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AbstractAbstract

No abstract available

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Journal Article

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Physics Today; v. 25(4); p. 30-38

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AbstractAbstract

[en] On the occasion of this International Conference on Fifty Years Research in Nuclear Fission, we summarize our present understanding of the fission process and the challenges that lie ahead. The basic properties of fission arise from a delicate competition between disruptive Coulomb forces, cohesive nuclear forces, and fluctuating shell and pairing forces. These static forces are primarily responsible for such experimental phenomena as deformed ground-state nuclear shapes, fission into fragments of unequal size, sawtooth neutron yields, spontaneously fissioning isomers, broad resonances and narrow intermediate structure in fission cross sections, and cluster radioactivity. However, inertial and dissipative forces also play decisive roles in the dynamical evolution of a fissioning nucleus. The energy dissipated between the saddle and scission points is small for low initial excitation energy at the saddle point and increases with increasing excitation energy. At moderate excitation energies, the dissipation of collective energy into internal single-particle excitation energy proceeds largely through the interaction of nucleons with the mean field and with each other in the vicinity of the nuclear surface, as well as through the transfer of nucleons between the two portions of the evolving dumbell-like system. These unique dissipation mechanisms arise from the Pauli exclusion principle for fermions and the details of the nucleon-nucleon interaction, which make the mean free path of a nucleon near the Fermi surface at low excitation energy longer than the nuclear radius. With its inverse process of heavy-ion fusion reactions, fission continues to yield surprises in the study of large-amplitude collective nuclear motion. 87 refs., 12 figs

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1989; 22 p; International conference on fifty years of research in nuclear fission; Berlin (Germany, F.R.); 3-7 Apr 1989; CONF-890491--7; Available from NTIS, PC A03/MF A01 - OSTI; 1 as DE89012733; Portions of this document are illegible in microfiche products.

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CROSS SECTIONS, DEFORMED NUCLEI, FERMIUM 258, FISSION, FISSION BARRIER, FISSION FRAGMENTS, FOKKER-PLANCK EQUATION, HALF-LIFE, HARTREE-FOCK METHOD, MICROSEC LIVING RADIOISOTOPES, NUCLEAR DEFORMATION, NUCLEAR POTENTIAL, POTENTIAL ENERGY, SPONTANEOUS FISSION, SPONTANEOUS FISSION RADIOISOTO, URANIUM 236

ACTINIDE NUCLEI, ALPHA DECAY RADIOISOTOPES, DECAY, DEFORMATION, DIFFERENTIAL EQUATIONS, ENERGY, EQUATIONS, EVEN-EVEN NUCLEI, FERMIUM ISOTOPES, HEAVY NUCLEI, ISOTOPES, NUCLEAR DECAY, NUCLEAR FRAGMENTS, NUCLEAR REACTIONS, NUCLEI, PARTIAL DIFFERENTIAL EQUATIONS, POTENTIALS, RADIOISOTOPES, URANIUM ISOTOPES, YEARS LIVING RADIOISOTOPES

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AbstractAbstract

[en] The author discusses future directions in the development of classical hydrodynamics for extended nucleons, corresponding to nucleons of finite size interacting with massive meson fields. This new theory provides a natural covariant microscopic approach to relativistic nucleus-nucleus collisions that includes automatically spacetime nonlocality and retardation, nonequilibrium phenomena, interactions among all nucleons, and particle production. The present version of the theory includes only the neutral scalar (σ) and neutral vector (ω) meson fields. In the future, additional isovector pseudoscalar (π

^{+}, π^{-}, π^{0}), isovector vector (ρ^{+}, ρ^{-}, ρ^{0}), and neutral pseudoscalar (η) meson fields should be incorporated. Quantum size effects should be included in the equations of motion by use of the spreading function of Moniz and Sharp, which generates an effective nucleon mass density smeared out over a Compton wavelength. However, unlike the situation in electrodynamics, the Compton wavelength of the nucleon is small compared to its radius, so that effects due to the intrinsic size of the nucleon dominatePrimary Subject

Secondary Subject

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1994; 6 p; 10. winter workshop on nuclear dynamics; Snowbird, UT (United States); 15-21 Jan 1994; CONF-940169--2; CONTRACT W-7405-ENG-36; Also available from OSTI as DE94007540; NTIS; US Govt. Printing Office Dep

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AbstractAbstract

No abstract available

Original Title

Macroscopic--microscopic method

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Journal Article

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Annual Review of Nuclear Science; v. 22 p. 65-120

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Madland, D.G.; Nix, J.R.

