Composition of cosmic rays in the knee region of the primary energy spectrum

The Soudan Surface-Underground Cosmic Ray Telescope is located at the Soudan iron mine in northern Minnesota. It consists of a coincidence arrangement of two detectors-one installed at the surface of the mine and the other located underground, at a vertical depth of 600 meters. Using such an arrangement, the energy and composition of a primary cosmic ray particle can be determined independently of one another. When a high energy cosmic ray enters the Earth's atmosphere, secondary particles are produced in successive interactions, creating an extensive air shower. Using the surface detector, the number of particles in the shower at the surface of the Earth can be counted and the energy of the primary particle estimated. Of all the particles that are created in a cosmic ray air shower, only the energetic muons are able to penetrate underground. The separations of the muons below ground are measured by the Soudan 1 detector and this serves as an indicator of the type of nucleus that initiated the shower. The Soudan surface-underground detector is sensitive to primary cosmic rays of energies between 1014 and 1018 eV. The data from the experiment were compared to the predictions of various cosmic ray composition models, within this energy range. The data supported a composition model that was proton dominated up to the highest energies measured. There was no indication of a shift in the composition towards heavier primaries as would be expected on the basis of some models