Nuclear optical model for 3He and 4He projectiles

The well-known ambiguities in the real part of the nuclear optical model potentials used to represent the elastic scattering of 3He and 4He projectiles by medium weight nuclei are investigated. Recent analyses of elastic scattering data for 3He and 4He indicated that large angle data with sufficient high energy could resolve the discrete ambiguities in the real optical potential. Differential elastic scattering cross sections for 3He and 4He projectiles were measured at 83.5 MeV and 82.0 MeV respectively on targets of 40Ca and 58Ni. Conventional optical model fits to the 3He elastic scattering data at 83.5 MeV exhibited a strong preference for the U approximately 120 MeV potential family. This potential family is shallower than that given by the single nucleon optical potential depth times the number of nucleons in the projectile. The inclusion of large angle 4He elastic scattering data at 82 MeV on 40Ca and 58Ni did not resolve the ambiguity in the optical model potentials and showed only a slight preference for the U approximately 130 MeV potential family. The energy dependence in the optical potential was studied for 3He from 21 to 83.5 MeV and for 4He from 20 to 140 MeV. Energy dependent terms in the optical potential were found to be considerably different for the low and high energy data for both projectiles. The energy mass dependences were examined for three families of potentials for low energy 3He scattering data. A crude theoretical model was used to estimate the mass dependence in the real strength parameter due to the finite size of the projectile. These analyses suggest that the U approximately 120 MeV potential family is slightly favored. This is consistent with the previous high energy analysis