A measurement of the cosmic ray spectrum and composition at the knee

J. W. Fowler, L. F. Fortson, C. C.H. Jui, D. B. Kieda, R. A. Ong, C. L. Pryke, P. Sommers

Research output: Contribution to journalArticlepeer-review

129 Scopus citations


The energy spectrum and primary composition of cosmic rays with energy between 3×1014 and 3×1016 eV have been studied using the CASA-BLANCA detector. CASA consisted of 957 surface scintillation stations; BLANCA consisted of 144 angle-integrating Cherenkov light detectors located at the same site. CASA measured the charged particle distribution of air showers, while BLANCA measured the lateral distribution of Cherenkov light. The data are interpreted using the predictions of the CORSIKA air shower simulation coupled with four different hadronic interaction codes. The differential flux of cosmic rays measured by BLANCA exhibits a knee in the range of 2-3 PeV with a width of approximately 0.5 decades in primary energy. The power law indices of the differential flux below and above the knee are -2.72±0.02 and -2.95±0.02, respectively. We present our data both as a mean depth of shower maximum and as a mean nuclear mass. A multi-component fit using four elemental species suggests the same composition trends exhibited by the mean quantities, and also indicates that QGSJET and VENUS are the preferred hadronic interaction models. We find that an initially mixed composition turns lighter between 1 and 3 PeV, and then becomes heavier with increasing energies above 3 PeV.

Original languageEnglish (US)
Pages (from-to)49-64
Number of pages16
JournalAstroparticle Physics
Issue number1
StatePublished - Mar 2001

Bibliographical note

Funding Information:
We acknowledge the invaluable assistance of the CASA–MIA collaboration as well as the University of Utah High-Resolution Fly's Eye (HiRes) group and the command and staff of the US Army Dugway Proving Ground. We thank D. Heck and the rest of the CORSIKA team for providing and maintaining their excellent program, and the authors of the hadronic interaction models to which it is linked. We thank C. Cassidy, J. Jacobs, J. Meyer, M. Pritchard, and K. Riley for helping with BLANCA's construction and K. Anderson and C. Eberhardy for calibration work. We especially wish to thank M. Cassidy for his essential contributions as our technician. JF and CP acknowledge fellowships from the William Grainger Foundation and the Robert R. McCormick Foundation, respectively. This work was supported by the US National Science Foundation. We would also like to thank S. Swordy for useful conversations.


  • 95.85.Ry
  • Cherenkov
  • Composition
  • Cosmic rays
  • Energy spectrum
  • Knee


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