  Many-body theory is developed to calculate the $\gamma$-spectra for positron
annihilation with valence and core electrons in the noble gas atoms. A proper
inclusion of correlation effects and core annihilation provides for an accurate
description of the measured spectra [Iwata \textit{et al.}, Phys. Rev. Lett.
{\bf 79}, 39 (1997)]. The theory enables us to calculate the enhancement
factors $\gamma_{nl}$, which describe the effect of electron-positron
correlations for annihilation on individual electron orbitals $nl$. We find
that the enhancement factors scale with the orbital ionization energy $I_{nl}$
(in electron-volt), as $\gamma_{nl}=1+\sqrt{A/I_{nl}}+(B/I_{nl})^{\beta}$,
where $A\approx 40$~eV, $B\approx 24$~eV and $\beta\approx 2.3$.
