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

Energy levels of light nuclei A = 12

The electric dipole strength distribution in ^{48}Ca between 5 and 25 MeV has been determined at RCNP, Osaka from proton inelastic scattering experiments at forward angles. Combined with photoabsorption data at higher excitation energy, this enables the first extraction of the electric dipole polarizability α_{D}(^{48}Ca)=2.07(22)  fm^{3}. Remarkably, the dipole response of ^{48}Ca is found to be very similar to that of ^{40}Ca, consistent with a small neutron skin in ^{48}Ca. The experimental results are in good agreement with ab initio calculations based on chiral effective field theory interactions and with state-of-the-art density-functional calculations, implying a neutron skin in ^{48}Ca of 0.14-0.20 fm.

Electric Dipole Polarizability of ^{48}Ca and Implications for the Neutron Skin.

Proton-induced knockout reactions with polarized and unpolarized beams

Nuclear Data Sheets for A=40☆

The nuclear spin response to intermediate energy protons

Cross sections, analyzing powers and spin-flip probabilities have been measured for inclusive inelastic scattering of 290 MeV protons from $^{54}\mathrm{Fe}$ at laboratory angles between 3.1\ifmmode^\circ\else\textdegree\fi{} and 20\ifmmode^\circ\else\textdegree\fi{}. The momentum transfers vary from small values (q\ensuremath{\sim}0.2 ${\mathrm{fm}}^{\mathrm{\ensuremath{-}}1}$) where individual giant resonances of low multipolarity are observed, to larger values (q\ensuremath{\sim}1.4 ${\mathrm{fm}}^{\mathrm{\ensuremath{-}}1}$) where quasielastic scattering dominates. Complete measurements of spin observables at 20\ifmmode^\circ\else\textdegree\fi{} are discussed, which show that present impulse approximation models based on either the Dirac or the Schr\"odinger equation for the nucleon are not capable of reproducing all the data. At all momentum and energy transfers the measured analyzing powers ${\mathit{A}}_{\mathit{y}}$ are smaller than predictions from nonrelativistic calculations. Relativistic calculations explain this reduction of ${\mathit{A}}_{\mathit{y}}$ for data near the quasielastic point (\ensuremath{\omega}=${\mathit{q}}^{2}$/2m) as an effect of the attractive scalar field in the nuclear medium; however, they fail to reproduce the observed slopes [d(${\mathit{A}}_{\mathit{y}}$)/d\ensuremath{\omega} at fixed angle], assuming for the nuclear response a simple Fermi gas model.

Nuclear response in the 54Fe(p

Cross sections and a complete set of seven independent spin observables have been measured for inclusive quasielastic scattering of 290-MEV protons from /sup 54/Fe at a quasifree momentum transfer of 1.36 fm/sup -1/ and energy transfers varying from 0--90 MeV. Comparison of the results with relativistic and nonrelativistic impulse-approximation models shows that Fermi-motion averaging, a realistic nuclear response function, and medium modifications of the NN interaction are essential ingredients of a successful theory.

Complete spin observables for quasielastic proton scattering from 54Fe at 290 MeV.

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The nuclear spin response to intermediate energy protons

Nuclear response in the 54Fe(p

Complete spin observables for quasielastic proton scattering from 54Fe at 290 MeV.