THE MASS-DEPENDENCE OF MESON-EXCHANGE CURRENTS IN THE REACTION (GAMMA,P)

A systematic study of the mass dependence of meson-exchange currents i n the reaction (gamma,p) has been carried out. This study became possi ble with the availability of new high resolution (gamma,p) data from C -12 to Pb-208. In order to determine the relative importance of meson- exchange currents the direct knockout part of the (gamma,p) cross sect ion has been constrained by results obtained with the (e,e'p) reaction in similar momentum ranges, It is found that the enhancement of the ( gamma,p) cross section due to meson-exchange currents reduces by a fac tor three in going from C-12 to Pb-208, This effect is tentatively att ributed to the increased importance of higher multipolarities in the d escription of the coupling to meson exchange currents in heavy nuclei.