where da 0 (t)/dt is the p° photoproduction differential cross section on a neutron or proton (taken equal) and D(b,z) is the .A-particle nucleon density function. We neglect the real part of the p-nucleon forward-scattering amplitude. The wave numbers for the photon and p° meson are k y and k p , respectively. Nuclear cm. motion and nuclear correlations are neglected. The effective nucleon number iV eff is a function of A, energy, and V At low enough energies (~2 GeV) the one-step process dominates. The two-step process is inhibited due to mismatch of the photon and p°-meson wave numbers because of the mass of the p° meson. One has then an incoherent production (1969). 21 K" Gottfried, Phys, Rev. Letters 18/ 1174(1967. 22 J 0 D. Bjorken, Stanford Linear Accelerator Center, Report No. SLAC-PUB-571, 1969 (unpublished). 23 J. D 0 Bjorken, private communication.