Compton scattering

The effect was discovered in 1923 by Arthur Holly Compton while researching the scattering of X-rays by light elements, and earned him the Nobel Prize for Physics in 1927.

The Compton effect significantly deviated from dominating classical theories, using both special relativity and quantum mechanics to explain the interaction between high frequency photons and charged particles.

Photons can interact with matter at the atomic level (e.g. photoelectric effect and Rayleigh scattering), at the nucleus, or with just an electron.

This implies that if the recoiling particle initially carried more energy than the photon, the reverse would occur.

The effect is significant because it demonstrates that light cannot be explained purely as a wave phenomenon.

[4] Thomson scattering, the classical theory of an electromagnetic wave scattered by charged particles, cannot explain shifts in wavelength at low intensity: classically, light of sufficient intensity for the electric field to accelerate a charged particle to a relativistic speed will cause radiation-pressure recoil and an associated Doppler shift of the scattered light,[5] but the effect would become arbitrarily small at sufficiently low light intensities regardless of wavelength.

Or the assumption that the electron can be treated as free is invalid resulting in the effectively infinite electron mass equal to the nuclear mass (see e.g. the comment below on elastic scattering of X-rays being from that effect).

2, the interaction between an electron and a photon results in the electron being given part of the energy (making it recoil), and a photon of the remaining energy being emitted in a different direction from the original, so that the overall momentum of the system is also conserved.

The electron gains no internal energy, respective masses remain the same, the mark of an elastic collision.

Whether Compton scattering is considered elastic or inelastic depends on which perspective is being used, as well as the context.

High-energy photons of 1.022 MeV and above may bombard the nucleus and cause an electron and a positron to be formed, a process called pair production; even-higher-energy photons (beyond a threshold energy of at least 1.670 MeV, depending on the nuclei involved), can eject a nucleon or alpha particle from the nucleus in a process called photodisintegration.

[8] In 1923, Compton published a paper in the Physical Review that explained the X-ray shift by attributing particle-like momentum to light quanta (Albert Einstein had proposed light quanta in 1905 in explaining the photo-electric effect, but Compton did not build on Einstein's work).

Compton found that some X-rays experienced no wavelength shift despite being scattered through large angles; in each of these cases the photon failed to eject an electron.

A photon γ with wavelength λ collides with an electron e in an atom, which is treated as being at rest.

Compton allowed for the possibility that the interaction would sometimes accelerate the electron to speeds sufficiently close to the velocity of light as to require the application of Einstein's special relativity theory to properly describe its energy and momentum.

At the conclusion of Compton's 1923 paper, he reported results of experiments confirming the predictions of his scattering formula, thus supporting the assumption that photons carry momentum as well as quantized energy.

At the start of his derivation, he had postulated an expression for the momentum of a photon from equating Einstein's already established mass-energy relationship of

, and thus hf can be substituted for pc for all photon momentum terms which arise in course of the derivation below.

, After scattering, the possibility that the electron might be accelerated to a significant fraction of the speed of light, requires that its total energy be represented using the relativistic energy–momentum relation Substituting these quantities into the expression for the conservation of energy gives This expression can be used to find the magnitude of the momentum of the scattered electron, Note that this magnitude of the momentum gained by the electron (formerly zero) exceeds the energy/c lost by the photon, Equation (1) relates the various energies associated with the collision.

yields Finally, since fλ = f′λ′ = c, It can further be seen that the angle φ of the outgoing electron with the direction of the incoming photon is specified by Compton scattering is of prime importance to radiobiology, as it is the most probable interaction of gamma rays and high energy X-rays with atoms in living beings and is applied in radiation therapy.

Compton suppression is used to detect stray scatter gamma rays to counteract this effect.

Magnetic Compton scattering is an extension of the previously mentioned technique which involves the magnetisation of a crystal sample hit with high energy, circularly polarised photons.

This means that the MCP is ideal for comparison with theoretical techniques such as density functional theory.

In X-ray astronomy, the accretion disk surrounding a black hole is presumed to produce a thermal spectrum.

This is surmised to cause the power law component in the X-ray spectra (0.2–10 keV) of accreting black holes.

[13] The effect is also observed when photons from the cosmic microwave background (CMB) move through the hot gas surrounding a galaxy cluster.

The CMB photons are scattered to higher energies by the electrons in this gas, resulting in the Sunyaev–Zel'dovich effect.

Some synchrotron radiation facilities scatter laser light off the stored electron beam.

This Compton backscattering produces high energy photons in the MeV to GeV range[14][15] subsequently used for nuclear physics experiments.

It is the non-linear version of inverse Compton scattering in which the conditions for multiphoton absorption by the charged particle are reached due to a very intense electromagnetic field, for example the one produced by a laser.