Photodisintegration of helium four

The 4He(γ,p)3H and 4He(γ,n)3He reactions from threshold up to E/sub γ/ approximately less than 170 MeV are considered. It is suggested that both the single nucleon-photon interaction and resonance photon absorption are important at low energies. However, as the photon energy increases these reaction mechanisms cannot explain the asymmetry shift toward the forward direction for the (γ,n) reaction. In addition, the absorption of photons in the intermediate and high energy range up to pion threshold is expected to occur predominately via the quasideuteron mechanism as is indicated from the reaction 4He(γ,npd). For the (γ,npd) amplitude, the breakup of 4He into a deuteron and quasideuteron is calculated using a 4He Gaussian ground state wave function fitted to the form factor, a quasideuteron S-wave scattered state and a deuteron effective range wave function. The photobreakup of the quasideuteron can be expressed in terms of the amplitude for the photobreakup of the deuteron as shown by Levinger. The photoamplitude for the (γ,npd) reaction is used to obtain the photoamplitude for the (γ,n) and (γ,p) reactions. The contribution to the (γ,p) and (γ,n) reactions from the quasideuteron mechanism is calculated as a rescattering of a nucleon-deuteron pair to form a triton. For both nucleon-triton reactions this yields angular distributions with forward peaking in agreement with the forward asymmetry obtained experimentally. The cross section at high energy appears to be dominated by the quasideuteron effect. Finally, the alternative quasideuteron calculation using nucleon-nucleon short-range-correlations is shown to be equivalent to the present quasideuteron calculation