Compton scattering

Compton scattering, discovered by Arthur Holly Compton, is the scattering of a photon by a charged particle, usually an electron. It results in a decrease in energy (increase in wavelength) of the photon (which may be an X-ray or gamma ray photon), called the Compton effect. Part of the energy of the photon is transferred to the recoiling electron. Inverse Compton scattering occurs when a charged particle transfers part of its energy to a photon.

Introduction

Compton en
Fig. 1: Schematic diagram of Compton's experiment. Compton scattering occurs in the graphite target on the left. The slit passes X-ray photons scattered at a selected angle. The energy of a scattered photon is measured using Bragg scattering in the crystal on the right in conjunction with ionization chamber; the chamber could measure total energy deposited over time, not the energy of single scattered photons.

Compton scattering is an example of inelastic scattering[1] of light by a free charged particle, where the wavelength of the scattered light is different from that of the incident radiation. In Compton's original experiment (see Fig. 1), the energy of the X ray photon (≈17 keV) was very much larger than the binding energy of the atomic electron, so the electrons could be treated as being free. The amount by which the light's wavelength changes is called the Compton shift. Although nuclear Compton scattering exists,[2] Compton scattering usually refers to the interaction involving only the electrons of an atom. The Compton effect was observed by Arthur Holly Compton in 1923 at Washington University in St. Louis and further verified by his graduate student Y. H. Woo in the years following. Compton earned the 1927 Nobel Prize in Physics for the discovery.

The effect is significant because it demonstrates that light cannot be explained purely as a wave phenomenon. [3] 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,[4] but the effect would become arbitrarily small at sufficiently low light intensities regardless of wavelength. Thus, light must behave as if it consists of particles, if we are to explain low-intensity Compton scattering. 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). Compton's experiment convinced physicists that light can be treated as a stream of particle-like objects (quanta called photons), whose energy is proportional to the light wave's frequency. But see the article on Julian Schwinger for Schwinger's different assessment of the necessity of any particles at all in a consistent QED or QCD.

As shown in Fig. 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. If the scattered photon still has enough energy, the process may be repeated. In this scenario, the electron is treated as free or loosely bound. Experimental verification of momentum conservation in individual Compton scattering processes by Bothe and Geiger as well as by Compton and Simon has been important in disproving the BKS theory.

Compton scattering is one of three competing processes when photons interact with matter. At energies of a few eV to a few keV, corresponding to visible light through soft X-rays, a photon can be completely absorbed and its energy can eject an electron from its host atom, a process known as the photoelectric effect. 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. Compton scattering is the most important interaction in the intervening energy region.

Description of the phenomenon

Compton-scattering
Fig. 2: A photon of wavelength comes in from the left, collides with a target at rest, and a new photon of wavelength emerges at an angle . The target recoils, carrying away an angle-dependent amount of the incident energy.

By the early 20th century, research into the interaction of X-rays with matter was well under way. It was observed that when X-rays of a known wavelength interact with atoms, the X-rays are scattered through an angle and emerge at a different wavelength related to . Although classical electromagnetism predicted that the wavelength of scattered rays should be equal to the initial wavelength,[5] multiple experiments had found that the wavelength of the scattered rays was longer (corresponding to lower energy) than the initial wavelength.[5]

In 1923, Compton published a paper in the Physical Review that explained the X-ray shift by attributing particle-like momentum to light quanta (Einstein had proposed light quanta in 1905 in explaining the photo-electric effect, but Compton did not build on Einstein's work). The energy of light quanta depends only on the frequency of the light. In his paper, Compton derived the mathematical relationship between the shift in wavelength and the scattering angle of the X-rays by assuming that each scattered X-ray photon interacted with only one electron. His paper concludes by reporting on experiments which verified his derived relation:

where
is the initial wavelength,
is the wavelength after scattering,
is the Planck constant,
is the electron rest mass,
is the speed of light, and
is the scattering angle.

The quantity h/mec is known as the Compton wavelength of the electron; it is equal to 2.43×10−12 m. The wavelength shift λ′λ is at least zero (for θ = 0°) and at most twice the Compton wavelength of the electron (for θ = 180°).

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.[5] Thus the magnitude of the shift is related not to the Compton wavelength of the electron, but to the Compton wavelength of the entire atom, which can be upwards of 10000 times smaller. This is known as "coherent" scattering off the entire atom since the atom remains intact, gaining no internal excitation.

