M. A. K. Glasmacher(a), M. A. Catanese(a), 1, M. C. Chantell(b), C. E. Covault(b), J. W. Cronin(b), B. E. Fick(b), L. F. Fortson(b), 2, J. W. Fowler(b), K. D. Green(b), 3, D. B. Kieda(c), J. Matthews(a), 4, B. J. Newport(b), 5, D. F. Nitz(a), 6, R. A. Ong(b), S. Oser(b), D. Sinclair(a) and J. C. van der Velde(a)

(a) Dept. of Physics, The University of Michigan, Ann Arbor, MI 48109, USA
(b) Enrico Fermi Institute, The University of Chicago, Chicago, IL 60637, USA
(c) Dept. of Physics, The University of Utah, Salt Lake City, UT 84112, USA

Received 22 February 1999; revised 24 March 1999; accepted 7 April 1999. ; Available online 10 November 1999.

Abstract
The mass composition of cosmic rays with primary energies between 10**14 eV and 10**16 eV has been studied using the surface and buried scintillators of the CASA-MIA air shower array. Near 10**14 eV, the composition of cosmic rays is in agreement with direct measurements, roughly half light elements (protons and helium) and half heavier elements. The average mass increases with energy, becoming heavier above 10**15 eV. The mass changes coincide with the spectral steepening of the energy spectrum known as the knee. There is evidence for rigidity dependence in the spectral change. A method of calculating the primary cosmic ray energy which is insensitive to the composition is employed to achieve these results.

The cosmic ray energy spectrum between 10**14 and 10**16 eV

M. A. K. Glasmacher(a), M. A. Catanese(a), 1, M. C. Chantell(b), C. E. Covault(b), J. W. Cronin(b), B. E. Fick(b), L. F. Fortson(b), 2, J. W. Fowler(b), K. D Green(b), 3, D. B. Kieda(c), J. Matthews(a), , *, B. J. Newport(b), 5, D. F. Nitz(a), R. A. Ong(b), S. Oser(b), D. Sinclair(a) and J. C. van der Velde(a)

(a) Dept. of Physics, The University of Michigan, Ann Arbor, MI 48109, USA
(b) Enrico Fermi Institute, The University of Chicago, Chicago, IL 60637, USA
(c) Dept. of Physics, The University of Utah, Salt Lake City, UT 84112, USA

Received 27 August 1998; revised 26 November 1998; accepted 4 December 1998. ; Available online 1 July 1999.

Abstract
The energy spectrum of cosmic rays with primary energies between 10**14 eV and 10**16 eV has been studied with the CASA-MIA air shower array. The measured differential energy spectrum is a power law (dj/dE E-y) with spectral indices of 2.66±0.02 below approximately 10**15 eV and 3.00±0.05 above. A new method is used for measuring primary energy derived from ground-based data in a compositionally insensitive way. In contrast with some previous reports, the "knee" of the energy spectrum does not appear sharp, but rather a smooth transition over energies from 10**15 eV to 3.0 × 10**15 eV.

Astroparticle Physics 10, 291, (1999)

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