I came across a worked example question in a particle physics book this morning that had a beam of neutrinos of energy 2.3MeV incident on a lead target. The point of the question was calculating the thickness of lead needed to reduce the beam intensity by a certain fraction - and the solution requires one to calculate the number density of "targets" in the lead. The solution given took the targets to be the nucleons within the lead nuclei, rather than the whole nuclei (the difference mathematically giving a factor of lead's atomic number 207). The reason given for this choice was at energy 2.3MeV, the neutrinos would be interacting with the nucleons. My question is: how can one tell what energy the interaction switches to be predominantly with the nucleons rather than the nuclei? To what energy must the incident neutrino energy be compared?