Nowadays it has become customary in nuclear physics to denote by “tradition” the approach that considers nucleons and mesons as the relevant degrees of freedom. It is the purpose of this chapter to review this “traditional” approach in the area of nuclear forces and their applications to nuclear structure.

The Meson Theory of Nuclear Forces and Nuclear Structure

A review is given of our present knowledge of collective spin-isospin excitations in nuclei. Most of this knowledge comes from intermediate-energy charge-exchange reactions and from inelastic electron- and proton-scattering experiments. The nuclear-spin dynamics is governed by the spin-isospin-dependent two-nucleon interaction in the medium. This interaction gives rise to collective spin modes such as the giant Gamow-Teller resonances. An interesting phenomenon is that the measured total Gamow-Teller transition strength in the resonance region is much less than a model-independent sum rule predicts. Two physically different mechanisms have been discussed to explain this so-called quenching of the total Gamow-Teller strength: coupling to subnuclear degrees of freedom in the form of $\ensuremath{\Delta}$-isobar excitation and ordinary nuclear configuration mixing. Both detailed nuclear structure calculations and extensive analyses of the scattering data suggest that the nuclear configuration mixing effect is the more important quenching mechanism, although subnuclear degrees of freedom cannot be ruled out. The quenching phenomenon occurs for nuclear-spin excitations at low excitation energies ($\ensuremath{\omega}\ensuremath{\sim}10\ensuremath{-}20$ MeV) and small-momentum transfers ($q\ensuremath{\le}0.5$ ${\mathrm{fm}}^{\ensuremath{-}1}$). A completely opposite effect is anticipated in the high ($\ensuremath{\omega}$, $q$)-transfer region ($0\ensuremath{\le}\ensuremath{\omega}\ensuremath{\le}500$ MeV, $0.5\ensuremath{\le}q\ensuremath{\le}3$ ${\mathrm{fm}}^{\ensuremath{-}1}$). The nuclear spin-isospin response might be enhanced due to the attractive pion field inside the nucleus. Charge-exchange reactions at GeV incident energies have been used to study the quasifree peak region and the $\ensuremath{\Delta}$-resonance region. An interesting result of these experiments is that the $\ensuremath{\Delta}$ excitation in the nucleus is shifted downwards in energy relative to the $\ensuremath{\Delta}$ excitation of the free proton. The physical origin of this shift is discussed, and it is shown that it may be related to the energy-dependent, attractive one-pion exchange interaction in the medium.

Nuclear spin and isospin excitations

Two Nucleon Forces and Nuclear Matter

New accelerator-driven technologies that utilize spallation neutrons, such as the production of tritium and the transmutation of radioactive waste, require accurate nuclear data to model the performance of the target/blanket assembly and to predict neutron production, activation, heating, shielding requirements, and material damage. To meet these needs, nuclear-data evaluations and libraries up to 150 MeV have been developed for use in transport calculations to guide engineering design. By using advanced nuclear models that account for details of nuclear structure and the quantum nature of the nuclear scattering, significant gains in accuracy can be achieved below 150 MeV, where intranuclear cascade calculations become less accurate. Evaluations are in ENDF-6 format for important target/blanket and shielding materials (isotopes of H, C, N, O, Al, Si, P, Ca, Cr, Fe, Ni, Cu, Nb, W, Hg, and Pb) for both incident neutrons and incident protons. The evaluations are based on measured data as well as predictions from the GNASH nuclear model code, which calculates cross sections using Hauser-Feshbach, exciton, and Feshbach-Kerman-Koonin preequilibrium models. Elastic scattering distributions and direct reactions are calculated from the optical model. All evaluations specify production cross sections and energy-angle correlated spectra of secondary light particles as well as production crossmore » sections and energy distributions of heavy recoils and gamma rays. A formalism developed to calculate recoil energy distributions is presented. The use of these nuclear data in the MCNPX radiation transport code is also briefly described. This code merges essential elements of the LAHET and MCNP codes and uses these new data below 150 MeV and intranuclear cascade collision physics at higher energies. Extensive comparisons are shown between the evaluated results and experimental cross-section data to benchmark and validate the evaluated library. In addition, integral benchmarks of calculated and measured kerma coefficients for neutron energy deposition and neutron transmission through an iron slab compared with MCNPX calculations are provided. These evaluations have been accepted into the ENDF/B-VI library as Release 6.« less

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Cross-Section Evaluations to 150 MeV for Accelerator-Driven Systems and Implementation in MCNPX

Charge symmetry, quarks and mesons

Comprehensive analyses of nucleon-nucleon elastic-scattering data below 1100 MeV laboratory kinetic energy are presented. The data base from which an energy-dependent solution and 22 single-energy solutions are obtained consists of 7223 pp and 5474 np data. A resonancelike structure is found to occur in the $^{1}\mathrm{D}_{2}$, $^{3}\mathrm{F}_{3}$, $^{3}\mathrm{P}_{2}$${\mathrm{\ensuremath{-}}}^{3}$${\mathrm{F}}_{2}$, and $^{3}\mathrm{F}_{4}$${\mathrm{\ensuremath{-}}}^{3}$${\mathrm{H}}_{4}$ partial waves; this behavior is associated with poles in the complex energy plane. The pole positions and residues are obtained by analytic continuation of the ``production'' piece of the T matrix obtained in the energy-dependent solution. The new phases differ somewhat from previously published VPI solutions, especially in I=0 waves above 500 MeV, where np data are very sparse. The partial waves are, however, based upon a significantly larger data base and reflect correspondingly smaller errors. The full data base and solution files can be obtained through a computer scattering analysis interactive dial-in (SAID) system at VPI, which also exists at many institutions around the world and which can be transferred to any site with a suitable computer system. The SAID system can be used to modify solutions, plan experiments, and obtain any of the multitude of predictions which derive from partial-wave analyses of the world data base.

Nucleon-nucleon partial-wave analysis to 1100 MeV.

The isospin reaction cross sections for $\mathrm{NN}$ single pion production are extracted from data for the processes $\mathrm{NN}\ensuremath{\rightarrow}\ensuremath{\pi}d$ and $\mathrm{NN}\ensuremath{\rightarrow}\mathrm{NN}\ensuremath{\pi}$. New data in conjunction with older existing data have precisely determined the components of the cross sections for the isospin one ($\mathrm{pp}$) initial state. Other new data for the $\mathrm{np}$ initial state and correct treatment of some older data indicate that the isospin zero reaction cross section is essentially zero below 1000 MeV, which indicates against the presence of inelastic isospin zero $\mathrm{NN}$ resonances.NUCLEAR REACTIONS Nucleon-nucleon scattering, $\ensuremath{\pi}$ production reaction cross sections, isospin decomposition, isospin cross sections deduced from data.

B.J. VerWest

Papers

Nucleon-nucleon partial-wave analysis to 1100 MeV.

NN single pion production cross sections below 1500 MeV

Determination of the πNN vertex function from np and pp charge exchange scattering

Field theoretical calculation of NN pion production

The measurement of KNN and KLL in at 800 MeV