Search for isovector giant monopole resonances via the Sn-124(He-3,tn) reaction

The (He-3,t) reaction on Sn-124 at E(He-3)=199 MeV and the subsequent decay by neutron emission at backward angles were studied in an attempt to distinguish isovector monopole strength (spin-flip and non-spin-flip) at excitation energies above 25 MeV from the nonresonant continuum. The present approach is based on the assumption that a large fraction of the contributions to the continuum result from quasifree processes and breakup-pickup processes which leave the nucleus in low-lying excited states below the threshold for neutron emission. It was found, however, that even at high "apparent" excitation energies (above 30 MeV) the branching ratio for decay by neutrons was as high as similar to 50%, indicating that a large part of the nonresonant continuum may not be due to quasifree processes involving valence neutrons only. No evidence for monopole strength at high excitation energies was found, although the experiment had sufficient sensitivity and accuracy to detect isovector monopole strength based on theoretical predictions calculated in the framework of normal-mode collective excitations and the distorted-wave Born approximation