Charge Exchange Reactions and Spin Modes
The recent (p, n) experiments at intermediate energy at the Indiana University Cyclotron1–6 have led to a major breakthrough in the understanding of spin correlations in nuclei. The (p,n) reaction for bombarding energies between 100 and 200 MeV has proven to be a unique tool for the study of spin excitations in nuclei. Fig.1 shows zero degree neutron spectra for Ep=200 MeV. 5 The spectra are seen to be dominated by a single peak which is interpreted as a collective l+-state, the giant Gamow-Teller state, the isospin-spin nuclear sound. The collective state carries a large fraction of the total Gamow-Teller strength, i.e. strength associated with the operator σμt_ that governs Gamow-Teller β-decay. The concentration of strength into a collective state was first discussed by K. Ikeda, S. Fujii and J.I. Fujita as early as 1963.7 In a shell model picture of e.g. 208Pb the G.T. operator has non-zero matrix elements with all 44 excess neutrons (and the 12 h11/2 neutrons in the next lower shell). The collective state then corresponds to a transition where all these nucleons coherently change direction of spin and isospin.
KeywordsHeavy Nucleus Neutron Spectrum Symmetry Energy Collective State Total Strength
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