D. Van Neck

The reaction ¹⁰B(γ,p) has been studied using tagged photons of mean energies Eγ=57.6 and 72.9 MeV. Angular distributions and derived single-particle momentum distributions for protons leading to the ground state of ⁹Be and higher excited states are compared to various calculations made using model parameters constrained by ¹⁰B(e,e′p)⁹Be and ⁹Be(p,p′)⁹Be measurements. The effects of varying final-state interactions (including channel couplings) and meson exchange currents are considered. A sizable discrepancy between direct-knockout calculations and the experimental results is observed. If meson exchange currents are included in an approximate fashion, a good description of the ¹⁰B(γ,p0)⁹Be data is found.

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.

We present a relativistic calculation of pion scattering, pion photoproduction and Compton scattering on the nucleon in the energy region of the Δ-resonance (upto 450 MeV photon lab energy), in a unified framework which obeys the unitarity constraint. It is found that the recent data on the cross section for nucleon Compton scattering determine accurately the parameters of the electromagnetic nucleon-Δ coupling. The pion-photoproduction partial-wave amplitudes, calculated with parameters fitted to the pion-nucleon and Compton scattering, agree well with the recent Arndt analysis.

The spectral function for finite nuclei is computed within the framework of the local-density approximation, starting from nuclear matter spectral functions obtained with a realistic nucleon-nucleon interaction. The spectral function is decomposed into a single-particle part and a ‘‘correlated’’ part; the latter is treated in the local-density approximation. As an application momentum distributions, quasiparticle strengths and overlap functions for valence hole states, and the light-cone momentum distribution in finite nuclei are computed.