The mass-dependence of meson-exchange currents in the reaction (gamma,p)

A systematic study of the mass dependence of meson-exchange currents in the reaction (γ,p) has been carried out. This study became possible with the availability of new high resolution (γ,p) data from ¹²C to ²⁰⁸Pb. In order to determine the relative importance of meson-exchange currents the direct knockout part of the (γ,p) cross section has been constrained by results obtained with the (e,e′p) reaction in similar momentum ranges. It is found that the enhancement of the (γ,p) cross section due to meson-exchange currents reduces by a factor three in going from ¹²C to ²⁰⁸Pb. This effect is tentatively attributed to the increased importance of higher multipolarities in the description of the coupling to meson exchange currents in heavy nuclei.