The Hoyle State (2021)

Summary

Nuclear physics is complicated compared to atomic physics, because the strong force is complicated compared to the electromagnetic force, and nucleons—protons and neutrons—are bag-like groupings of quarks and gluons held together by the strong force. They resemble elastic bags that attract each other. They jostle each other in the nucleus… governed by the rules of quantum mechanics. To begin to understand such a complex thing as a nucleus, people started with approximate models. In 1930 George Gamow introduced the ‘liquid drop model’, which was further developed by Niels Bohr, John Archibald Wheeler and Carl F. von Weizsäcker. The idea is to treat the nucleus as a droplet of an incompressible fluid with some surface tension—but again, quantum-mechanically. Another model, more reminscent of atomic physics, is the shell model. Here neutrons are protons are treated as moving in a potential well (which is actually created by their interaction with each other). Since protons and neutrons each separately obey the Pauli exclusion principle, there are—approximately—separate shells for each kind of particle, which fill up when their reaches a so-called magic number 2, 8, 20, 28, 50, 82, 126, … Thus, nuclei with a magic number of either protons or neutrons are especially stable, and ‘doubly magic’ nuclei, with a magic number of protons and a a magic number of neutrons, are even more stable: think of helium-4, oxygen-16, calcium-40, nickel-56, or lead-208 (with 82 protons and 126 neutrons). Yet another interesting approximation is to think of a nucleus as made of smaller nuclei, especially these four: • the deuteron: a deuteron, the nucleus of deuterium or 2H, is a proton and neutron stuck together. • the triton: a triton, the nucleus of tritium or 3H, is a proton and two neutrons stuck together. • the helion: a helion, the nucleus of 3He, is two protons and a neutron stuck together. • the alpha particle: an alpha particle, the nucleus of 4He, is two protons and two neutrons s...