Transitional radiation

Transition radiation is electromagnetic radiation that arises when a charged, highly relativistic particle passes the interface of two media with different permittivities while passing through matter . The energy of this radiation is typically between 5  keV and 15 keV, i.e. in the range of the X-ray spectrum . ${\displaystyle \varepsilon }$

Explanation

Various models can be used to explain the transition radiation. Even if the individual explanations are different, they do not contradict one another.

With the so-called "mirror charge model" , the transition radiation is explained by the fact that the charged particle generates a mirror charge in the medium of the other permittivity, which together with the approaching particle charge represents a variable dipole . This changeable dipole emits photons .

A second perspective looks at the time-varying dipoles that the charged particle induces on its way in the respective medium. All these time-varying dipoles in a plane perpendicular to the direction of movement of the charged particle emit their wave trains simultaneously. Due to the phase difference of the wave trains transmitted at different locations, however, there is usually destructive interference . Since the wave trains are emitted staggered in time along the direction of movement, the resulting phase differences mean that the radiation only interferes constructively at the boundary surface in a volume aligned in the direction of the particle path.

Another form of explanation shows that the emitted radiation corresponds to the difference between the two solutions of the (inhomogeneous) Maxwell equations for electromagnetic fields, each considered in one of the two media. To put it clearly: Since the electrical field of the observed particle is different in the two media, it has to "shake off" this difference when passing the interface.

properties

The intensity of the electromagnetic radiation emitted mainly in the forward direction is given by the Lorentz factor , the charge of the particle and the plasma frequencies and the two media. The radiated energy is therefore directly proportional to . The maximum of the angular distribution lies in the forward direction at the emission angle . For reasons of symmetry, however, there is no emission directly in the direction of particle movement. ${\displaystyle I}$ ${\displaystyle I={\frac {\gamma \,q^{2}\,(\omega _{1}-\omega _{2})^{2}}{3c}}}$ ${\displaystyle \gamma ={\frac {E}{mc^{2}}}}$ ${\displaystyle q}$ ${\displaystyle \omega _{1}}$ ${\displaystyle \omega _{2}}$ ${\displaystyle \gamma }$ ${\displaystyle \theta ={\frac {1}{\gamma }}}$

In contrast to the Tscherenkov effect , the transition radiation does not show any threshold behavior, so that, according to classical calculations, a radiation intensity other than zero can be expected even for low particle speeds. In terms of quantum mechanics, this can be interpreted as a very low but non-zero photon emission probability.

use

Transition radiation is used in high-energy physics to detect and identify high-energy particles (especially electrons and hadrons) from energies of around 1 GeV in transition radiation detectors (TRD). The dependence of the radiation intensity on the Lorentz factor allows conclusions to be drawn about the particle energy if the particle mass is known. If, on the other hand, the particle energy is known, the mass of the particle can be determined and thus the particle can be identified. ${\displaystyle \gamma }$

Historical

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

Individual evidence

1. ↑ ^{a b} Jochen Schnapka: Double track detection using the cathode readout on the ZEUS transition radiation detector. Archived from the original on June 26, 2007. Info: The archive link was automatically inserted and not yet checked. Please check the original and archive link according to the instructions and then remove this notice. In: Bonn University (Ed.): Diploma thesis University of Bonn . October 1998. Retrieved February 2, 2008.  
2. ↑ Frank Hagenbuck: Development of a novel imaging method for digital subtraction radiography with transition radiation at the Mainz microtron MAMI . In: Mainz University (Hrsg.): PhD Johannes Gutenberg University Mainz . March 2002.
3. ↑ John D. Jackson: Classical Electrodynamics . de Gruyter, 2002, ISBN 3-11-016502-3 .
4. ^ Rudolf Bock: The Particle Detector BriefBook. April 9, 1998, archived from the original on June 7, 2008 ; Retrieved November 18, 2013 . 
5. ^ VL Ginsburg and IM Frank, J. Exp. Theoret. Phys. (USSR) 16 (1946), p. 15.