Magnetic dipole and Gamow–Teller modes: quenching, fine structure and astrophysical implications

Abstract

The magnetic dipole (M1) and Gamow–Teller (GT) response are prime examples to illustrate the importance of configuration mixing for an understanding of elementary excitation modes of the nucleus. Starting from the ’’classical’’ problem of quenching - whose proper description is still beyond the capabilities of microscopic models after all those years — I want to address some current developments of the field. Mandatory for the progress are high-resolution data from electron and hadron scattering and charge-exchange reactions.In medium-mass fp-shell nuclei, the detailed knowledge of the M1 and GT strength distribution provides a stringent test of state-of-the-art shell-model calculations, validating their applicability in astrophysical network calculations. As an example, it is demonstrated that high-precision M1 data on N = 28 isotones from electron scattering at Darmstadt permit the extraction of neutral-current neutrino-nucleus scattering cross sections important for supernova dynamics and nucleosynthesis.Fine structure of the GT mode is not only observed in light and medium-mass nuclei, but also in the GT resonance observed in a heavy nucleus like 90Nb studied in the 90Zr(3He, t) reaction at Osaka with a resolution ΔE ≃ 50 keV (FWHM). Novel methods, based on wavelet transforms, to extract scales characterizing the fine structure are presented. This in turn permits an interpretation of the physics underlying the phenomenon. These methods can also be used to extract spin- and parity-resolved level densities in a nearly model-independent way, again important to test models used in various astrophysical scenarios.As a final example, the influence of configuration mixing on the GT strength distribution at low energies is investigated for the heavy odd-odd nuclei 138La and 180Ta. The nucleosynthesis of these exotic nuclides, amongst the rarest in nature, is a long-standing problem. A likely source are charged-current neutrino-nucleus reactions which would be dominated by the GT response. However, the main GT resonance lies above the particle threshold and, therefore, does not contribute. Recent measurements of the GT strength distributions in 138La and 180Ta below the particle threshold and their astrophysical implications are discussed.