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 similar to 0.1 PeV is consistent with expectations from previous d irect measurements. At higher energies, there is little change in comp osition up to similar to 5 PeV. Above this energy, (X-max is deeper th an expected for a constant elemental composition, which implies that t he overall elemental composition is becoming lighter above the knee re gion. These results disagree with the idea that cosmic rays should bec ome on average heavier above the knee. Instead, they suggest a transit ion to a qualitatively different population of particles above 5 PeV.