SUPPRESSION OF NUCLEON RESONANCES IN THE TOTAL PHOTOABSORPTION ON NUCLEI

We analyze the recent nuclear-photoabsorption data which have shown the disappearance on nuclei of the resonances higher than the Delta. We discuss how the Fermi motion, the collision broadening, and the Pauli blocking might distort the shape of nucleon resonances in the nuclear medium, and show that the Fermi motion and the collision broadening play the major role. From a fit to the photoabsorption data on uranium we have estimated the total cross sections for scattering of the resonances P-33(1232), D-13(1520), F-15(1680), and D-33(1700) on nucleons. The estimate for the P-33-resonance cross section is in fair agreement with the value derived from the NN --> Delta N cross sections, while those for the other resonances are high although do not violate the unitarity limits.