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Inelastic Electron Scattering, Fine Structure of M1 Giant Resonances and Gamow-Teller States

Recent progress in obtaining detailed fine structure distributions of magnetic giant resonances in nuclei using high resolution inelastic electron scattering at low energy is discussed. Specific examples chosen are the medium heavy nuclei 40,42,44,48Ca in which M1 excitations are due to neutron spin-flip transitions and the N = 28 isotones 50Ti, 52Cr and 54Fe where in addition also proton excitations contribute to the measured M1 strength. It is found that the M1 strength in the investigated energy region is very fragmented and considerably quenched in comparison to predictions of shell model calculations in a model space that includes up to 2p-2h excitations. The amount of strength which might have escaped experimental detection is estimated with the help of a fluctuation analysis technique and the comparison of experimentally deduced strength distributions with a statistical one. The remaining quenching still leaves ample room for theoretical speculations and it is shown for two specific examples of M1 transitions in 48Ca and 88Sr that the Δ(1232)-hole model accounts for part of this quenching. Based on this mechanism the virtual Δ-isobar admixture in 1+ spin-isospin excitations in complex nuclei is estimated. Furthermore, the results from inelastic electron scattering are compared to the results from (p, n) charge exchange reactions exciting Gamow-Teller states and to recent high resolution (p, p') results. The similarities and differences in the excitation of spin modes is pointed out by using 42Ca, 48Ca and 51V as examples. Finally, the old problem of M1 strength in 208Pb is revisited and the results of a form factor measurement of a recently discovered low lying Jπ = 1+ state by nuclear resonance fluorescence are presented.