  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.

Astrophysics