The similarities in the experimental indications for multinucleon mechanisms in (γ, p) and (e, e′p) processes are pointed out. For both types of reactions, the substantial role of two-nucleon emission processes for transitions to high excitation energies in the residual nucleus is stressed. A microscopic model for the calculation of the two-body knockout contributions to the inclusive (γ,N) reaction is presented. It is based on an unfactorized formalism for the calculation of electromagnetically induced two-nucleon emission cross sections. The model is shown to yield a reasonable description of the overall behaviour of the 12C(γ, p) and 12C(γ, n) data at high excitation energies in the residual nucleus. In the calculations, effects from non-resonant and resonant pion exchange currents are included. Photoabsorption on these currents are predicted to produce the major contributions to the exclusive 16O(γ, n0) 15O process at photonenergies above the pion threshold. Double differential cross sections for photon induced pp and pn emission from 16O are calculated and compared with the data. 24.10.Eq,25.20.-x,21.60.Jz Typeset using REVTEX

It is pointed out that shell-model effects are likely to affect the energy dependence of the ${\mathrm{\ensuremath{\Delta}}}_{33}$ propagator in finite nuclei. \textcopyright{} 1996 The American Physical Society.

The 12C(γ,p) differential cross-section in the Δ-resonance region has been measured using tagged photons. Data are presented for 20 MeV wide photon energy bins at 200, 300, 400 and 500 MeV and for proton angles from 52° to 128°. The present 12C(γ,p) measurements are compared with previous experiments carried out at Frascati, Tokyo and Bonn. Comparisons are also made with theoretical calculations developed at Gent and Valencia which provide an interpretation of the photon reaction mechanism in terms of the major contributing processes. Coincidence data with correlated neutrons, protons and pions are also presented.

The role of π- and ϱ-meson exchange currents and dynamic Δ-isobar currents with both π- and ϱ-exchange are studied in two-nucleon knockout reactions. The Gottfried approximation is tested for these different absorption mechanisms by making a comparison between a factorized (Gottfried) model and an unfactorized model. For the unfactorized model, analytical expressions are given for the different absorption mechanisms by using harmonic-oscillator wavefunctions. The formalism is applied to the 16O(γ,pn) reaction in the photon-energy range Eγ = 60–300 MeV. Over the whole energy range, the ϱ-meson degree of freedom is found not to be negligible. It is demonstrated that the unfactorized treatment leads to a sizeable reduction of the cross section and to a change of the shape of the angular cross section.

The similarities in the experimental indications for multinucleon mechanisms in (gamma, p) and (e, ep) processes are pointed out. For both types of reactions, the substantial role of two-nucleon emission processes for transitions to high excitation energies in the residual nucleus is stressed. A microscopic model for the calculation of the two-body knockout contributions to the inclusive (gamma, N) reaction is presented. It is based on an unfactorized formalism for the calculation of electromagnetically induced two-nucleon emission cross sections. The model is shown to yield a reasonable description of the overall behavior of the C-12(gamma, p) and C-12(gamma, n) data at high excitation energies in the residual nucleus. In the calculations, effects from nonresonant and resonant pion exchange currents are included. Photoabsorption on these currents is predicted to produce the major contributions to the exclusive O-16(gamma, n0)O-15 process at photonenergies above the pion threshold. Double differential cross sections for photon induced pp and pn emission from O-16 are calculated and compared with the data.

Calculations have been performed for the O-16(gamma,pn) and the O-16(gamma,pp) reaction in the photon-energy range E(gamma) = 60-300 MeV. Besides the contribution from the more common photoabsorption on the pionic degrees of freedom, we have investigated the influence of heavier meson exchange (rho, sigma, omega) and intermediate DELTA creation with pi and rho exchange. Whereas the pi meson is found to set the main trends, the rho meson is found not to be discardable in a theoretical description of the (gamma,pn) reaction. The incorporation of an energy dependence and a decay width in the DELTA propagator is observed to be essential in order to arrive at a more realistic description of (gamma, NN) reactions at higher photon energies.

The role of one-pion-exchange currents in intermediate-energy photonuclear reactions of the type A(gamma,p)B is studied within a mean-field approach. Calculations are presented for the C-12(gamma,p) reaction leading to the low-lying states of B-11 including states with a predominant hole character and states with a more complicated two-hole-one-particle (2h-1p) structure. The level of agreement with the data for decay to the 2h-1p states suggests the important role of meson exchange currents in (gamma,p) reactions at intermediate energies.

Calculations have been performed for the 16O(γ,pn) reaction in the photon-energy range Eγ=50–300MeV. Effects due to both one-pion exchange currents and the Δ have been included. We discriminate between different couplings of the initial proton-neutron (pn) pair involved in the reaction. At lower photon energies where the pion exchange currents dominate the cross section it is found that pn pairs in the finite nucleus do not fully behave like a quasideuteron. Therefore, we feel that the factorized form of the (γ,pn) cross section should be used with reservation.