Study of proton spectra in the high-energy photoreactions of carbon and beryllium

Momentum spectra of protons photo-produced from light nuclei have been measured using a tagged photon facility at the 1.3 GeV electron synchrotron in Tokyo. The range of incident photon energy is between 360 and 600 MeV. Protons emitted from Be and C are detected by a magnetic spectrometer at lab angles 230, 550 and 1300 with an angular spread of +-50. The range of proton momentum is between 0.3 and 1.2 GeV/c. A two-peaked structure in proton spectra is observed both for Be and C at 230 and 550. As photon energy increases, the structure becomes less pronounced. These peaks are interpreted as those due to protons coming from two reactions γ + 'N' → p + π and γ + 'd' → p + n, where 'N' and 'd' are quasi-free nucleon and deuteron in the target nucleus, respectively. A quasi-free particle model calculation in which 'N' and 'd' are taken into account describes the two-peaked structure very well, confirming the correctness of this interpretation. At 1300, the spectra decrease monotonically as momentum increases, and no clear structure due to 'd' is observed. A simple explanation can not be found for the spectra at 1300. (author)