(p,n) reaction to ground- and excited-state analogs on the samarium isotopes: Importance of two-phonon coupling effects

The (p,n) reaction to ground- and excited-state analogs on /sup 144,148,150,152/Sm is investigated at bombarding energies of 20.0, 24.5, and 26.0 MeV. Ground-state analog (p,n) angular distributions are measured at all three bombarding energies. Excited-analog 2+ and 3- (p,n) angular distributions are obtained at 24.5 and 26.0 MeV. For 152Sm, the ground- and 2+ excited-analog neutron groups are not resolved, but an angular distribution for one 4+ excited-analog transition is obtained. Coupled-channel calculations indicate substantial three-step destructive interference which reduces the ground-analog cross sections and accounts substantially for the departure of these cross sections from a simple proportionality on neutron excess. A corresponding effect on the 2+ excited-analogs due to a set of four-step processes proceeding through one- and two-phonon states interfering destructively with the dominant two-step excited-analog mechanism is found