Los Alamos National Lab., NM (USA)

Los Alamos National Lab., NM (USA)

AbstractAbstract

[en] We present a new method for calculating the prompt fission neutron spectrum N(E) and average prompt neutron multiplicity anti nu/sub p/ as functions of the fissioning nucleus and its excitation energy. The method is based on standard nuclear evaporation theory and takes into account (1) the motion of the fission fragments, (2) the distribution of fission-fragment residual nuclear temperature, (3) the energy dependence of the cross section sigma/sub c/ for the inverse process of compound-nucleus formation, and (4) the possibility of multiple-chance fission. We use a triangular distribution in residual nuclear temperature based on the Fermi-gas model. This leads to closed expressions for N(E) and anti nu/sub p/ when sigma/sub c/ is assumed constant and readily computed quadratures when the energy dependence of sigma/sub c/ is determined from an optical model. Neutron spectra and average multiplicities calculated with an energy-dependent cross section agree well with experimental data for the neutron-induced fission of

^{235}U and the spontaneous fission of^{252}Cf. For the latter case, there are some significant inconsistencies between the experimental spectra that need to be resolved. 29 referencesPrimary Subject

Source

1983; 36 p; Specialists meeting on yields and decay data of fission product nuclides; Upton, NY (USA); 24-27 Oct 1983; CONF-8310104--2; Available from NTIS, PC A03/MF A01 as DE84001724

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Report

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Conference; Numerical Data

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ACTINIDE NUCLEI, ALPHA DECAY RADIOISOTOPES, BARYON REACTIONS, BARYONS, CALIFORNIUM ISOTOPES, DATA, DECAY, ELEMENTARY PARTICLES, EVEN-EVEN NUCLEI, FERMIONS, FISSION NEUTRONS, HADRON REACTIONS, HADRONS, HEAVY NUCLEI, INFORMATION, ISOTOPES, MATHEMATICAL MODELS, NEUTRONS, NUCLEAR DECAY, NUCLEAR FRAGMENTS, NUCLEAR MODELS, NUCLEAR REACTIONS, NUCLEI, NUCLEON REACTIONS, NUCLEONS, NUMERICAL DATA, RADIOISOTOPES, SPECTRA, TARGETS, YEARS LIVING RADIOISOTOPES

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Sierk, A.J.; Nix, J.R.

Los Alamos Scientific Lab., N.Mex. (USA)

Los Alamos Scientific Lab., N.Mex. (USA)

AbstractAbstract

[en] In the framework of a hydrodynamical model, three important aspects of heavy-ion collisions are investigated: the potential energy of a nucleus as a function of deformation, the dynamical coupling between collective shape modes, and the effect of the transfer of collective energy into single-particle excitations. The dependence of potential energy on shape has the effect of preventing fusion of heavy ions unless the nuclear system can be brought inside its fission saddle point. For increasing mass number A and angular momentum the fission saddle-point shape becomes more compact than a touching-ion configuration, leading to a rapid drop of predicted fusion cross sections in the vicinity of A = 200. For nuclei with A approximately greater than 200, the coupling between collective shape modes increases the kinetic energy needed by colliding ions to coalesce to a compact shape. This increases the energy required for fusion. In addition to exciting collective shape oscillations during heavy-ion collisions, some of the initial kinetic energy is converted into internal single-particle excitation energy. Two possible mechanisms for this conversion are discussed: ordinary (two-body) viscosity, which arises from collisions between individual nucleons, and one-body dissipation, which arises from nucleon collisions with the moving potential wall. Dynamical calculations using either of these dissipation mechanisms reproduce experimental fission-fragment kinetic energies for nuclei throughout the periodic table. Many of the experimentally observed features of strongly damped heavy-ion collisions are reproduced by dynamical calculations using relatively small values of the ordinary two-body viscosity coefficient, although some discrepancies remain. The analogous calculations using one-body dissipation are not yet done. 8 figures, 1 table

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1976; 24 p; Symposium on macroscopic features of heavy ion collisions; Argonne, Illinois, United States of America (USA); 1 Apr 1976; CONF-760424--11; Available from NTIS. $3.50.

