Abstract
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, Xmax, in the atmosphere as a function of particle energy. The value of Xmax near the instrument threshold of ~0.1 PeV is consistent with expectations from previous direct measurements. At higher energies, there is little change in composition up to ~5 PeV. Above this energy, Xmax is deeper than expected for a constant elemental composition, which implies that 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.
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