The cosmic ray composition between 10(14) and 10(16) eV

The mass composition of cosmic rays with primary energies between 10(14) eV and 10(16) eV has been studied using the surface and buried scintillators of the CASA-MIA air shower array. Near 10(14) eV, the composition of cosmic rays is in agreement with direct measurements, roughly half light elements (protons and helium) and half heavier elements. The average mass increases with energy, becoming heavier above 10(15) eV. The mass changes coincide w ith the spectral steepening of the energy spectrum known as the knee. There is evidence for rigidity dependence in the spectral change. A method of ca lculating the primary cosmic ray energy which is insensitive to the composi tion is employed to achieve these results. (C) 1999 Elsevier Science B.V. A ll rights reserved.