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Nix, J.R.; Sierk, A.J.

Los Alamos Scientific Lab., N.Mex. (USA)

Los Alamos Scientific Lab., N.Mex. (USA)

AbstractAbstract

[en] The statics and dynamics of very-heavy-ion reactions are discussed, with special emphasis on those aspects associated with the production of superheavy nuclei. Cross sections for forming compound nuclei in symmetric heavy-ion collisions are calculated by use of the criterion that the dynamical trajectory for the fusing system must pass inside the fission saddle point in a multidimensional space in order to form a compound nucleus. (5 figures) (auth)

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1 Sep 1975; 11 p; Seminar on reactions of heavy ions with nuclei and synthesis of new elements; Dubna, USSR; 23 Sep 1975; CONF-750938--1

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Bush, B.W.; Nix, J.R.

Los Alamos National Lab., NM (United States). Funding organisation: USDOE, Washington, DC (United States)

Los Alamos National Lab., NM (United States). Funding organisation: USDOE, Washington, DC (United States)

AbstractAbstract

[en] We discuss a new approach to relativistic nucleus-nucleus collisions based on classical hadrodynamics for extended nucleons, corresponding to nucleons of finite size interacting with massive meson fields. This theory provides a natural covariant microscopic approach to relativistic nucleus-nucleus collisions that includes automatically spacetime nonlocality and retardation, nonequilibrium phenomena, interactions among all nucleons, and particle production. Inclusion of the finite nucleon size cures the difficulties with preacceleration and runaway solutions that have plagued the classical theory of self-interacting point particles

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1992; 6 p; 8. winter workshop on nuclear dynamics; Jackson Hole, WY (United States); 18-25 Jan 1992; CONF-920113--1; CONTRACT W-7405-ENG-36; OSTI as DE92008482; NTIS; INIS; US Govt. Printing Office Dep

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Los Alamos National Lab., NM (United States). Funding organisation: USDOE, Washington, DC (United States)

AbstractAbstract

[en] The interaction of high-energy cosmic rays with nuclei in spacecraft shielding and the human body is important for manned interplanetary missions and is not well understood either experimentally or theoretically. We present a new theoretical approach to this problem based on classical hadrodynamics for extended nucleons, which treats nucleons of finite size interacting with massive meson fields. This theory represents the classical analogue of the quantum hadrodynamics of Serot and Walecka without the assumptions of the mean-field approximation and point nucleons. It provides a natural covariant microscopic approach to collisions between cosmic rays and nuclei that automatically includes space-time non-locality and retardation, nonequilibrium phenomena, interactions among all nucleons, and particle production. Unlike previous models, this approach is manifestly Lorentz covariant and satisfies a priori the basic conditions that are present when cosmic rays collide with nuclei, namely an interaction time that is extremely short and a nucleon mean-free path, force range, and internucleon separation that are all comparable in size. We review the history of classical meson-field theory and derive the classical relativistic equations of motion for nucleons of finite size interacting with massive scalar and vector meson fields

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Jan 1993; 50 p; CONTRACT W-7405-ENG-36; OSTI as DE93006263; NTIS; INIS; US Govt. Printing Office Dep

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Moeller, P.; Nix, J.R.

Los Alamos National Lab., NM (United States). Funding organisation: USDOE, Washington, DC (United States)

Los Alamos National Lab., NM (United States). Funding organisation: USDOE, Washington, DC (United States)

AbstractAbstract

[en] We present some new results on heavy-element nuclear-structure properties calculated on the basis of the finite-range droplet model and folded-Yukawa single-particle potential. Specifically, we discuss calculations of nuclear ground-state masses and microscopic corrections, α-decay properties, β-decay properties, fission potential-energy surfaces, and spontaneous-fission half-lives. These results, obtained in a global nuclear-structure approach, are particularly reliable for describing the stability properties of the heaviest elements

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1993; 12 p; Actinides '93; Santa Fe, NM (United States); 19-24 Sep 1993; CONF-930905--7; CONTRACT W-7405-ENG-36; Also available from OSTI as DE94000674; NTIS; US Govt. Printing Office Dep

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