On the Interpretation of Neutron-Proton Scattering Data by the Schwinger Variational Method

Variational methods developed by Schwinger are applied to neutron-proton scattering at energies below 10 Mev. $S$-wave scattering alone is considered, and the tensor force is not taken into account. An expansion is obtained for the phase shift in powers of the energy. The coefficients can be evaluated explicity from the wave function. The first term of the series is related to Fermi's scattering length, the second term involves an "effective range." The third and higher terms turn out to be negligible.