On the calculation of charge-exchange effects in a relativistic model for (g n) reactions

The charge-exchange contribution to photoneutron emission is calculated in a relativistic model. The emission is regarded as a two-step process consisting of a direct electromagnetic knockout of a proton followed by a (p,n) charge-exchange reaction. The latter step is effected by the isovector components of the scalar and vector mean field optical potentials. A novel procedure, in which the pertinent integral is reduced to a simpler integral in combination with a differential equation, is adopted to evaluatethe transition amplitude. We find that the charge-exchange contribution is comparable in magnitude to the direct contribution over a range of photon energies. There is some dependence on the optical and binding potentials used to calculate nucleon wave functions. As expected, the sum of direct and charge-exchange contributions is not sufficient to account for the observed cross sections, confirming the importance of including meson exchange and isobar currents.