On the cosmic ray energy spectrum “knees”

Abstract

Primary cosmic ray energy spectrum around and above 1 PeV is of great interest due to its non-power law behavior found many years ago using the indirect EAS (Extensive Air Shower) method. The method is based on secondary particles measuring on Earth’s surface under a thick atmosphere. Traditionally people use detectors sensitive to ionization produced mostly by electromagnetic component and so called “knee” was found for EAS size spectrum many years ago. Later it was assigned to a steepening of cosmic ray spectrum at 3-5 PeV. Recently some new “knees” were claimed by high altitude experiments, for primary protons and helium: at ∼200-300 TeV (Tibet ASγ) and at ∼700 TeV (ARGO-YBJ) thus widening the “knee” region from ∼0.2 to 5 PeV and demonstrating disagreement with the existing experimental data. The natural explanation of such a strange spectrum behavior can be found in the phenomenological approach to the knee problem.