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FNWI --- IMAPP Department of Astrophysics
Radboud University > Faculty of Science > Department of Astrophysics

Radiation processes

Cherenkov radiation 
(Image courtesy Argonne National Laboratory and Idaho National Laboratory)The emission of electromagnetic radiation tends to be formulated in terms of one or more of the classical named radiation processes, most commonly blackbody, synchrotron, transition, and Cherenkov radiation, bremsstrahlung etc. However, these terms are simply short-hand for the radiation observed in a precise set of physical circumstances, e.g. ultra-relativistic particles spiralling infinitely in magnetic fields in the case of synchrotron radiation, or a particle travelling an infinite distance in a uniform dielectric in the case of Cherenkov radiation. They are not fundamentally distinct physical processes. In a complex physical situation - such as a cascade of particles from a cosmic-ray interaction in the Earth's atmosphere, or even something as simple as a particle spiralling in a magnetic field in a dielectric - the distinction between these radiation processes becomes both blurred and arbitrary. In such circumstances, it is best not to artificially distinguish between processes and simply stick to the well-known (and mostly correct) maxim ``accelerated charged particles produce electromagnetic radiation''.

To handle complex radiating systems in a logical fashion, researchers in Nijmegen are developing the `end-point' methodology, by which radiation is calculated from an instantaneous particle acceleration/deceleration event. In this way, the dependence of the radiation on the acceleration is made explicit, avoiding the “is it Cherenkov? Is it transition?” questions which have plagued previous calculations. Complex physical situations can be described via a super-position of these 'end-points'. This methodology is being used to calculate the radiation from cosmic-ray and neutrino interactions in the Earth's atmosphere and the Moon, and has already been used to show that the radiation from the Askaryan Effect in a dense medium is not (as previously thought) coherent Cherenkov radiation, but in fact coherent bremsstrahlung.

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