C14(e+,e-)14O reaction between 19 and 80 MeV

Cross-section measurements are reported for the C14(e+,e-)14O reaction to the double isobaric analog state and for nonanalog states at 5 to 10 MeV excitation. The extrapolated zero-degree cross sections are 2.3+0.5, 3.7+0.4, 2.2+0.3, and 0.9+0.3 1/4b/sr at 19.0, 29.1, 64.4, and 79.5 MeV, respectively. The nonanalog-state cross sections are found to rise sharply with increasing beam energy above 50 MeV, in contrast to those of the double isobaric analog state transition. Calculations with a phenomenological isospin-dependent optical potential must include an isotensor term, which accounts for transitions through nonanalog intermediate states or other short-range correlations, in order to describe the energy and angular trends of the data. Microscopic calculations in the second-order distorted-wave impulse approximation fail to reproduce the cross sections near 50 MeV unless the effects of short-range correlations beyond those contained in the model are incorporated.