Photoproduction of charged and neutral pions in nuclei in the Δ-resonance region

Photonuclear reactions induced by tagged photons in the energy range around the Δ-resonance are discussed. The total hadronic photoabsorption cross section (σγtot) shows a clear evidence for Δ-excitation in nuclei with a universal behaviour for a wide range of nuclear mass numbers. Next, the partitioning into specific reaction channels is studied. The dominant reaction mechanisms are pion production and γ-absorption on a correlated nucleon pair (quasi-deuteron effect), which are clearly visible in inclusive pion and proton energy spectra from light nuclei. The A-dependences of σγtot and the emission of pions and protons are compared. Whereas pions are strongly absorbed in heavy nuclei, the emission of protons is enhanced, supporting the picture of a two-step γ-absorption mechanism (volume π-production followed by π-absorption). (orig.)