A Study of Cosmic Ray Composition in the Knee Region using Multiple Muon   Events in the Soudan 2 Detector
  Deep underground muon events recorded by the Soudan 2 detector, located at a depth of 2100 meters of water equivalent, have been used to infer the nuclear composition of cosmic rays in the "knee" region of the cosmic ray energy spectrum. The observed muon multiplicity distribution favors a composition model with a substantial proton content in the energy region 800,000 - 13,000,000 GeV/nucleus. 
S.M. Kasahara and the Soudan 2 collaboration