Optical model for low-energy neutrons on 60Ni

A previously published s-wave scattering function for 1--450 keV neutrons on 60Ni is averaged for comparison to the scattering from an optical model potential. The scattering length R' is found to be 5.5 +- 0.03 fm at 225 keV. Averaging of the scattering function (both by integration with a normalized weight function and by use of an analytical approximation) produces shape elastic and compound nucleus cross sections which are then fitted by adjustment of the real and imaginary well depths in both spherical and vibrational optical models with a Woods-Saxon real well (r0 = 1.21 fm, a0 = 0.66 fm) and a surface derivative imaginary well (r/sub D/ = 1.21 fm, a/sub D/ = 0.48 fm). The fitted depths are V0 = 48 MeV and W/sub D/ = 29 MeV for the spherical potentials, and V0 = 50 MeV and W/sub D/ = 24 MeV for the vibrational potentials. Uncertainties are +- 5 MeV. From an upper limit on the p-wave strength function the W/sub D/ for p waves is found to be 1.5 MeV for the vibrational model. Thus, the imaginary potential is l dependent for the assumed geometry. For s waves the vibrational model gives a good fit also with W/sub D/ = 1.5 MeV and V0 = 54.4 MeV; however, with that V0 the 2p states are bound too deeply in 61Ni and the 3s size resonance is predicted at too low a mass