Total hadronic photoabsorption cross section on nuclei in the nucleon resonance region

The total photoabsorption cross section for Li-7, C, Al, Cu, Sn, Pb has been measured in the energy range 300-1200 MeV at Frascati with the jet-target tagged photon beam. A 4 pi NaI crystal detector and a lead-glass shower counter were used, respectively, to measure hadronic events and to reject the electromagnetic background. Data above 600 MeV clearly indicate a broadening of higher nucleon resonance peaks in nuclei and a reduction of the absolute value of the cross section per nucleon with respect to the free-nucleon case. This large broadening suggests a strong influence of the nuclear medium in the resonance propagation and interaction, while the systematic reduction of the measured cross sections might be due to a depletion of the resonance excitation strength and to the onset of the shadowing effect around 1 GeV. Moreover, our systematic study indicates that also the Delta-resonance excitation parameters are not the same for all nuclei, being its mass and width increasing with the nuclear density.