PROTON KNOCKOUT FROM O-16 AND RELATIVISTIC RECOIL EFFECTS IN THE DIRAC IMPULSE APPROXIMATION

Recoil effects in exclusive nucleon knockout reactions, such as the p2 p reaction, are examined within a relativisitic distorted-wave impulse approximation based on the Dirac equation. Starting from a tree-level Feynman diagram in which the Bethe-Salpeter propagator is replaced by the three-dimensional ''smooth'' propagator, a prescription is develo ped which preserves the Dirac equation and which reduces to the correc t results in the non-relativistic and infinite target-mass limits. Dis tortions are incorporated in the scheme in a manner consistent with th e plane-wave result. Within the scheme, the wave functions of the sing le-particle bound state and the various distorted waves are evaluated in different two-body and three-body centre-of-momentum frames, which are then boosted, in principle, to a common reference frame. It is;fou nd that this feature of the recoil prescription, i.e., the evaluation of the various wave functions in the appropriate centre-of-momentum fr ames, in conjunction with energy-momentum conservation, is the single most important feature of the recoil prescription. When recoil effects are neglected, the various centre-of-momentum frames coincide with on e another, and the imposition of energy-momentum conservation then cau ses the momentum dependence of the bound-state wave function to be inc orrectly folded into the p2p matrix element. Other recoil effects, mai nly relativistic in nature, are found to be numerically less important . The scheme is applied to an analysis of the p2p reaction on O-16 at 200 MeV incident energy. Overall, the results give a reasonably good a ccount of the existing TRIUMF data, although at certain angle pairs, t he calculated cross sections are somewhat too low. The cross sections exhibit a dependence on the bound-state momentum distribution similar to that observed in earlier calculations for a Ca-40 target. Unlike th e Ca-40 results, however, the oxygen results also show a significant d ependence on the elastic distorting potentials used, indicating that t he elastic data for oxygen does not sufficiently constrain the oxygen optical potential to fix its role in the p2p reaction.