A Study of the Nuclear Dependence of the Coherent Photoproduction Reaction in a Relativistic Approach Typeset Using Revt E X 1

We study the nuclear (or A) dependence of the coherent photoproduction reaction in a relativistic impulse approximation approach. We use a standard relativistic parameterization of the elementary amplitude, based on a set of four Lorentz-and gauge-invariant amplitudes, to calculate the coherent cross section from 4 He, 12 C, and 40 Ca. In contrast to nonrelativistic treatments our approach maintains the full relativistic structure of the process. The nuclear structure aaects the process through the ground-state tensor density. This density is sensitive to relativistic eeects and depends on A in a diierent manner than the vector density used in nonrelativistic approaches. This peculiar dependence results in 4 He having a cross section signiicantly smaller than that of 12 C|in contrast to existent nonrelativistic calculations. Distortion eeects are incorporated through an-nucleus optical potential that is computed in a simple \tt" approximation.