| Popis: |
Cross-section measurements are reported for the /sup 14/C(..pi../sup +/,..pi../sup /minus//)/sup 14/O 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 ..mu..b/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. |
