## 2 Detection Techniques  
We can divide the experimental methods adopted to measure fluxes and elemental composition of CRs into two categories: _'direct'_ and _'indirect'_ measurements. Generally speaking, for all particle types:  
* the higher the energy, the lower the flux;
* the lower the flux, the larger the required detector area. The direct measurements, in principle, detect and directly identify the primary particles with detectors outside the atmosphere (on board of stratospheric balloons or satellites), since the atmosphere behaves as a shield (see below). Since the CR flux rapidly decreases with increasing energy and the size of detectors is constrained by the weight that can be carried in flight, their _'aperture'_ (i.e., the acceptance measured in m\({}^{2}\)-sr) is small and determines a maximum energy (of the order of a few hundred TeV/nucleon), at which a statistically significant detection is possible. In fact, the number of detected events is given by the CR flux times the detection area times the total observation time. Therefore, the detection area limits the smallest measurable flux. In addition, the limited volume of the detectors makes the containment of showers induced by high-energy nuclei difficult, thus limiting the energy resolution of the instruments in direct measurements. At higher energies, the flux is so low (about 1 particle/m\({}^{2}\)/year around 10\({}^{15}\) eV) that the only chance is to have earth-based detectors of large area, operating for long times. In that case, the atmosphere is considered as a target, and we study the primary properties in an _'indirect'_ way, through the measurement of secondary particles produced in the interaction of the primary particle with the nuclei of the atmosphere, the so-called _'Extensive Air Shower'_ (EAS). Approaching the hundred TeV energy region, even in space-borne experiments, the energy assignment is indirect since it is generally based on the energy deposition of particles produced in the interaction of primaries in the detector itself. The reconstruction of the total energy is then obtained by comparison with some model prediction, and therefore, at least in that region, the boundary line between 'direct' and 'indirect' experiments is more uncertain. In fact, important results obtained by 'direct' methods are conflicting due to some still unknown systematic uncertainties probably related to the interaction model used to assign the energy. A neutrino energy-dependent component must be estimated via Monte Carlo simulations, an evaluation which adds some additional model dependency for 'indirect' measurements. At the ground, the study of CRs is based on the reconstruction and interpretation of EAS observables in the different components, electromagnetic (e.m.), muonic and hadronic, Cherenkov photons, nitrogen fluorescence, radio emission. Therefore, different detectors must be used to detect different observables. Two different approaches are exploited:  
* Arrays, to sample the shower tail particles reaching the ground. In High Energy Particle language, a shower array is a _\"Tail Catcher Sampling Calorimeter\"_. The atmosphere is the absorber and the detectors at ground are the device to measure a (poor) calorimetric signal. Arrays are wide field of view detectors able to observe most of the overhead sky with a duty cycle of \(\sim\)100%. Measurements are limited by large shower-to-shower fluctuations. * Telescopes, to detect Cherenkov photons or nitrogen fluorescence and observe the EAS longitudinal profile. The atmosphere acts as a _\"Homogeneous Calorimeter\"_. The duty cycle is low (\(\sim\)10-15%) because telescopes can be operated only during clear moonless nights and the field of view is small (a few degrees). On the contrary, pointing capability and energy resolution are excellent.