Photo-excitation of nucleon resonances from complex nuclei

Photoproduction of [n] mesons off 12C, 40Ca, 93Nb, and natPb nuclei has been
measured with the tagged-photon-beam technique for incident photon energies
between 0.6 and 2.2 GeV. The experiment was performed at the Bonn ELSA ac-
celerator with the combined setup of the Crystal Barrel and TAPS calorimeters.
It aimed at the in-medium properties of the S11(1535) nucleon resonance and
the study of the absorption properties of nuclear matter for [n] mesons. Careful
consideration was given to contributions from [np] final states and secondary pro-
duction mechanisms of [n]-mesons e.g. from inelastic [p]N reactions of intermediate
pions. The analysis of the mass number scaling shows that the nuclear absortpion
cross section [o]N[n] for [n] mesons is constant over a wide range of the [n] momen-
tum. The comparison of the excitation functions to data off the deuteron and to
calculations in the framework of a BUU-model show no unexplained in-medium
modifications of the S11(1535).