23.4 Nuclear Radii, Strong-Force Evidence, and Closest Approach

Because nuclear mass is approximately proportional to (A) and volume is also proportional to (A), average nuclear density is roughly constant across many nuclides. That surprising result is one signature of saturation behavior in nuclear binding: each nucleon interacts strongly with nearby neighbors, not with every nucleon at long range.

At moderate energies, alpha particles are repelled before touching the nuclear surface, so Rutherford electrostatic treatment works well. At higher energies and smaller separation, measured scattering starts to deviate, signaling that another interaction is becoming relevant. Those deviations are evidence for the strong nuclear force.

The strong force must be attractive at nuclear distances and negligible at much larger distances. If it were long-range like Coulomb, atomic and macroscopic matter would behave completely differently. If it were purely repulsive, nuclei could not exist. The scattering evidence and nuclear stability together force the short-range attractive picture.

A useful interpretation is barrier language: Coulomb repulsion creates an external energy barrier for positive projectiles approaching a positive nucleus. If projectile energy is too low, turning point distance stays outside strong-force range. If energy or tunneling conditions are favorable, particles reach distances where strong attraction and nuclear processes become possible.