Transition radiation

Transition radiation is electromagnetic radiation that is produced when a charged, hochrelativistisches particles during passage through matter passes through the interface of two media with different permittivities. The energy of this radiation is typically between 5 keV and 15 keV, ie in the region of the X-ray spectrum.

Explanation

To explain the transition radiation different models can be used. Even if the individual explanatory approaches are different, they do not contradict each other.

With the so-called " image-charge model " the transition radiation is explained that the charged particle creates an image charge in the medium of other permittivity, which together with the approaching particle charge constitutes a variable dipole. This variable dipole radiates photons.

A second perspective considers the time-varying dipoles which induces the charged particles on its way in the respective medium. All of these time-varying dipole in a plane perpendicular to the movement direction of the charged particle emitting their waveforms simultaneously. Due to the phase difference of the transmitted wave trains at different locations, however, is usually in the destructive interference. Since the wave trains are emitted with a time delay along the moving direction, the resulting phase differences result, interferes constructively radiation only in an aligned in the direction of the particle volume of the interface.

Another form of the statement turns out that the emitted radiation of the difference between the two solutions of ( inhomogeneous ) Maxwell's equations for electromagnetic fields, in each case viewed in a corresponding of the two media. Illustratively stated: Since the electric field of the subject particle in the two media varies, it must be " shake off " the difference in passing the interface.

Properties

The intensity of the light emitted mainly in the forward direction is given by the electromagnetic radiation with the Lorentz factor, the charge of the particle and the plasma frequencies of the two media. The radiated energy is thus directly proportional. The maximum of the angular distribution is in the forward direction at the emission angle. For reasons of symmetry, there is no emission directly in the direction of particle motion.

Unlike the Cherenkov effect the transition radiation shows no threshold behavior so that according to the classical account even for low particle velocities a nonzero radiation intensity is to be expected. In quantum mechanics can interpret as a very low but non-zero photon emission probability.

Use

Transition radiation in high energy physics for the detection and identification of high-energy particles (particularly electrons and hadrons ) from energies of about 1 GeV in transition radiation detectors (English Transition Radiation Detector TRD short ) are used. Due to the dependence of the radiation intensity from the Lorentz factor can be given a known mass of particles infer the particle. In contrast, the particle is known, the mass of the particle can be determined, and thus the particles can be identified.

Historical

The theory of transition radiation, as it was published in 1946 by Ginsburg and Frank, explained the Lilienfeld radiation as a form of transition radiation.

Swell

• Electrodynamics
• Particle Physics