The excitation of the electric-dipole giant resonance in ${\mathrm{C}}^{12}$ and ${\mathrm{O}}^{16}$ was studied by measuring the spectrum of electrons inelastically scattered at 180°. Experiments were made with incident electrons of 40, 55, and 70 MeV; combining these data with the known photon absorption cross section a form factor for the giant resonance cross section is obtained for momentum transfers up to 120 MeV/c. The form factor for the combined strength of the main giant resonance in ${\mathrm{C}}^{12}$ and ${\mathrm{O}}^{16}$ which is concentrated between 20 and 25 MeV has a very characteristic shape going through a shallow minimum and increasing again with the momentum transfer. A comparison of the data with calculations of Lewis and Walecka using an extended shell model of the giant resonance indicates quantitative agreement, while collective models (such as the Goldhaber-Teller and Steinwedel-Jensen) fail to explain the data. The cross sections display also some fine structure. In ${\mathrm{C}}^{12}$ peaks are seen at 18.1, 19.5, 24, and 34 MeV; in ${\mathrm{O}}^{16}$ the peaks are at 19, 22.5, and 25.5 MeV.

• Received 23 January 1964

DOI:https://doi.org/10.1103/PhysRev.134.B963