Dispersive approach to the nuclear compton amplitude and exchange effects

We discuss the limitation of the standard approach to exchange current effects from photonuclear sum rules. We propose to look for an energy-independent contribution to the real part of the forward Compton-scattering amplitude below pion threshold. Experimental data on photonuclear cross-sections are used to calculate the real part of the amplitude, by means of an once-subtracted dispersion relation. The analysis is made for2H,7Li and9Be at energies varying between 50 and 150 MeV. A remarkably stable value ofK=0.60 is extracted for deuterium. For 1p shell nuclei, the main uncertainty is due to the lack of data above 300 MeV.