When a neutron or proton enters an atomic nucleus, it behaves like a ball hitting a rough wall: it can bounce off in different ways, depending on where it hits. Physicists describe such collisions using the “optical model”—it replaces the complex nucleus with an effective potential, as if we described the wall by a single average height rather than every indentation.
The new model, called the East Lansing Model (ELM), refines this potential for neutrons and protons. Its distinctive feature is a new way of accounting for the difference between neutrons and protons in the nucleus. The authors used data on neutron and proton scattering on spherical nuclei with mass number A ≥ 40 and beam energies of 10–100 MeV. It turned out that adding data on (p,n) reactions (where a proton turns into a neutron) by itself does not change the parameters much, but introducing a new parameter related to the “neutron skin” (the excess of neutrons on the nucleus surface) makes the model more accurate.
The model is especially useful for nuclei far from stability, up to the boundaries of the proton and neutron drip lines (where nuclei can no longer hold extra nucleons). ELM extrapolations give smaller uncertainties than other global optical potentials, which is important for astrophysics and the physics of rare isotopes.
The optical model of the nucleus got its name from an analogy with the scattering of light on a translucent sphere: the nucleus partially absorbs and partially scatters particles, like frosted glass does with light.