Compton Scattering

Compton Scattering is the collision process between a X-ray or a gamma ray and a bound atomic electron where only part of the energy of the electromagnetic ray is transferred to the electron.

The effect was at first observed by Arthur Holly Compton in 1923 at Washington University in St. Louis and explained in his article "A Quantum Theory of the Scattering of X-ray by Light Elements". Compton was rewarded the 1927 Nobel Prize in Physics for this discovery.

Arthur H. Compton treated the x-ray photons as particles and applied conservation of energy and conservation of momentum to the collision of a photon with a stationary electron.. He used the Planck relationship and the relativistic energy expression to derive the standard Compton formula:

<math>\Delta \lambda = \frac{h}{m_e c} (1-\cos \theta)</math>

Here, Δλ denotes the difference between the wavelengths of the incoming and the scattered ray, while θ is the angle of scattering.

The Compton Experiment

A collimated beam of high-energy photons, e.g. emitted from a radioactive ¹³⁷Cs source or an X-ray emitter hits a target, e.g. a rod of graphite. A scintillation counter is used to measure the number and the energy of the deflected photons at various angles.

Classical Expectation

The number of photons should vary with the angle of the deflection, while the energy (i.e., the wavelength) of the photons should remain unchanged.

Observation

While indeed many of the scattered photons have an unchanged wavelength, there are some which lost energy, i.e., their wavelength lengthened. This lengthening depends on the angle of the deflection only.

The probability for Compton scattering is approximately proportional to the atomic number Z, and for energies greater than 500 keV approximately proportional to <math>\frac{1}{E^\gamma}</math>, the energy of the gamma ray photon.