In Compton's original experiments the wavelength shift given above was the directly-measurable observable. In modern experiments it is conventional to measure the energies, not the wavelengths, of the scattered photons. For a given incident energy , the outgoing final-state photon energy, , is given by

Derivation of the scattering formula

ComptonEnergy
Fig. 3: Energies of a photon at 500 keV and an electron after Compton scattering.

A photon γ with wavelength λ collides with an electron e in an atom, which is treated as being at rest. The collision causes the electron to recoil, and a new photon γ' with wavelength λ' emerges at angle θ from the photon's incoming path. Let e' denote the electron after the collision. 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 to the quantized photon energies of , which Einstein had separately postulated. If , the equivalent photon mass must be . The photon's momentum is then simply this effective mass times the photon's frame-invariant velocity c. For a photon, its momentum , and thus hf can be substituted for pc for all photon momentum terms which arise in course of the derivation below. The derivation which appears in Compton's paper is more terse, but follows the same logic in the same sequence as the following derivation.

The conservation of energy merely equates the sum of energies before and after scattering.

Compton postulated that photons carry momentum;[5] thus from the conservation of momentum, the momenta of the particles should be similarly related by

in which () is omitted on the assumption it is effectively zero.

The photon energies are related to the frequencies by

where h is Planck's constant.

Before the scattering event, the electron is treated as sufficiently close to being at rest that its total energy consists entirely of the mass-energy equivalence of its (rest) mass ,

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. The electron's momentum change involves a relativistic change in the energy of the electron, so it is not simply related to the change in energy occurring in classical physics. The change of the magnitude of the momentum of the photon is not just related to the change of its energy; it also involves a change in direction.

Solving the conservation of momentum expression for the scattered electron's momentum gives

Making use of the scalar product yields the square of its magnitude,

In anticipation of being replaced with , multiply both sides by ,

After replacing the photon momentum terms with , we get a second expression for the magnitude of the momentum of the scattered electron,

Equating the alternate expressions for this momentum gives

which, after evaluating the square and canceling and rearranging terms, further yields

Dividing both sides by yields

Finally, since = 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

Applications

Compton scattering

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.[6]

In material physics, Compton scattering can be used to probe the wave function of the electrons in matter in the momentum representation.

Compton scattering is an important effect in gamma spectroscopy which gives rise to the Compton edge, as it is possible for the gamma rays to scatter out of the detectors used. Compton suppression is used to detect stray scatter gamma rays to counteract this effect.

Magnetic Compton scattering

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. By measuring the scattered photons' energy and reversing the magnetisation of the sample, two different Compton profiles are generated (one for spin up momenta and one for spin down momenta). Taking the difference between these two profiles gives the magnetic Compton profile (MCP), given by - a one-dimensional projection of the electron spin density.

where is the number of spin-unpaired electrons in the system, and are the three-dimensional electron momentum distributions for the majority spin and minority spin electrons respectively.

Since this scattering process is incoherent (there is no phase relationship between the scattered photons), the MCP is representative of the bulk properties of the sample and is a probe of the ground state. This means that the MCP is ideal for comparison with theoretical techniques such as density functional theory. The area under the MCP is directly proportional to the spin moment of the system and so, when combined with total moment measurements methods (such as SQUID magnetometry), can be used to isolate both the spin and orbital contributions to the total moment of a system. The shape of the MCP also yields insight into the origin of the magnetism in the system.[7]

Inverse Compton scattering

Inverse Compton scattering is important in astrophysics. In X-ray astronomy, the accretion disk surrounding a black hole is presumed to produce a thermal spectrum. The lower energy photons produced from this spectrum are scattered to higher energies by relativistic electrons in the surrounding corona. This is surmised to cause the power law component in the X-ray spectra (0.2-10 keV) of accreting Black Holes.

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. Observations of the Sunyaev-Zel'dovich effect provide a nearly redshift-independent means of detecting galaxy clusters.

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[8] subsequently used for nuclear physics experiments.

