QUANTUM-MECHANICAL EQUIVALENT-PHOTON SPECTRUM FOR HEAVY-ION PHYSICS

In a previous paper, we calculated the fully quantum-mechanical cross section for electromagnetic excitation during peripheral heavy-ion col lisions. Here, we examine the sensitivity of that cross section to the detailed structure of the projectile and target nuclei. At the transi tion energies relevant to nuclear physics, we find the cross section t o be weakly dependent on the projectile charge radius and to be sensit ive to only the leading momentum-transfer dependence of the target tra nsition form. factors. We exploit these facts to derive a quantum-mech anical ''equivalent-photon spectrum'' valid in the long-wavelength lim it. This improved spectrum includes tile effects of projectile size, t he finite longitudinal momentum transfer required by kinematics, and t he response of the target nucleus to the off-shell photon.