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 particle ^ { -1 } .
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 particle ^ { -1 } and exhibit nearly the same spectral index ( 2.65 \pm 0.05 ) for all elements .