See also

References

  1. ^ Elastic or inelastic scattering? The incident photon loses energy in the lab frame, which centuries of practice had identified with inelastic scattering—even though, in the c.m. frame, the respective masses remaining the same, no new species are created and kinetic energy is conserved, the mark of an elastic collision. As a result, HEP and nuclear physicists prefer to emphasize elasticity, while atomic and molecular physicists use "inelastic".
  2. ^ P. Christillin (1986). "Nuclear Compton scattering". J. Phys. G: Nucl. Phys. 12 (9): 837–851. Bibcode:1986JPhG...12..837C. doi:10.1088/0305-4616/12/9/008.
  3. ^ Griffiths, David (1987). Introduction to Elementary Particles. Wiley. pp. 15, 91. ISBN 0-471-60386-4.
  4. ^ C. Moore (1995). "Observation of the Transition from Thomson to Compton Scattering in Optical Multiphoton Interactions with Electrons" (PDF).
  5. ^ a b c d Taylor, J.R.; Zafiratos, C.D.; Dubson, M.A. (2004). Modern Physics for Scientists and Engineers (2nd ed.). Prentice Hall. pp. 136–9. ISBN 0-13-805715-X.
  6. ^ Camphausen KA, Lawrence RC. "Principles of Radiation Therapy" in Pazdur R, Wagman LD, Camphausen KA, Hoskins WJ (Eds) Cancer Management: A Multidisciplinary Approach. 11 ed. 2008.
  7. ^ Malcolm Cooper (14 October 2004). X-Ray Compton Scattering. OUP Oxford. ISBN 978-0-19-850168-8. Retrieved 4 March 2013.
  8. ^ "GRAAL home page". Lnf.infn.it. Retrieved 2011-11-08.

Further reading

External links

Compton edge

In spectrophotometry, the Compton edge is a feature of the spectrograph that results from the Compton scattering in the scintillator or detector. When a gamma-ray scatters off the scintillator but escapes, only some fraction of its energy is registered by the detector. The amount of energy deposited in the detector depends on the scattering angle of the photon, leading to a spectrum of energies each corresponding to a different scattering angle. The highest energy that can be deposited, corresponding to full back-scatter, is called the Compton edge.

Compton telescope

A Compton telescope (also known as Compton camera or Compton imager) is a gamma-ray detector which utilizes Compton scattering to determine the origin of the observed gamma rays.

Compton cameras are usually applied to detect gamma rays in the energy range where Compton scattering is the dominating interaction process, from a few hundred keV to several MeV. They are applied in fields such as astrophysics, nuclear medicine, and nuclear threat detection.

In astrophysics, the most famous Compton telescopes was COMPTEL aboard the Compton Gamma-ray Observatory, which pioneered the observation of the gamma-ray sky in the energy range between 0.75 and 30 MeV. A potential successor is NCT - the Nuclear Compton Telescope.

Compton wavelength

The Compton wavelength is a quantum mechanical property of a particle. It was introduced by Arthur Compton in his explanation of the scattering of photons by electrons (a process known as Compton scattering). The Compton wavelength of a particle is equal to the wavelength of a photon whose energy is the same as the mass (see mass–energy equivalence) of that particle.

The standard Compton wavelength, λ, of a particle is given by

where h is the Planck constant, m is the particle's mass, and c is the speed of light. The significance of this formula is shown in the derivation of the Compton shift formula.

The CODATA 2014 value for the Compton wavelength of the electron is 2.4263102367(11)×10−12 m. Other particles have different Compton wavelengths.

Cosmic-ray observatory

A cosmic-ray observatory is a scientific installation built to detect high-energy-particles coming from space called cosmic rays. This typically includes photons (high-energy light), electrons, protons, and some heavier nuclei, as well as antimatter particles. About 90% of cosmic rays are protons, 9% are alpha particles, and the remaining ~1% are other particles.

It is not yet possible to build image forming optics for cosmic rays, like a Wolter telescope for lower energy X-rays, although some cosmic-ray observatories also look for high energy gamma rays and x-rays. Ultra-high-energy cosmic rays (UHEC) pose further detection problems. One way of learning about cosmic rays is using different detectors to observe aspects of a cosmic ray air shower.

Methods of detection for gamma-rays:

Scintillation detectors

Solid state detectors

Compton scattering

Pair telescopes

Air Cerenkov detectorsFor example, while a visible light photon may have an energy of a few eV, a cosmic gamma ray may exceed a TeV (1,000,000,000,000 eV). Sometimes cosmic gamma rays (photons) are not grouped with nuclei cosmic rays.

