= Uniform-sphere Coulomb displacement energy
{title2=$\Delta E_C=3\alpha\hbar c\,\Delta(Z^2)/(5R)$}
The electrostatic self-energy of a uniformly charged sphere of sharp radius $R$ is $3Z^2\alpha\hbar c/(5R)$. For mirror nuclei, subtract the changed neutron/proton rest-mass contribution from their nuclear mass difference before identifying the residual with a <nuclear Coulomb energy> difference. The inferred $R$ is a sharp-sphere radius; the corresponding <nuclear charge radius> defined as an rms radius is $\sqrt{3/5}R$. Replacing $Z^2$ by $Z(Z-1)$ removes a proton self-interaction approximation and shifts the estimate.
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