Ca40 photoneutron cross section above the giant dipole resonance

The Ca40(γ,n) reaction cross section has been measured with tagged photons with energies ranging from 31.2 to 102 MeV, for neutron emission angles of 45°, 60°, 75°, 90°, and 135°. Exclusive differential cross sections for the Ca40(γ,n) and Ca40(γ,p) reactions are found to be of a similar magnitude, but are not well predicted by recent random-phase-approximation and relativistic direct knock-out calculations. A quasideuteron model calculation with spin-orbit splitting of single-particle shells and an improved nucleon transparency correction is found to be consistent with the data. However, it is shown that this does not establish that the photoabsorption matrix element for the deuteron is necessarily similar to that of a correlated np-pair in nuclear matter. This result is generalized to question the predicted sensitivity to short-range correlations for photonuclear reactions calculated within the Gottfried factorization.