  When Enrico Fermi discovered slow neutrons, he accounted for their great
efficiency in inducing radioactivity by merely mentioning the well-known
scattering cross-section between neutrons and protons. He did not refer to
capture cross-section, at that early stage. It is put forward that a
thermodynamic approach to neutron-proton radiative capture then widely debated
might underlie his early accounts. Fermi had already met with a similar
approach, and repeatedly used it.
