The Elemental Composition of High-Energy Cosmic Rays: Measurements with   TRACER
  TRACER ('Transition Radiation Array for Cosmic Energetic Radiation') is a balloon borne instrument that has been developed to directly measure the composition and energy spectra of individual heavy elements up to 10^15 eV per particle. TRACER achieves a large geometric factor (5 m^2 sr) through the use of a Transition Radiation Detector utilizing arrays of single wire proportional tubes. TRACER has measured the energy spectra of the elements O, Ne, Mg, Si, S, Ar, Ca, and Fe. The energy spectra reach energies in excess of 10^14 eV per particle and exhibit nearly the same spectral index (2.65 +/- 0.05) for all elements. 
P.J. Boyle