate particle transport. To test the overall ability to calculate radionuclide yields, a thick-target measurement was carried out and the results compared to detailed calculations. Although numerous measurements of thin-target spallation yields have been made, there have been only a few measurements on thick systems. The most complete study showed results for Pb and U systems. In this contribution, the authors report on measurements made for a stopping-length W target. Special efforts were made to measure short-lived isotopes, and reliable data on isotopes with two or three minute half-lives were obtained.
Date: July 1, 1994
Creator: Ullmann, J. L.; Staples, P. & Butler, G.
Partner: UNT Libraries Government Documents Department

The Feshbach-Kerman-Koonin multistep reaction theory and its application to data evaluation

Description: The development of the multistep nuclear reaction theory of Feshbach, Kerman, and Koonin (FKK) is reviewed, with particular attention to recent work. We present methodologies for unambiguously separating multistep direct and multistep compound contributions, and analyze neutron inelastic scattering by {sup 93}Nb at 14: MeV to illustrate the approaches. Modification of the FKK theory to include transitions from the multistep direct to the multistep compound chain (``P{yields}Q transitions``) is discussed. We also describe developments to the theory in order to calculate inelastic cross sections for incident nucleon energies to 200 MeV. At these high energies multiple preequilibrium emission processes must be included, and a theory for their determination is described and compared with experimental measurements of proton reactions on {sup 90}Zr. The usefulness of the FKK theory for a range of nuclear data applications, including intermediate energy reactions of importance in Accelerator Transmutation of Waste, reactions on biologically-important elements for cancer radiation treatment, and reactions of importance in fusion technology, is assessed.
Date: June 1, 1994
Creator: Hodgson, P. E. & Chadwick, M. B.
Partner: UNT Libraries Government Documents Department

Measurement of double differential charged-particle emission cross sections for reactions induced by 26 MeV protons and FKK model analysis

Description: Double differential charged-particle emission cross sections of proton-induced reactions have been measured for {sup nat}C, {sup 27}Al, {sup nat}Si, {sup 98}Mo, {sup 106}Pd, {sup 159}Tb and {sup 181}Ta at energies around 26 MeV. Several (p,p{prime}) and (p,n) data for {sup 98}Mo and {sup 106}Pd in the incident energy range from 12 to 26 MeV are analysed in terms of the Feshbach-Kerman-Koonin model, in order to study preequilibrium nucleon emission from nucleon-induced reactions.
Date: June 1, 1994
Creator: Watanabe, Y.; Aoto, A. & Kashimoto, H.
Partner: UNT Libraries Government Documents Department

Nuclear Science Division: 1993 Annual report

Description: This report describes the activities of the Nuclear Science Division for the 1993 calendar year. This was another significant year in the history of the Division with many interesting and important accomplishments. Activities for the following programs are covered here: (1) nuclear structure and reactions program; (2) the Institute for Nuclear and Particle Astrophysics; (3) relativistic nuclear collisions program; (4) nuclear theory program; (5) nuclear data evaluation program, isotope project; and (6) 88-inch cyclotron operations.
Date: June 1, 1994
Creator: Myers, W. D.
Partner: UNT Libraries Government Documents Department

A review of physics models in the LAHET{trademark} code

Description: This paper discusses the various physics options available in LAHET, including the Bertini and ISABEL intranuclear cascade models, the multistage, multistep preequilibrium exciton model, and the level density parameter options. The interdependence, the interaction with evaporation/fission models, and the sensitivity of results to the choice of options in shown. The particular options used in the code comparisons benchmark calculations are compared with the other options possible.
Date: June 1, 1994
Creator: Prael, R. E.
Partner: UNT Libraries Government Documents Department

Simplified simulation of Boltzmann-Langevin equation

Description: We briefly recall the Boltzmann-Langevin model of nuclear dynamics. We then summarize recent progress in deriving approximate analytical expressions for the associated transport coefficients and describe a numerical method for simulating the stochastic evolution of the phase-space density.
Date: June 1, 1994
Creator: Ayik, S. & Randrup, J.
Partner: UNT Libraries Government Documents Department

Improvements to the nuclear model code GNASH for cross section calculations at higher energies

Description: The nuclear model code GNASH, which in the past has been used predominantly for incident particle energies below 20 MeV, has been modified extensively for calculations at higher energies. The model extensions and improvements are described in this paper, and their significance is illustrated by comparing calculations with experimental data for incident energies up to 160 MeV.
Date: May 1, 1994
Creator: Young, P. G. & Chadwick, M. B.
Partner: UNT Libraries Government Documents Department

Investigation of the generation of several long-lived radionuclides of importance in fusion reactor technology: Report on a Coordinated Research Program sponsored by the International Atomic Energy Agency

Description: The IAEA initiated a Coordinated Research Program (CRP) in 1988 to obtain reliable information for 16 long-lived activation reactions of special importance to fusion reactor technology: {sup 27}Al (n, 2n){sup 26}Al, {sup 63}Cu(n,p){sup 63}Ni, {sup 94}Mo(n,p) {sup 94}Nb, {sup 109}Ag(n,2n){sup 108m}Ag, {sup 179}Hf(n,2n) {sup 178m2}Hf, {sup 182}W(n,n{sup `}a){sup 178m2}Hf, {sup 151}Eu(n,2n) {sup 150}gEu, {sup 153}Eu(n,2n){sup 152+m2}Eu, {sup 159}Tb(n, 2n){sup 158}Tb, {sup 158}Dy(n,p){sup 158}Tb, {sup 193}Ir(n,2n) {sup 192m2}Ir, {sup 187}Re(n,2n){sup 186m}Re, {sup 62}Ni(n{gamma}) {sup 63}Ni, {sup 98}Mo(n,{gamma}){sup 99}Mo({beta}-){sup 99}Tc, {sup 165}Ho(n,{gamma}) {sup 166m}Ho and {sup 191}Ir(n,{gamma}){sup 192m2}Ir. this paper documents progress achieved from the start of the program through mid- 1993.
Date: May 1, 1994
Creator: Smith, D. L. & Pashchenko, A. B.
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

The 1992 FRDM mass model and unstable nuclei

Description: We discuss the reliability of a recent global nuclear-structure calculation in regions far from {beta} stability. We focus on the results for nuclear masses, but also mention other results obtained in the nuclear-structure calculation, for example ground-state spins. We discuss what should be some minimal requirements of a nuclear mass model and study how the macroscopic-microscopic method and other nuclear mass models fullfil such basic requirements. We study in particular the reliability of nuclear mass models in regions of nuclei that were not considered in the determination of the model parameters.
Date: February 1, 1994
Creator: Moeller, P. & Nix, R. J.
Partner: UNT Libraries Government Documents Department

Mass model for unstable nuclei

Description: We present some essential features of a macroscopic-microscopic nuclear-structure model, with special emphasis on the results of a recent global calculation of nuclear masses. We discuss what should be some minimal requirements of a nuclear mass model and study how the macroscopic-microscopic method and other nuclear mass models fulfil such basic requirements. We study in particular the reliability of nuclear mass models in regions of nuclei that were not considered in the determination of the model parameters.
Date: February 1, 1994
Creator: Moeller, P. & Nix, J. R.
Partner: UNT Libraries Government Documents Department