The partial ($\gamma , p$) cross section of carbon in which the residual boron nucleus is left in the ground state has been measured with a thin-crystal proton spectrometer. This cross section is shown to decrease from about 10 mb at the giant resonance peak (22 Mev) to about 0.1 mb near 60-Mev photon energy. Angular distributions measured at five energies exhibit an asymmetry around 90° which increases rapidly with increasing energy. The partial cross section to the first excited state of boron is (7±16)% of the ground-state cross section. The partial cross section to one or more excited states of boron about 5 Mev above the ground state is comparable with the ground-state cross section above 30-Mev photon energy. In addition to the cross-section data, the measurements provide a sensitive means of calibrating the energy scales of electron accelerators at energies in the 25- to 50-Mev region.

• Received 31 December 1958

DOI:https://doi.org/10.1103/PhysRev.114.1101