Transition radiation
Encyclopedia
Optical Transition radiation is produced by relativistic charged particles when they cross the interface of two media of different dielectric constant
s. The emitted radiation is the homogeneous difference between the two inhomogeneous solutions of Maxwell's equations of the electric and magnetic fields of the moving particle in each medium separately. In other words, since the electric field of the particle is different in each medium, the particle has to "shake off" the difference when it crosses the boundary. The total energy loss of a charged particle on the transition depends on its Lorentz factor
and is mostly directed forward, peaking at an angle of the order of 
relative to the particle's path. The intensity of the emitted radiation is roughly proportional to the particle's energy .
Optical transition radiation is emitted both in the forward direction and reflected by the interface surface. In case of a foil having an angle at 45 degrees with respect to a particle beam, the particle beam's shape can be visually seen at an angle of 90 degrees. More elaborate analysis of the emitted visual radiation may allow for the determination of
and emittance.
The characteristics of this electromagnetic radiation
makes it suitable for particle discrimination, particularly of electron
s and hadron
s in the momentum range between and .
The transition radiation photon
s produced by electrons have wavelengths in the X-ray range, with energies typically in the range from 5 to . However, the number of produced photons per interface crossing is very small: for particles with
, about 0.8 X-ray photons are detected. Usually several layers of alternating materials or composites are used to collect enough transition radiation photons for an adequate measurement—for example, one layer of inert material followed by one layer of detector (e.g. microstrip gas chamber), and so on.
By placing interfaces (foils) of very precise thickness and foil separation, coherence effects will modify the transition radiation's spectral and angular characteristics. This allows a much higher number of photons to be obtained in a smaller angular "volume". Applications of this X-ray source are limited by the fact that the radiation is emitted in a cone, with a minimum intensity at the center. X-ray focusing devices (crystals/mirrors) are not easy to build for such radiation patterns.
Dielectric constant
The relative permittivity of a material under given conditions reflects the extent to which it concentrates electrostatic lines of flux. In technical terms, it is the ratio of the amount of electrical energy stored in a material by an applied voltage, relative to that stored in a vacuum...
s. The emitted radiation is the homogeneous difference between the two inhomogeneous solutions of Maxwell's equations of the electric and magnetic fields of the moving particle in each medium separately. In other words, since the electric field of the particle is different in each medium, the particle has to "shake off" the difference when it crosses the boundary. The total energy loss of a charged particle on the transition depends on its Lorentz factor
Lorentz factor
The Lorentz factor or Lorentz term appears in several equations in special relativity, including time dilation, length contraction, and the relativistic mass formula. Because of its ubiquity, physicists generally represent it with the shorthand symbol γ . It gets its name from its earlier...
and is mostly directed forward, peaking at an angle of the order of relative to the particle's path. The intensity of the emitted radiation is roughly proportional to the particle's energy .Optical transition radiation is emitted both in the forward direction and reflected by the interface surface. In case of a foil having an angle at 45 degrees with respect to a particle beam, the particle beam's shape can be visually seen at an angle of 90 degrees. More elaborate analysis of the emitted visual radiation may allow for the determination of
and emittance.The characteristics of this electromagnetic radiation
Electromagnetic radiation
Electromagnetic radiation is a form of energy that exhibits wave-like behavior as it travels through space...
makes it suitable for particle discrimination, particularly of electron
Electron
The electron is a subatomic particle with a negative elementary electric charge. It has no known components or substructure; in other words, it is generally thought to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton...
s and hadron
Hadron
In particle physics, a hadron is a composite particle made of quarks held together by the strong force...
s in the momentum range between and .
The transition radiation photon
Photon
In physics, a photon is an elementary particle, the quantum of the electromagnetic interaction and the basic unit of light and all other forms of electromagnetic radiation. It is also the force carrier for the electromagnetic force...
s produced by electrons have wavelengths in the X-ray range, with energies typically in the range from 5 to . However, the number of produced photons per interface crossing is very small: for particles with
, about 0.8 X-ray photons are detected. Usually several layers of alternating materials or composites are used to collect enough transition radiation photons for an adequate measurement—for example, one layer of inert material followed by one layer of detector (e.g. microstrip gas chamber), and so on.By placing interfaces (foils) of very precise thickness and foil separation, coherence effects will modify the transition radiation's spectral and angular characteristics. This allows a much higher number of photons to be obtained in a smaller angular "volume". Applications of this X-ray source are limited by the fact that the radiation is emitted in a cone, with a minimum intensity at the center. X-ray focusing devices (crystals/mirrors) are not easy to build for such radiation patterns.
External links
- Article on transition radiation: http://rkb.home.cern.ch/rkb/PH14pp/node194.html