Transition radiation

Optical Transition radiation is produced by relativistic charged particles when they cross the interface of two media of different dielectric constants. 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 γ = E / mc² and is mostly directed forward, peaking at an angle of the order of 1 / γ relative to the particle's path. The intensity of the emitted radiation is roughly proportional to the particle's energy E.

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 γ and emittance.

The characteristics of this electromagnetic radiation makes it suitable for particle discrimination, particularly of electrons and hadrons in the momentum range between 1 GeV/c and 100 GeV/c. The transition radiation photons produced by electrons have wavelengths in the X-ray range, with energies typically in the range from 5 to 15 keV. However, the number of produced photons per interface crossing is very small: for particles with $\gamma = 2\times 10^3$, 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.

See also

• Transition radiation detector

External links

• Article on transition radiation: http://rkb.home.cern.ch/rkb/PH14pp/node194.html

Sources

• Interference phenomenon in optical transition radiation and its application to particle beam diagnostics and multiple-scattering measurements, L. Wartski et al., Journal of Applied Physics -- August 1975 -- Volume 46, Issue 8, pp. 3644-3653.