Composition of cosmic rays at ultra high energies

摘要

We present measurements of the cosmic ray (CR) composition above 1015eV, performed with the Yakutsk extensive air shower array. Almost forty four thousand events above 1017eV observed by Cherenkov detectors areselected for the analysis of the depth of maximum of the longitudinal development of air showers induced by CRs. Theinterpretation of these results in terms of CR mass composition is given. It is shown that mean logarithm of the CR atomicnumber h A() i is characterized by the peak value h ln A i 2.5 achieved at 1017eV and its substantial decreasewithin the energy interval 1017- 1018eV to h ln A i 1 at > 1018eV. Such a behavior has to be considered as indicationfor the transition from galactic CR component, which is produced in galactic supernova remnants, to extragalactic CRcomponent at = 1017-1018eV.