  This paper explores the transition between Compton Scattering and Inverse
Compton Scattering (ICS), which is characterized by an equal exchange of energy
and momentum between the colliding particles (electrons and photons). This
regime has been called Symmetric Compton Scattering (SCS) and has the unique
property of cancelling the energy-angle correlation of scattered photons, and,
when the electron recoil is large, transferring mono-chromaticity from one
colliding beam to the other, resulting in back-scattered photon beams that are
intrinsically monochromatic. The paper suggests that large-recoil SCS or
quasi-SCS can be used to design compact intrinsic monochromatic gamma-ray
sources based on compact linacs, thus avoiding the use of GeV-class electron
beams together with powerful laser/optical systems as those typically required
for ICS sources.
