Andringa, Sofia2019-02-042019-02-042014http://dx.doi.org/10.22323/1.180.0391http://hdl.handle.net/10400.26/26567The Pierre Auger Observatory measures cosmic rays with energies between 10 17 . 5 eV and 10 20 eV, based on air shower sampling at ground, complemented with shower development measurements with a smaller duty-cycle. The cross-section for the primary interaction of 10 18 eV protons with air has been measured by analysing the maximum of shower development in the atmosphere. This corresponds to a centre-of-mass energy of 57 TeV, and the LHC results show the same evolution of the proton-proton cross-section at intermediate energies. The depth of shower maximum is sensitive to cross-section and primary mass. Its energy evolution indicates a change towards the behavior expected for heavier primaries or larger cross-sections. We will show also the results on other observables related to primary nuclear mass composition.engjournal article2019-02-04