Gamow-teller strength distributions in Sc48 by the Ca48(p,n) and Ti48(n,p) reactions and two-neutrino double-β decay nuclear matrix elements

The double-differential cross sections for the Ca48(p,n) and Ti48(n,p) reactions were measured at 300MeV. A multipole decomposition technique was applied to the spectra to extract the Gamow-Teller (GT) components. The integrated GT strengths up to an excitation energy of 30MeV in Sc48 are 15.3±2.2 and 2.8±0.3 in the (p,n) and (n,p) spectra, respectively. In the (n,p) spectra additional GT strengths were found above 8MeV where shell models within the fp shell-model space predict almost no GT strengths, suggesting that the present shell-model description of the nuclear matrix element of the two-neutrino double-β decay is incomplete.