= First-forbidden unique beta-decay phase space
{title2=$C(E)\propto p_e^2+E_\nu^2$}
A first-forbidden unique <beta decay> has $|\Delta J|=2$ with a <parity> change. Its leading lepton shape factor contains two additional powers of momentum relative to an <allowed beta decay>. In the massless, no-Coulomb limit, $\int_0^Q E^2(Q-E)^2[E^2+(Q-E)^2]dE=2Q^7/105$. Thus its microscopic reduced nuclear matrix element multiplies a seventh-power phase-space factor, rather than the fifth power in the <Sargent rule>. A fifth-power comparison across different forbiddenness classes instead extracts convention-dependent effective strengths.
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