Abstract
The impulse approximation expansion of the nucleon-nucleus scattering operator in terms of nucleon-nucleon scattering operators is rearranged so that it is given by the sum of a term representing repeated scatterings from the same nuclear nucleon and a term representing repeated scatterings from two different nuclear nucleons. When the expansion is limited to double scattering, the result explains some well-known experimental phenomena, namely, the parallelism between the elastic differential scattering cross section and the Born approximation cross section, and the sharp dip in the polarization at the Born approximation diffraction zero. 1 > The usual procedure is to neglect the binding energy of the nuclear nucleons com pared to the kinetic energy of the incident particle (impulse approximation), to assume that the nucleon-nucleus potential is a sum over the individual nucleon-nucleon potentials, and then to express these two-body potentials in terms of the two-body scattering matrices. The resultant optical model potential, linear in the two-body scattering amplitude, can then be used to calculate the nucleon-nucleus scattering cross section and polarization. The nucleon-nucleus scattering calculated in the Born approximation from the optical model potential is then linear in the nucleon-nucleon scattering and can be interpreted as the scattering of the incident particle by a single nucleon in the nucleus, the nucleus then recoiling as a whole. We shall often refer to the first Born approximation and the single scattering amplitude interchangeably. It has been shown that the polarization predicted by the optical model potential can be computed quite adequately at very small scattering angles, using the Born approximation scattering amplitude/> and several attempts have been made to use this result to distinguish among the various proposed sets of nucleon-nucleon phase shifts, 3 > usually without success. 4 > The polarization cal-
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