Ratio of the isovector effective interaction strengths ‖Jστ/Jτ‖2 between 200 and 450 MeV from the 14C(p, n) 14N reaction

The cross section at 0° for the reaction 14 C(p, n) 14 N has been measured at energies of 200, 300, 400 and 450 MeV using a nucleon charge-exchange facility recently commissioned at TRIUMF. Energy resolution of 1 MeV or better was obtained at each energy, permitting a reliable measurement of the cross section ratio for the Gamow-Teller transition to the 1 + state at 3.95 MeV and the Fermi transition to the 0 + state at 2.31 MeV. The deried values of ‖ J στ / J τ ‖ 2 are in clear disagreement with impulse approximation calculations using a free NN t -matrix.