The (γ, p) reaction at E ∼60 MeV

A study of the (γ, p) reaction on C, N, Al and V targets, at photon energies E ∼60 MeV, is reported. Angular distributions in the range t = 60°-120° are presented for transitions leading to the ground state and several low-lying states in each residual nucleus. The relative population of residual nucleus slates is similar to those found in (e, e'p) and (d, He) experiments, apart from the well-known exception of the peak at 6.8 MeV in the C(γ, p) reaction. Direct-knockout calculations fail to account for the magnitude of the (γ, p) results, which indicates the need for calculations that include effects such as meson exchange or short-range correlations. Strength seen at higher excitation energies is attributed to inclusive (γ, p) events. The excitation-energy spectra have been compared with quasi-deuteron model calculations which assume a (γ, np) reaction where the neutron goes undetected. The shape of the V(γ, p) excitation spectrum is compared to a pre-equilibrium model calculation. The results appear to indicate that two-nucleon absorption of the photon is important.