PHOTON-INDUCED PROTON KNOCKOUT FROM PB-208

Cross sections have been measured for the reaction Pb-208(gamma,p) leading to the low-lying (1/2(+), 3/2(+)) and (11/2(-), 5/2(+)) doublets 7/2(+) state in (207)T1 using a high energy-resolution tagged photon beam of 41 to 57 MeV. The data are compared with results obtained with the reaction Pb-208(e,e'p) in the same recoil-momentum range. The recoil-momentum distributions derived from the (gamma,p) data do not coincide with those obtained from the (e,e'p) data. Distorted-wave impulse-approximation (DWIA) calculations of which the input is constrained by the (e,e'p) data underestimate the (gamma,p) data by typically one order of magnitude. An estimate of meson-exchange currents (MEG) effects using the Siegert theorem brings the DWIA-calculations close to the data, Random-phase approximation calculations also give a fair account of the data, but in this case MEC-effects are predicted to be less important.