Gamow-Teller Transitions Ruling the Supernova Explosion

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In the core-collapse stage of type II supernovae, weak-interaction processes of pf-shell nuclei play important roles. Therefore, studies of electron capture and beta decay caused by charged currents and neutrino-nucleus scattering caused by neutral currents are of great astrophysical interest. The charged-current processes are dominated by Fermi and Gamow-Teller (GT) transitions, but the knowledge for the important GT transitions in pf-shell nuclei is very poor. Direct information on the GT transition strength B(GT) can be derived from beta-decay measurements. However, B(GT) values were derived for at most a few low-lying states with large ambiguities.Charge-exchange (CE) reactions can access to GT transitions at higher excitations. In particular, it was shown that measurements at scattering angles around 0 deg and at intermediate beam energies above 100 MeV/nucleon were good probes of GT transitions. A development in precise beam matching techniques realized an energy resolution of about 30 keV in intermediate energy (311e, t) reactions at 0 deg. With this one-order of magnitude better resolution, we can now study GT and Fermi states that were unresolved in the pioneering (p, n) reactions. We performed experiments on T=1 targets 46Ti, 50Cr, 54Fe, and Ni isotopes 58Ni, 6ONi, 62Ni, and 64 Ni, and some additional reference nuclei. The B(GT) values have been extracted for some of the nuclei. The obtained strength distributions were compared with results of theoretical calculations. It was found that the modern shell-model calculations can reproduce the gross features of the distribution, but not the details.We plan to extend studies for a wider range of pf-shell nuclei, and the results will put restrictions on the theoretical studies of supernova explosion and also for the element synthesis.