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 with the spectral steepening of the energy spectrum known as the knee. There is evidence for rigidity dependence in the spectral change. A method of calculating the primary cosmic ray energy which is insensitive to the composition is employed to achieve these results.