Electron scattering

Electron scattering occurs when electrons are deviated from their original trajectory. This is due to the electrostatic forces within matter interaction or, if an external magnetic field is present, the electron may be deflected by the Lorentz force. This scattering typically happens with solids such as metals, semiconductors and insulators; and is a limiting factor in integrated circuits and transistors.The application of electron scattering is such that it can be used as a high resolution microscope for hadronic systems, that allows the measurement of the distribution of charges for nucleons and nuclear structure. The scattering of electrons has allowed us to understand that protons and neutrons are made up of the smaller elementary subatomic particles called quarks.Electrons may be scattered through a solid in several ways:

Not at all: no electron scattering occurs at all and the beam passes straight through.

Single scattering: when an electron is scattered just once.

Plural scattering: when electron(s) scatter several times.

Multiple scattering: when electron(s) scatter very many times over.The likelihood of an electron scattering and the proliferance of the scattering is a probability function of the specimen thickness to the mean free path.

Gamma ray

A gamma ray or gamma radiation (symbol γ or ), is a penetrating electromagnetic radiation arising from the radioactive decay of atomic nuclei. It consists of the shortest wavelength electromagnetic waves and so imparts the highest photon energy. Paul Villard, a French chemist and physicist, discovered gamma radiation in 1900 while studying radiation emitted by radium. In 1903, Ernest Rutherford named this radiation gamma rays based on their relatively strong penetration of matter; he had previously discovered two less penetrating types of decay radiation, which he named alpha rays and beta rays in ascending order of penetrating power.

Gamma rays from radioactive decay are in the energy range from a few keV to ~8 MeV, corresponding to the typical energy levels in nuclei with reasonably long lifetimes. The energy spectrum of gamma rays can be used to identify the decaying radionuclides using gamma spectroscopy. Very-high-energy gamma rays in the 100–1000 TeV range have been observed from sources such as the Cygnus X-3 microquasar.

Natural sources of gamma rays originating on Earth are mostly as a result of radioactive decay and secondary radiation from atmospheric interactions with cosmic ray particles. However there are other rare natural sources, such as terrestrial gamma-ray flashes, that produce gamma rays from electron action upon the nucleus. Notable artificial sources of gamma rays include fission, such as occurs in nuclear reactors, as well as high energy physics experiments, such as neutral pion decay and nuclear fusion.

Gamma rays and X-rays are both electromagnetic radiation and they overlap in the electromagnetic spectrum, the terminology varies between scientific disciplines. In some fields of physics, they are distinguished by their origin: Gamma rays are created by nuclear decay, while in the case of X-rays, the origin is outside the nucleus. In astrophysics, gamma rays are conventionally defined as having photon energies above 100 keV and are the subject of gamma ray astronomy, while radiation below 100 keV is classified as X-rays and is the subject of X-ray astronomy. This convention stems from the early man-made X-rays, which had energies only up to 100 keV, whereas many gamma rays could go to higher energies. A large fraction of astronomical gamma rays are screened by Earth's atmosphere.

Gamma rays are ionizing radiation and are thus biologically hazardous. Due to their high penetration power, they can damage bone marrow and internal organs. Unlike alpha and beta rays, they pass easily through the body and thus pose a formidable radiation protection challenge, requiring shielding made from dense materials such as lead or concrete.

Greydon Square

Eddie Collins (born September 28, 1981), better known by his stage name Greydon Square, is an American West Coast hip hop emcee, producer and sound engineer from Compton, California. He is a former U.S. Army soldier and Iraq War veteran who is also an outspoken atheist. He promotes discussion on philosophical and scientific issues.

HERA (particle accelerator)

HERA (German: Hadron-Elektron-Ringanlage, English: Hadron-Electron Ring Accelerator) was a particle accelerator at DESY in Hamburg. It began operating in 1992. At HERA, electrons or positrons were collided with protons at a center of mass energy of 318 GeV. It was the only lepton-proton collider in the world while operating. Also, it was on the energy frontier in certain regions of the kinematic range. HERA was closed down on 30 June 2007.The HERA tunnel is located under the DESY site and the nearby Volkspark around 15 to 30 m underground and has a circumference of 6.3 km. Leptons and protons were stored in two independent storage rings on top of each other inside this tunnel.

