PHOTON-INDUCED PROTON KNOCKOUT FROM PB-208 AND C-12

Cross sections have been measured for the reactions Pb-208(gamma,p) an d C-12(gamma,p) leading to the low-lying and continuum states in Tl-20 7 and B-11 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) r eaction in the same recoil-momentum range and with various theoretical calculations. A comparison of the (e,e'p) and (gamma,p) data in terms of a reduced cross section versus momentum does not show a systematic overlap of the (e,e'p) and (gamma,p) results, i.e. no scaling is obse rved. Distorted-wave impulse-approximation (DWIA) calculations, of whi ch the input is constrained by the (e,e'p) data, and random-phase appr oximation calculations (RPA) underestimate the C-12(gamma,p) data by t ypically a factor five depending on angle and missing energy. For Pb-2 08 the difference between the calculations and the data for the low-ly ing states is much less, but an increasing discrepancy between the dat a and the calculations is found in the continuum. The inclusion of mes on-exchange currents (MEG) brings the calculations for both nuclei on average closer to the data, but MEC effects are considerably less impo rtant for Pb-208 as compared to C-12. Finally, a remarkable similarity is observed between the continuum response of the (gamma,p) and (e,e' p) reactions on Pb-208 despite the difference in the electromagnetic c ouplings involved. (C) 1997 Published by Elsevier Science B.V.