Compton scattering
An incident X-ray photon of energy E travels to the right and strikes a loosely bound outer-shell ("free") electron of an atom. The photon is deflected upward through angle theta as a scattered photon of lower energy E-prime (longer wavelength). A recoil (Compton) electron is ejected forward and downward through angle phi. A dashed line marks the original photon direction. Notes state that a larger scatter angle means greater photon energy loss (maximum at 180 degrees), the recoil electron stays in the forward hemisphere, a scattered photon always remains, and the interaction probability is proportional to electron density and nearly independent of atomic number Z.
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