There are four interaction regions, which were used by the experiments H1, ZEUS, HERMES and HERA-B. All these experiments were particle detectors.

Leptons (electrons or positrons) were pre-accelerated to 450 MeV in the linear accelerator LINAC-II. From there they were injected into the storage ring DESY-II and accelerated further to 7.5 GeV before their transfer into PETRA, where they were accelerated to 14 GeV. Finally they were injected into their storage ring in the HERA tunnel and reached a final energy of 27.5 GeV. This storage ring was equipped with warm (non-superconducting) magnets keeping the leptons on their circular track by a magnetic field of 0.17 Tesla.

Protons were obtained from originally negatively charged hydrogen ions and pre-accelerated to 50 MeV in a linear accelerator. They were then injected into the proton synchrotron DESY-III and accelerated further to 7 GeV. Then they were transferred to PETRA where they were accelerated to 40 GeV. Finally, they were injected into their storage ring in the HERA tunnel and reached their final energy of 920 GeV. The proton storage ring used superconducting magnets to keep the protons on track.

The lepton beam in HERA became naturally transversely polarised through the Sokolov-Ternov effect. The characteristic build-up time expected for the HERA accelerator was approximately 40 minutes. Spin rotators on either side of the experiments changed the transverse polarisation of the beam into longitudinal polarisation. The positron beam polarisation was measured using two independent polarimeters, the transverse polarimeter (TPOL) and the longitudinal polarimeter (LPOL). Both devices exploit the spin-dependent cross section for Compton scattering of circularly polarised photons off positrons to measure the beam polarisation. The transverse polarimeter was upgraded in 2001 to provide a fast measurement for every positron bunch, and position-sensitive silicon strip and scintillating-fibre detectors were added to investigate systematic effects.

On 30 June 2007 at 11:23 pm, HERA was shut down, and dismantling of the four experiments started. HERA's main pre-accelerator PETRA was converted into a synchrotron radiation source, operating under the name PETRA-III since August 2010.

Inelastic scattering

In chemistry, nuclear physics, and particle physics, inelastic scattering is a fundamental scattering process in which the kinetic energy of an incident particle is not conserved (in contrast to elastic scattering). In an inelastic scattering process, some of the energy of the incident particle is lost or increased. Although the term is historically related to the concept of inelastic collision in dynamics, the two concepts are quite distinct; inelastic collision in dynamics refers to processes in which the total macroscopic kinetic energy is not conserved. In general, scattering due to inelastic collisions will be inelastic, but, since elastic collisions often transfer kinetic energy between particles, scattering due to elastic collisions can also be inelastic, as in Compton scattering.

J-phenomenon

The J-phenomenon was a hypothetical form of X-ray behaviour similar to X-ray fluorescence. It was reported and studied by C. G. Barkla but other scientists were not persuaded that that this was a different mechanism from other known effects such as Compton scattering and so the theory was not successful.

James Douglas Beason

James Douglas Beason, from the Air Force Research Laboratory, was awarded the status of Fellow of the American Physical Society after being nominated nominated by the APS in 2000. The honor was for his advancement of national science policy, especially for his impact throughout the government on basic research. In addition, he has fundamentally advanced science in his work toward solving the relativistic Compton scattering kernel, and inventing innovative techniques for simulating lasers and plasmas.

James Paul Miller

James Paul Miller from the Boston University, was awarded the status of Fellow in the American Physical Society, after they were nominated by their Division of Nuclear Physics in 1995, for the development of a high resolution NaI detector and the performance of pioneering experiments on nuclear Compton scattering and radiative kaon capture utilizing this device which paved the way for the design and construction of other high resolution calorimeters.

Latifa Elouadrhiri

Latifa Elouadrhiri is a Moroccan experimental physicist and researcher at Thomas Jefferson National Accelerator Facility studying elementary particle physics and nuclear physics.

She has worked significantly with the CLAS collaboration in Jefferson Lab's Hall B, performing 3D imaging of nucleons.

Additionally, she is the spokesperson of the Deeply Virtual Compton Scattering (DVCS) experiment, studying Generalized Parton Distributions

.Elouadrhiri is a fellow of the American Physical Society, and has served as a member of the Department of Energy (DOE) Nuclear Science Advisory Committee (NSAC) and the DOE Nuclear Physics (NP) Committee of Visitors (COV).

