= Nuclear rotational inertia relative to a rigid sphere
{title2=$\mathcal I=\hbar^2J(J+1)/(2E_J)$}
The excitation <energy> in an ideal $K=0$ <nuclear rotational band> determines an effective <moment of inertia> through the displayed equation. Comparing it with $2MR^2/5$ tests the rigid-sphere scale. A sequence of approximately constant $E_J/[J(J+1)]$ supports the rotational interpretation; systematic variation indicates that the effective inertia changes along the band. Collective rotation need not have a rigid-body inertia, notably when <nuclear pairing> is important.
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