Measurement of the Depth of Maximum of Extensive Air Showers above 1018 eV

J. ABRAHAM (GARCIA, B. Y 400 AUTORES DE LA COL. INTERNACIONAL PIERRE AUGER)

PHYSICAL REVIEW LETTERS

AMER PHYSICAL SOC

Año: 2010

0031-9007

We describe the measurement of the depth of maximum, Xmax, of the longitudinal developmentof air showers induced by cosmic rays. Almost four thousand events above 1018 eV observed bythe fluorescence detector of the Pierre Auger Observatory in coincidence with at least one surfacedetector station are selected for the analysis. The average shower maximum was found to evolve withenergy at a rate of (106+35−21) g/cm2/decade below 1018.24±0.05 eV and (24±3) g/cm2/decade abovethis energy. The measured shower-to-shower fluctuations decrease from about 55 to 26 g/cm2. Theinterpretation of these results in terms of the cosmic ray mass composition is briefly discussed.