Mihai Gavrilă

Mihai Gavrilă (Romanian pronunciation: [miˈhaj ɡaˈvrilə]; b. October 16, 1929, Cluj) is a Romanian quantum physicist, member of the Romanian Academy since 1974. He made fundamental contributions to quantum theories of electromagnetic interactions with atoms. His parents were Ion and Florica Gavrilă (née Vișoiu). His father taught medicine and his mother taught English at the University of Cluj.

Nucleon spin structure

Nucleon spin structure describes the partonic structure of nucleon (proton and neutron) intrinsic angular momentum (spin). The key question is how the nucleon's spin, whose magnitude is 1/2ħ, is carried by its constituent partons (quarks and gluons). It was originally expected before the 1980s that quarks carry all of the nucleon spin, but later experiments contradict this expectation. In the late 1980s, the European Muon Collaboration (EMC) conducted experiments that suggested the spin carried by quarks is not sufficient to account for the total spin of the nucleons. This finding astonished particle physicists at that time, and the problem of where the missing spin lies is sometimes referred to as the proton spin crisis.

Experimental research on these topics has been continued by the Spin Muon Collaboration (SMC) and the COMPASS experiment at CERN, experiments E142, E143, E154 and E155 at SLAC, HERMES at DESY, experiments at JLab and RHIC, and others. Global analysis of data from all major experiments confirmed the original EMC discovery and showed that the quark spin did contribute about 30% to the total spin of the nucleon. A major topic of modern particle physics is to find the missing angular momentum, which is believed to be carried either by gluon spin, or by gluon and quark orbital angular momentum. This fact is expressed by the sum rule,

The gluon spin components are being measured by many experiments. Quark and gluon angular momenta will be studied by measuring so-called generalized parton distributions (GPD) through deeply virtual compton scattering (DVCS) experiments, conducted at CERN (COMPASS) and at Jefferson Lab, among other laboratories.

Perley Ason Ross

Perley Ason Ross (6 April 1883 – 13 March 1939) was a U.S. experimental physicist who worked, carefully and without seeking publicity, at some essential problems in the behaviour of X-rays.Born in Panacea, Missouri he was awarded his PhD from Stanford University in 1911, becoming a full professor there in 1927, after a year at Cornell University.Some of his principal studies included:

Scattering of X-rays by matter;

Development of the Ross differential filter for X-ray spectroscopy;

X-ray polarization;

Compton scattering; and

Radiative Auger effect.His daughter, Betsy, married fellow Stanford physicist William Webster Hansen.

Sunyaev–Zeldovich effect

The Sunyaev–Zel'dovich effect (named after Rashid Sunyaev and Yakov B. Zel'dovich and often abbreviated as the SZ effect) is the distortion of the cosmic microwave background radiation (CMB) through inverse Compton scattering by high-energy electrons in galaxy clusters, in which the low energy CMB photons receive an average energy boost during collision with the high-energy cluster electrons. Observed distortions of the cosmic microwave background spectrum are used to detect the density perturbations of the universe. Using the Sunyaev–Zel'dovich effect, dense clusters of galaxies have been observed.

Thomson scattering

Thomson scattering is the elastic scattering of electromagnetic radiation by a free charged particle, as described by classical electromagnetism. It is just the low-energy limit of Compton scattering: the particle kinetic energy and photon frequency do not change as a result of the scattering. This limit is valid as long as the photon energy is much smaller than the mass energy of the particle: , or equivalently, if the wavelength of the light is much greater than the Compton wavelength of the particle.

X-ray

X-rays make up X-radiation, a form of electromagnetic radiation. Most X-rays have a wavelength ranging from 0.01 to 10 nanometers, corresponding to frequencies in the range 30 petahertz to 30 exahertz (3×1016 Hz to 3×1019 Hz) and energies in the range 100 eV to 100 keV. X-ray wavelengths are shorter than those of UV rays and typically longer than those of gamma rays. In many languages, X-radiation is referred to with terms meaning Röntgen radiation, after the German scientist Wilhelm Röntgen who discovered these on November 8, 1895, who usually is credited as its discoverer, and who named it X-radiation to signify an unknown type of radiation. Spelling of X-ray(s) in the English language includes the variants x-ray(s), xray(s), and X ray(s).

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