Astroparticle Physics

6.1 Charged Component of Primary Cosmic Rays 85collisions) and if the muon content and lateral distribution ofmuons in extensive air showers are taken as a criterion forthe identity of the primary particle, then one would arriveat the conclusion that the chemical composition of primarycosmic rays cannot be very different from the compositionbelow the knee (< 10 15 eV). Some experiments, however,seem to indicate that the iron fraction of primary cosmicrays increases with energy beyond the knee. This could justbe a consequence of galactic containment: the iron knee isexpected to occur at higher energies compared to the protonknee.Even though cosmic rays have been discovered about 90years ago, their origin is still an open question. It is generallyassumed that active galactic nuclei, quasars, or supernovaexplosions are excellent source candidates for high-energycosmic rays, but there is no direct evidence for this assumption.In the energy range up to 100 TeV individual sourceshave been identified by primary gamma rays. It is conceivablethat gamma rays of these energies are decay products ofelementary particles (π 0 decay, Centaurus A?), which havebeen produced by those particles that have been originallyaccelerated in the sources. Therefore, it would be interestingto see the sources of cosmic rays in the light of theseoriginally accelerated particles.This, however, presents a serious problem: photons andneutrinos travel on straight lines in galactic and intergalacticspace, therefore pointing directly back to the sources.Charged particles, on the other hand, are subject to the influenceof homogeneous or irregular magnetic fields. Thiscauses the accelerated particles to travel along chaotic trajectoriesthereby losing all directional information beforefinally reaching Earth. Therefore, it is of very little surprisethat the sky for charged particles with energies below10 14 eV appears completely isotropic. The level of observedanisotropies lies below 0.5%. There is some hope that forenergies exceeding 10 18 eV a certain directionality could befound. It is true that also in this energy domain the galacticmagnetic fields must be taken into account, however, the deflectionradii are already rather large. The situation is evenmore complicated because of a rather uncertain topologyof galactic magnetic fields. In addition, one must in principleknow the time evolution of magnetic fields over thelast ≈ 50 million years because the sources can easily resideat distances of > 10 Mpc (̂= 32.6 million light-years).For simultaneous observation of cosmic-ray sources in theorigin of cosmic rayshadronic originof energetic γ s?isotropy of charged particlesanisotropy