A New Measurement of Cosmic Ray Composition at the Knee

The Dual Imaging Cerenkov Experiment (DICE) was designed and operated for making elemental composition measurements of cosmic rays near the knee of the spectrum at several PeV. Here we present the first results using this experiment from the measurement of the average location of the depth of shower maximum, < X max >, in the atmosphere as a function of particle energy. The value of < X max > near the instrument threshold of ∼ 0.1PeV is consistent with expectations from previous direct measurements. At higher energies there is little change in composition up to ∼ 5PeV. Above this energy < X max > is deeper than expected for a constant elemental composition implying the overall elemental composition is becoming lighter above the knee region. These results disagree with the idea that cosmic rays should become on average heavier above the knee. Instead they suggest a transition to a qualitatively different population of particles above 5 PeV. Subject headings: cosmic rays, supernova remnants, instrumentation:detectors, ISM:abundances Enrico Fermi Institute, U. of Chicago, Chicago, IL 60637, USA Department of Physics, U. of Utah, Salt Lake City, UT 84112, USA