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On Closed Shells in Nuclei

Description: It has been suggested in the past that special numbers of neutrons or protons in the nucleus form a particularly stable configuration.{sup1} The complete evidence for this has never been summarized, nor is it generally recognized how convincing this evidence is. That 20 neutrons or protons (Ca{sup40}) form a closed shell is predicted by the Hartree model. A number of calculations support this fact.{sup2} These considerations will not be repeated here. In this paper, the experimental facts indicating a particular stability of shells of 50 and 82 protons and of 50, 82, and 126 neutrons will be listed.
Date: February 1, 1948
Creator: Mayer, M.G.
Partner: UNT Libraries Government Documents Department

Application of simple ramsauer model to neutron total cross sections

Description: The simple nuclear Ramsauer model has been used successfully to fit neutron cross sections for three decades, but has not been widely used because the foundations of the model seem to be so unrealistic. We have shown that the Glauber calculations with the inclusion of refraction and optical model calculations essentially validate this simple model for neutron total cross sections in the neutron energy range of 5-50 MeV. This model yields a simple formula for parameterizing the energy dependence of the neutron cross section. We have applied the model to nuclei ranging from vanadium to bismuth. With the addition of a single parameter, we can improve these fits to less than 1.5%.
Date: April 29, 1997
Creator: Bauer, R.W.; Anderson, J.D.; Grimes, S.M. & Madsen, V.A.
Partner: UNT Libraries Government Documents Department

Empirical model for shell-corrected level densities

Description: An empirical model for calculating level densities of closed and near- closed shell nuclei has been developed and tested. This method is based on the calculation of shell plus pairing corrections for each relevant nuclide. A new version of the ALICE code is used to extract these corrections from the Myers-Swiatecki mass formula and to apply them to the calculation of effective excitations in level densities. The corrections are applied in a backshifted fashion to assure correct threshold dependence. We compare our calculated results with experimental data for the production of 56Ni and 88Y to test shell corrections near f7/c closure and the N=50 neutron shell. We also compare our results with those using pure Fermi gas (plus pairing) level densities, and with the more computationally intensive model of Kataria and Ramamurthy.
Date: April 29, 1997
Creator: Ross, M.A. & Blann, M.
Partner: UNT Libraries Government Documents Department

Nuclear interaction potential in a folded-Yukawa model with diffuse densities

Description: The folded-Yukawa model for the nuclear interaction potential is generalized to diffuse density distributions which are generated by folding a Yukawa function into sharp generating distributions. The effect of a finite density diffuseness or of a finite interaction range is studied. The Proximity Formula corresponding to the generalized model is derived and numerical comparison is made with the exact results. (8 figures) (auth)
Date: September 1, 1975
Creator: Randrup, J.
Partner: UNT Libraries Government Documents Department

Finite range Droplet Model

Description: A treatment of nuclear masses and deformations is described which combines the Droplet Model with the folding model surface and Coulomb energy integrals. An additional exponential term, inspired by the folding model, but treated here as an independent contribution with two adjustable parameters, is included. With this term incorporated, the accuracy of the predicted masses and fission barriers was improved significantly, the ability of the Droplet Model to account for isotope shifts in charge radii was retained, and the tendency of the Droplet Model to over-predict the surface-tension squeezing of light nuclei was rectified. 20 references, 4 figures.
Date: August 1, 1984
Creator: Moeller, P.; Myers, W.D.; Swiatecki, W.J. & Treiner, J.
Partner: UNT Libraries Government Documents Department

Axial asymmetry, finite particle number and the IBA

Description: Although the IBA-1 contains no solutions corresponding to a rigid triaxial shape, it does contain an effective asymmetry. This arises from zero point motion in a ..gamma..-soft potential leading to a non-zero mean or rms ..gamma... Three aspects of this feature will be discussed: (1) The relation between IBA-1 calculations and the corresponding ..gamma... This point is developed in the context of the Consistent Q Formalism (CQF) of the IBA. (2) The dependence of this asymmetry on boson number, N, and the exploitation of this dependence to set limits on both the relative and absolute values of N for deformed nuclei. (3) The relation between this asymmetry and the triaxiality arising from the introduction of cubic terms into the IBA Hamiltonian. Various observables will be inspected in order both to determine their sensitivity to these two structural features and to explore empirical ways of distinguishing which origin of asymmetry applies in any given nucleus. 16 references.
Date: January 1, 1984
Creator: Casten, R.F.
Partner: UNT Libraries Government Documents Department

Nuclear model tests with neutron resonance data

Description: 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.
Date: July 1, 1997
Creator: Mughabghab, S.F. & Dunford, C.
Partner: UNT Libraries Government Documents Department

{sup 3}He- and {sup 4}He-induced nuclear fission -- A test of the transition state method

Description: Fission in {sup 3}H and {sup 4}He induced reactions at excitation energies between the fission barrier and 140 MeV has been investigated. Twenty-three fission excitation functions of various compound nuclei in different mass regions are shown to scale exactly according to the transition state prediction once the shell effects are accounted for. New precise measurements of excitation functions in a mass region where shell effects are very strong, allow one to test the predictions with an even higher accuracy. The fact that no deviations from the transition state method have been observed within the experimentally investigated excitation energy regime allows one to assign limits for the fission transient time. The precise measurement of fission excitation functions of neighboring isotopes enables one to experimentally estimate the first chance fission probability. Even if only first chance fission is investigated, no evidence for fission transient times larger than 30 zs can be found.
Date: January 1, 1997
Creator: Rubehn, T.; Jing, K.X.; Moretto, L.G.; Phair, L.; Tso, K. & Wozniak, G.J.
Partner: UNT Libraries Government Documents Department

