On the scent of the knee - air shower measurements with KASCADE
  Detailed investigations of extensive air showers have been performed with the data measured by the KASCADE experiment. The results allow to evaluate hadronic interaction models, used in simulations to interpret air shower data. The all-particle spectrum of cosmic rays and their mass composition, as well as individual spectra for groups of elements have been reconstructed. The results suggest, the knee in the all-particle cosmic--ray energy spectrum is caused by a rigidity-dependent cut-off of individual element groups. 
KASCADE Collaboration: J.R. Hoerandel, et al