$\gamma$-ray spectra and enhancement factors for positron annihilation   spectra with core-electrons
  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 $\beta\approx 2.3$. 
D. G. Green and G. F. Gribakin
physics.atom-ph cond-mat.other