The photoproton cross section of ${\mathrm{C}}^{12}$ was measured from 20.2 to 22.4 MeV using monochromatic gamma rays from the ${\mathrm{T}}^3(p, \gamma ){\mathrm{He}}^4$ reaction. The energy of the gamma ray was varied by changing the energy of the incident protons. The gamma rays were monitored by a 3-in.-diam by 4-in.-long sodium iodide crystal. A 2-in.-diam by ½-in.-thick anthracene crystal was used for both the irradiated sample and the proton detector. The protons originating in the anthracene crystal were identified by a pulse-shape discrimination technique. The peak energy of the giant resonance was found to be equal to or greater than 22.4 MeV and the cross section at 22.4 MeV was found to be 13 mb. The cross section curve showed structure at 20.57, 20.95, and 21.38 MeV, with integrated cross sections of approximately 0.3, 0.6, and 1.2 MeV-mb, respectively. The cross-section curve also suggested structure in the vicinity of 20 and 22 MeV. The energy dependence of the ($\gamma$, $p_0$) cross section differs from that of the ($\gamma , n$) reaction.

• Received 18 June 1964

DOI:https://doi.org/10.1103/PhysRev.136.B660