The binding curve and iron

Plotting the binding energy per nucleon $B/A$ as a function of the mass number $A$ gives one of the most important curves in all of physics, the binding curve (or Aston curve). It rises rapidly for light nuclei (deuterium, tritium, helium, lithium), reaches a maximum around ${}^{56}\text{Fe}$ with $B/A \approx 8{,}8\;\text{MeV/nucleon}$, and then slowly falls off towards heavy nuclei.

The binding-energy-per-nucleon curve. Light nuclei gain stability by fusing (green arrow), heavy ones by splitting (yellow arrow): both processes move towards the maximum at iron.

Two enormous consequences follow from this shape, which are the two ways of extracting energy from nuclei:

• Nuclei heavier than iron can split into two lighter nuclei, closer to the maximum, releasing energy: this is fission.
• Nuclei lighter than iron can fuse into heavier nuclei, also closer to the maximum, releasing energy: this is fusion.

Iron, the terminal state

Iron sits at the top of the curve and is therefore the “terminal state” of both processes: from iron onwards, neither fission nor fusion spontaneously release energy. This has a cosmic consequence: massive stars fuse ever-heavier elements up to iron, after which the iron core can no longer fuse to produce energy and collapses, giving rise to supernovae, neutron stars and black holes.

Collegamenti

Argomenti: Fisica nucleare Concetti: Energia di legame nucleare · Fissione nucleare · Fusione nucleare Competenze: Lettura dei grafici

Esercizi collegati: Curva di Aston e direzione dell’energia · Perché la fissione conviene · Consumo di idrogeno del Sole