Problem 2092

Photonuclear cross sections are tabulated in GEANT4 as a function of the nuclear mass number (A) of the nucleus, but the G4PhotoNuclearCrossSection class only provides functionality to retrieve the cross section as a function of the atomic number (Z), from which the average nuclear mass is looked up before determining the cross section. The (integer) difference between the average nuclear mass and the atomic number is taken as the neutron number (N), and the mass number is then calculated as A = Z + N, after which the cross section can be calculate. While this is generally incorrect for any given isotope, it is especially problematic for heavy isotopes of hydrogen, for which a special branch exists to (correctly) return a cross section of exactly 0 for photonuclear reactions on a single proton below the pion threshold (~140 MeV). The current implementation returns a cross section of exactly 0 for low energy reactions on all isotopes of hydrogen, not just A = 1, which is incorrect.