The strong nuclear force

Inside a nucleus the protons are crammed together at distances of the order of $10^{-15}\;\text{m}$, all positively charged. At such small distances the repulsive Coulomb force between two protons is gigantic: the law $F = k\,e^2/r^2$ grows as the inverse square of the distance, and at nuclear scale it becomes enormous. Why, then, does the nucleus not explode instantly?

The answer is that there exists a new fundamental interaction, the strong nuclear force, which attracts all nucleons to one another — protons and neutrons alike. Two of its features make it suited to act as nuclear glue:

• It is extremely intense at short range: at nuclear distances it is about $100$ times stronger than the Coulomb repulsion, and so it wins out, holding the nucleus together.
• It has a very short range: its action practically vanishes beyond $\sim 2\;\text{fm}$ ($2\cdot 10^{-15}\;\text{m}$). A nucleon feels the strong force only from its immediate neighbours, not from the whole nucleus.

It is precisely this limited range that explains why nuclei cannot grow indefinitely. The strong force binds only adjacent nucleons (it saturates), while the electric repulsion of the protons acts over long range and adds up over all pairs. In very large nuclei the Coulomb repulsion eventually prevails over the strong cohesion, and the nucleus becomes unstable: this is the deep root of the radioactivity of heavy nuclei and their tendency towards fission.

Collegamenti

Argomenti: Fisica nucleare Oggetti: Nucleo atomico

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