In 1958, 46 years after the discovery of cosmic rays by Hess, Kulikov and Khristiansen measured the “knee” of the cosmic ray spectrum, which is associated with a change of the index of the power law. Many decades passed, before in 2003 the KASCADE experiment clarified post hoc that this knee is caused by a change in the spectra of light cosmic ray mass groups. The knowledge about the composition of cosmic rays and the existence of specific structures in the energy spectrum around the knee is of fundamental importance for understanding things like the transition from galactic to extragalactic origin of cosmic rays, or the cosmic ray acceleration and propagation in general.
The KASCADE-Grande experiment, an extension of the original KASCADE experiment, is dedicated to measure the energy spectra for elemental groups of cosmic rays at one or two orders of magnitude higher energies than its predecessor, namely in the energy range from 1016 eV to 1018 eV. It thereby enables the verification of a possible second knee expected at approximately 1017 eV.
This work's primary objective is to determine the energy spectra of different cosmic ray mass groups by means of unfolding techniques applied to the KASCADE-Grande dataset. This analysis yields strong indications for a knee-like structure in the spectrum of the heavy component of cosmic rays (represented by iron) at about 8 * 1016 eV. For the first time, it is demonstrated experimentally that the sought-after second knee exists and is related to a change in the spectra of heavy cosmic ray mass groups. This insight contributes considerably to our understanding of cosmic ray physics.