“…The Nγ → Y transitions in the Coulomb field of the nucleus were discussed by Pomeranchuck and Shmushkevich [8] . The pγ → Σ + excitation in crystals based on the approach [6], [7] and [8] was considered recently in [9] and it was claimed that the law P Σp (N) ∝ N 2 , holds up to the crystal thicknesses N = L/d ≃ 10 7 [9]. However, the plane wave Born approximation is not self-contained, it does not allow to assess what is the upper limit on N and, as a matter of fact, it grossly overestimates the enhancement factor.…”