Nuclear Excitation by Scattering of 40-Mev Electrons

The electron energy spectra resulting from the scattering of 40-Mev primary electrons were measured for the purpose of studying nuclear excitations. Targets of ${\mathrm{Li}}^6$, Be, C, and Si were employed at scattering angles of 132° and 160°; Pb was studied at a single angle of 160°. In addition to the elastic peaks, all spectra show peaks corresponding to excitation of the target nucleus into well-defined energy states. Peaks corresponding to known levels in ${\mathrm{Li}}^6$ at 2.18 and 3.56 Mev, in ${\mathrm{Be}}^9$ at 2.43 Mev, and in ${\mathrm{C}}^{12}$ at 15.1 Mev, were measured and analyzed by a virtual photon theory to give values of ($4_{-1.5\mathrm{}}^{+3\mathrm{}}$)×${10}^{-4\mathrm{}}$, 6.2±0.6, 0.13±0.03, and (${40}_{-6\mathrm{}}^{+8\mathrm{}}$) ev for the values of their respective radiation widths to the ground states. Other well-defined peaks were observed at excitation energies of 16.9 Mev in Be, 11.6 Mev in Si, and 4.2 Mev in Pb. Broad peaks corresponding to the excitation of the giant resonance were observed in C, Si, and Pb, with maxima at 23, 20, and 15 Mev, respectively, and integrated cross sections of 75, 125, and 6500 Mev-mb, respectively. These cross sections are uncertain by a factor of approximately two because they depend on arbitrary methods used in subtracting the continuum of low-energy electrons and on arbitrary assumptions about nuclear form factors.