E.C. Aschenauer, C. Van den Abeele, J.-O. Adler, B.-E. Andersson, L.J. de Bever, I. Bobeldijk, D. Branford, S.A. Bulychjov, T. Davinson, K. Hansen, D.G. Ireland, L. Isaksson, D. Ivanov, D.G. Johnstone, A. Khanov, L. Lapikás, R.A. Lindgren, G. De Meyer, D. Van Neck, B. Nilsson, H. Ruijter, D. Ryckbosch, A. Sandell, A.C. Shotter, B. Schröder, V. Van der Sluys, G. van der Steenhoven, P. Svensson, M.A. van Uden, R.E. Van de Vyver

Nuclear Physics A

Published while none of the authors were employed at the CMM

Abstract

Cross sections have been measured for the reactions ^{208}Pb(γ,p) and ^{12}C(γ,p) leading to the low-lying and continuum states in ^{207}Tl and ^{11}B 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 ^{12}C(γ,p) data by typically a factor five depending on angle and missing energy. For ^{208}Pb 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 ^{208}Pb as compared to ^{12}C. Finally, a remarkable similarity is observed between the continuum response of the (γ,p) and (e,e′p) reactions on ^{208}Pb despite the difference in the electromagnetic couplings involved.