Fermi’s favorite figure: the history of the pseudopotential concept in atomic physics and neutron physics

In the early 1930’s, Fermi wrote two papers in which he introduced the concepts of “scattering length” and “pseudopotential.” Since that time, these terms have become universally associated with low energy scattering phenomena. Even though the two papers are very different—one in atomic physics, the other in neutron physics—a simple figure underlies both. The figure appears many times in Fermi’s work. We review how the two papers came about and briefly discuss modern developments of the work that Fermi initiated with these two remarkable papers.