Nuclear Excitation by 180° Electron Scattering

We have studied magnetic multipole transitions in ${\mathrm{Be}}^9$, ${\mathrm{B}}^{10}$, ${\mathrm{B}}^{11}$, ${\mathrm{C}}^{12}$, ${\mathrm{N}}^{14}$, ${\mathrm{O}}^{16}$, and ${\mathrm{Si}}^{28}$ by measuring the energy spectra resulting from the scattering of 41.5-MeV primary electrons through an angle of 180°. At this angle of scattering, electric multipole transitions were greatly suppressed, and in addition, the background radiation tail accompanying the elastic peak was minimized. Inelastic electron scattering cross sections were obtained by comparing the inelastic peaks to the electron-proton elastic scattering peak and radiation widths were deduced by using virtual photon theory. Inelastic scattering peaks corresponding to excitation energies of 2.4 and 14.7 MeV were measured for ${\mathrm{Be}}^9$; 7.9, 11.8, and 14.0 MeV for ${\mathrm{B}}^{10}$; 2.1, 4.4, 4.9, 7.3, 9.1, 10.4, and 12.9 MeV for ${\mathrm{B}}^{11}$; 15.1 MeV for ${\mathrm{C}}^{12}$; 9.2 and 10.5 MeV for ${\mathrm{N}}^{14}$; and 11.6 MeV for ${\mathrm{Si}}^{28}$. No excitations were observed for ${\mathrm{O}}^{16}$ below 15 MeV, and no excitations were observed in ${\mathrm{Ca}}^{40}$ below the giant resonance for 160° electron scattering.