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 A-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 {co~ 10-20 MeV) and smallmomentum transfers {q < 0.5 fm~ *). A completely opposite effect is anticipated in the high (&>,#)-transfer region (0 < (o < 500 MeV, 0.5 < q < 3 fm~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 A-resonance region. An interesting result of these experiments is that the A excitation in the nucleus is shifted downwards in energy relative to the A 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.

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The 2 H(p, n) charge exchange reaction at Tp = 790 MeV is used to study the ∆(1232)-nucleon (∆N ) interaction in the ∆ resonance excitation energy region. For the ∆N potential, a meson exchange model is adopted where π, ρ, ω, and σ meson exchanges are taken into account. The deuteron disintegration below and above pion threshold is calculated using a coupled channel approach. Various observables, such as the inclusive cross section, the quasifree ∆ decay, the coherent pion production, and the two-nucleon breakup are considered. It is shown that these observables are influenced by the dynamical treatment of the ∆ degrees of freedom. Of special interest is the coherent pion decay of the ∆ resonance which is studied by means of the exclusive reaction 2 H(p, nπ + ) 2 H. Both the peak energy and the magnitude of the coherent pion production cross section depend very sensitively on the strength of the ∆N potential. The coherent pions have a peak energy of ω = 300 MeV and a strongly forward peaked angular distribution.

An antisymmetrized second-order microscopic calculation of the imaginary optical potential for "Ca(n,n) is made using random-phase approximation transition densities to the intermediate excited states. An optical Green's function is used for the intermediate projectile propagator. Both inelastic and (n,p) charge exchange intermediate states of the nucleus are included and a finite range effective projectile-target nucleon interaction is used. The local approximation to the calculated imaginary optical potential is surface peaked but at a smaller radius than most of the phenomenological potentials, and the depth is somewhat smaller. Collectivity and intermediate charge-exchange states are shown to play an important role.NUCLEAR REACTIONS (n, n) scattering, calculation of optical potential; E = 30 MeV. culation of the optical potential using a phenomenological effective interaction of 6-function type, the random-phase approximation (RPA) vectors of Gillet-Sanderson' for the intermediate target states, and a free-particle propagator for the intermediate projectile. Similar in approach are 179