= Nuclear quadrupole moment
{title2=$Q=\int\rho(\mathbf r)(3z^2-r^2)\,d^3r$}
The <electric quadrupole moment> of an <atomic nucleus>, conventionally specified in its stretched <nuclear spin> state. It measures a nonspherical nuclear charge distribution. Its interaction with the electronic electric-field gradient contributes to <hyperfine structure> and generally changes the magnetic-dipole <Landé interval rule>. A permanent rank-two moment vanishes for <nuclear spin> less than one.
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