Compton scattering
In physics, Compton scattering, or the Compton effect, is the name used for what happens to the energy (or frequency or wavelength) of an X-ray or gamma ray photon when it interacts with matter; the wavelength increases (or energy/frequency decreases) as it scatters off electrons. This scattering is one of the main things that happen when gamma rays meet matter. The Compton effect was studied by Arthur Holly Compton  in 1923.