Baryon-baryon mixing in hypernuclei

Description: Implications of few-body hypernuclei for the understanding of the baryon-baryon interaction are examined. Octet-octet coupling effects not present in conventional, non strange nuclei are the focus. The need to identify strangeness {minus}2 hypernuclei to test model predictions is emphasized.
Date: May 1998
Creator: Gibson, B. F.
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

QCD scales and chiral symmetry in finite nuclei

Description: The authors report on this progress in the calculation of nuclear ground-state properties using effective Lagrangians whose construction is constrained by QCD scales and chiral symmetry. Good evidence is found that QCD and chiral symmetry apply to finite nuclei.
Date: October 1, 1997
Creator: Madland, D.G.
Partner: UNT Libraries Government Documents Department

Theory of neutron emission in fission

Description: A survey of theoretical representations of two of the observables in neutron emission in fission is given, namely, the prompt fission neutron spectrum N(E) and the average prompt neutron multiplicity {bar {nu}}{sub p}. Early representations of the two observables are presented and their deficiencies are discussed. This is followed by summaries and some examples of recent theoretical models for the calculation of these quantities. Emphasis is placed upon the predictability and accuracy of the recent models. In particular, the dependencies of N(E) and {bar {nu}}{sub p} upon the fissioning nucleus and its excitation energy are treated in detail for the Los Alamos model. Recent work in the calculation of the prompt fission neutron spectrum matrix N(E, E{sub n}), where E{sub n} is the energy of the neutron inducing fission, is then discussed. Concluding remarks address the current status of the ability to calculate these observables with confidence, the direction of future theoretical efforts, and limitations to current (and future) approaches. This paper is an extension of a similar paper presented at the International Centre for Theoretical Physics in 1996.
Date: August 1998
Creator: Madland, D. G.
Partner: UNT Libraries Government Documents Department

Interaction, currents and the electromagnetic structure of light nuclei

Description: The authors discuss the traditional model of the nucleus as a system of nucleons interacting via effective interactions and currents, and present results obtained within this framework for the electromagnetic structure of the ground- and low-lying-states of A=3-6 nuclei.
Date: June 1, 1998
Creator: L.E. Marcucci, R. Schiavilla
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

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

Competing degrees of freedom in nuclear structure theory. Final Report for 1999-2002

Description: The central focus of this research was the interplay between three generic classes of degrees of freedom relevant to nuclear structure theory: single-particle degrees of freedom, collective degrees of freedom, and statistical degrees of freedom, which can be thought of as an incoherent mean field or a thermal bath.
Date: July 4, 2003
Creator: Johnson, Calvin W.
Partner: UNT Libraries Government Documents Department

Experimental test of nuclear magnetization distribution and nuclear structure models

Description: Models exist that ascribe the nuclear magnetic fields to the presence of a single nucleon whose spin is not neutralized by pairing it up with that of another nucleon; other models assume that the generation of the magnetic field is shared among some or all nucleons throughout the nucleus. All models predict the same magnetic field external to the nucleus since this is an anchor provided by experiments. The models differ, however, in their predictions of the magnetic field arrangement within the nucleus for which no data exist. The only way to distinguish which model gives the correct description of the nucleus would be to use a probe inserted into the nucleus. The goal of our project was to develop exactly such a probe and to use it to measure fundamental nuclear quantities that have eluded experimental scrutiny. The need for accurately knowing such quantities extends far beyond nuclear physics and has ramifications in parity violation experiments on atomic traps and the testing of the standard model in elementary particle physics. Unlike scattering experiments that employ streams of free particles, our technique to probe the internal magnetic field distribution of the nucleus rests on using a single bound electron. Quantum mechanics shows that an electron in the innermost orbital surrounding the nucleus constantly dives into the nucleus and thus samples the fields that exist inside. This sampling of the nucleus usually results in only minute shifts in the electron´┐Ż s average orbital, which would be difficult to detect. By studying two particular energy states of the electron, we can, however, dramatically enhance the effects of the distribution of the magnetic fields in the nucleus. In fact about 2% of the energy difference between the two states, dubbed the hyperfine splitting, is determined by the effects related to the distribution of ...
Date: February 26, 1999
Creator: Beirsdorfer, P; Crespo-Lopez-Urrutia, J R & Utter, S B
Partner: UNT Libraries Government Documents Department

Covariance matrices for nuclear cross sections derived from nuclear model calculations.

Description: The growing need for covariance information to accompany the evaluated cross section data libraries utilized in contemporary nuclear applications is spurring the development of new methods to provide this information. Many of the current general purpose libraries of evaluated nuclear data used in applications are derived either almost entirely from nuclear model calculations or from nuclear model calculations benchmarked by available experimental data. Consequently, a consistent method for generating covariance information under these circumstances is required. This report discusses a new approach to producing covariance matrices for cross sections calculated using nuclear models. The present method involves establishing uncertainty information for the underlying parameters of nuclear models used in the calculations and then propagating these uncertainties through to the derived cross sections and related nuclear quantities by means of a Monte Carlo technique rather than the more conventional matrix error propagation approach used in some alternative methods. The formalism to be used in such analyses is discussed in this report along with various issues and caveats that need to be considered in order to proceed with a practical implementation of the methodology.
Date: January 10, 2005
Creator: Smith, D. L.
Partner: UNT Libraries Government Documents Department