Photon-induced proton knockout from 208Pb and 12C

Cross sections have been measured for the reactions ²⁰⁸Pb(γ,p) and ¹²C(γ,p) leading to the low-lying and continuum states in ²⁰⁷Tl and ¹¹B using a high energy-resolution tagged photon beam of 41 to 57 MeV. The data are compared with results obtained with the (e,e′p) reaction in the same recoil-momentum range and with various theoretical calculations. A comparison of the (e,e′p) and (γ,p) data in terms of a reduced cross section versus momentum does not show a systematic overlap of the (e,e′p) and (γ,p) results, i.e. no scaling is observed. Distorted-wave impulse-approximation (DWIA) calculations, of which the input is constrained by the (e,e′p) data, and random-phase approximation calculations (RPA) underestimate the ¹²C(γ,p) data by typically a factor five depending on angle and missing energy. For ²⁰⁸Pb the difference between the calculations and the data for the low-lying states is much less, but an increasing discrepancy between the data and the calculations is found in the continuum. The inclusion of meson-exchange currents (MEC) brings the calculations for both nuclei on average closer to the data, but MEC effects are considerably less important for ²⁰⁸Pb as compared to ¹²C. Finally, a remarkable similarity is observed between the continuum response of the (γ,p) and (e,e′p) reactions on ²⁰⁸Pb despite the difference in the electromagnetic couplings involved.