TOTAL HADRONIC PHOTOABSORPTION CROSS-SECTION ON NUCLEI IN THE NUCLEONRESONANCE REGION

The total photoabsorption cross section for Li-7, C, Al, Cu, Sn, Pb ha s 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 even ts and to reject the electromagnetic background. Data above 600 MeV cl early indicate a broadening of higher nucleon resonance peaks in nucle i and a reduction of the absolute value of the cross section per nucle on with respect to the free-nucleon case. This large broadening sugges ts a strong influence of the nuclear medium in the resonance propagati on and interaction, while the systematic reduction of the measured cro ss sections might be due to a depletion of the resonance excitation st rength and to the onset of the shadowing effect around 1 GeV. Moreover , our systematic study indicates that also the Delta-resonance excitat ion parameters are not the same for all nuclei, being its mass and wid th increasing with the nuclear density.