One form of this process is responsible for the Sunyaev-Zel'dovich effect, while its so-called y-parameter can be used to identify galaxy clusters.  
One form of this process is responsible for the Sunyaev-Zel'dovich effect, while its so-called y-parameter can be used to identify galaxy clusters.  It was first identified in 1922 using a molybdenum source, a block of graphite and a Bragg interferometer. It reduces to classical Thomson scattering for massive targets, while it corresponds to classical Rayleigh scattering in the long-wavelength limit. For 10 points, name this process in which an X-ray or gamma ray scatters off an electron resulting in a change in wavelength.
One form of this process is responsible for the Sunyaev-Zel'dovich effect, while its so-called y-parameter can be used to identify galaxy clusters.  
{Compton} scattering [or {Compton} effect]