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

The energy spectrum and primary composition of cosmic rays with energy betw een 3 x 10(14) and 3 x 10(16) eV have been studied using the CASA-BLANCA de tector. CASA consisted of 957 surface scintillation stations; BLANCA consis ted 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 coup led 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 e nergy. The power law indices of the differential flux below and above the k nee 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 nu clear mass. A multi-component fit using four elemental species suggests the same composition trends exhibited by the mean quantities, and also indicat es 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. (C) 2001 El sevier Science B.V. All rights reserved.