Beyond the Point Ps Approximation
  In application to positron annihilation spectroscopy, Ps atom is considered not as a point particle, but as a finite size e+ e- pair localized in a bubble-state in a medium. Variation of the internal Coulombic e+ -e- attraction vs. the bubble radius is estimated. 
Sergey V. Stepanov and Dmitry S. Zvezhinskiy and Vsevolod M. Byakov
cond-mat.other physics.chem-ph