Changes of the cosmic-ray mass composition in the 10^{14} - 10^{16} eV energy range

Changes of the cosmic-ray mass composition at the `knee' of the cosmic-ray flux spectrum near 10^{15} eV energy are investigated using data from ten Cosmic Ray Tracking (CRT) detectors and the HEGRA air-shower array on La Palma, Canary Islands. The analysis is based on the angular distribution of muons in air showers. Results can be easily expressed in terms of of primary cosmic rays. We find a rise of below 10^{15} eV, consistent with direct measurements. Simple cosmic-ray composition models are presented which are fully consistent with our results as well as the JACEE flux and composition measurements and the flux measurements of the Tibet AS-gamma collaboration.Comment: 22 pages, 11 Postscript figures, 2 tables. Accepted for publication in Astroparticle Physic