Excitation of the electric dipole giant resonance by inelastic electron scattering at 180°

Abstract

The cross section for the excitation of the nuclear giant resonance by electrons is calculated in a model in which the protons are assumed to oscillate as a unit against the neutrons (Goldhaber-Teller model). The longitudinal component of the field of the electrons is shown to have a cross section that agrees with results obtained previously by Fallieros, Ferrell and Pal. The transverse component is shown to have a form factor proportional to the elastic scattering form factor. Preliminary measurements in C, Al and Kr with 41.5 MeV electrons scattered at 180°, where only the transverse component is present, show cross sections smaller by about a factor of four than the calculated ones. For C and Al, these results are not surprising, because the Goldhaber-Teller model predicts a γ-absorption cross section that exhausts the dipole sum-rule and photonuclear experiments have shown that light elements fall considerably short of this. The discrepancy in the case of Kr could result from the failure of the Born approximation used in the theory. Another interesting possibility is that the Goldhaber-Teller model previously thought to be a fairly good description of the giant resonance in medium and heavy nuclei, is not correct in detail.