Differential cross sections of the 12,13C(p,p)12,13C and 12,13C(p,n)12,13N reactions near 180 degree

The recoil detection method has been used to measure the differential cross sections at nucleon backward angles, 160 degree endash 180 degree (cm), for the 12,13C(p,p)12,13C and 12,13C(p,n)12,13N reactions at an incident proton energy of 200 MeV. These are the first reported data in this angular region. The elastic scattering cross sections show a shallow minimum at 180 degree and are about a factor of 15 larger than those predicted by optical model calculations using standard parameters. The (p,n) cross sections are lower than the (p,p), flat within the rather limited statistics, and about a factor of 5 greater than predicted. The backward-angle elastic scattering cross sections are selectively sensitive to the real central potential and can be brought into fairly good agreement with experiment without destroying the fit at more forward angles by greatly reducing both its depth and diffuseness. Adding a Majorana exchange term in the real central and spin orbit potential to a phenomenological potential that fits previously reported lower-angle 12C elastic scattering data does not significantly alter these fits while introducing a peak at 180 degree. copyright 1996 The American Physical Society