## 1 Introduction  
Cosmic rays (CRs) are the most outstanding example of accelerated particles and represent about 1% of the total mass of the universe [1]. The riddle of the origin of this radiation has been unsolved for more than a century. The study of CRs is based on two complementary approaches [2]:  
1. Measurement of energy spectrum, elemental composition and anisotropy in the CR arrival direction distribution, the three basic parameters crucial for understanding the origin, acceleration, and propagation of radiation. 2. Search of their sources through the observation of neutral radiation (photons and neutrinos), which points back to the emitting sources not being affected by the magnetic fields, in a multi-messenger approach. We note that, however, photons and neutrinos do not necessarily point back to their sources (see, for example, the the Ref. [3]). In Figure 1, the primary CR all-particle energy spectrum (namely, the number of nuclei as a function of total energy) is shown. The spectrum exceeds \(10^{20}\) eV, showing a few basic characteristics [2]:  
A power-law behaviour \(\sim\)E\({}^{-27}\) up to the so-called _\"knee\"_, a small downwards bend around a few PeV (\(1\) PeV = \(10^{15}\) eV);  
a power-law behaviour \(\sim\)E\({}^{-3.1}\) beyond the knee, with a downwards bend near \(10^{17}\) eV, sometimes referred to as the _\"second knee\"_;  
a transition back to a power-law \(\sim\)E\({}^{-2.7}\) (the so-called _\"ankle\"_) around \(10^{18.7}\) eV;  
a cutoff, probably due to extra-galactic CR interactions with the Cosmic Microwave Background (CMB), around \(10^{19.7}\) eV (the Greisen-Zatsepin-Kuzmin effect). Despite the differences in flux, emphasized by multiplying the differential spectrum by E\({}^{3}\), all the measurements of the all-particle energy spectrum are in fair agreement when taking into account the statistical, systematic and energy scale uncertainties. Nevertheless, uncertainties affecting flux measurements could be underestimated for a number of reasons discussed in this document. In a conservative approach, the spread of different results provides a more realistic estimate of the uncertainty. All the observed features are believed to carry fundamental information that sheds light on the key questions of the origin, acceleration and propagation of CRs. However, from the all-particle results alone, it is not possible to understand the origin of different features. All models concerning sources, acceleration and propagation of the primary flux, differ considerably for what concerns expected elemental composition as a function of the energy. A measurement of the chemical composition is therefore crucial to disentangle between different hypotheses. The main structure is the _\"knee\"_ observed for the first time by [PERSON] in 1948 in the experiment which first located individual shower cores from symmetry of the fired detectors [4,5]. The knee as a feature connected to the end of the Galactic CR flux was first suggested in 1959 by [PERSON] and [PERSON] [6]. They speculated that particles above \(10^{16}\) eV may have a _\"metagalactic origin\"_. Consequently, the observed spectrum is a superposition of the spectra of particles of galactic and metagalactic origin. In 1962, [PERSON] and [PERSON] [7] reported the first observation of two spectral kinks (in both N\({}_{e}\) and N\({}_{\mu}\) spectra) correlating them to a steepening of the primary energy spectrum. All experiments observed the knee at about 4 \(\times\) 10\({}^{15}\) eV but a general consensus about the chemical component responsible for such a feature does not exist yet because experimental results are still conflicting, as will be discussed in Section 5. Determining elemental composition in the knee energy region is crucial to understand where Galactic CR spectrum ends and to give a solid basis to CR models up to the highest observed energies. The maximum energy at which the various nuclei are accelerated should be subject to a rigidity cutoff, as proposed originally by [PERSON] [8].