Cosmic ray composition at high energies: The TRACER project

Abstract The TRACER instrument ( Transition Radiation Array for Cosmic Energetic Radiation ) is designed to measure the individual energy spectra of cosmic ray nuclei in long duration balloon flights. The large geometric factor of TRACER (5 m 2 sr) permits statistically significant measurements at particle energies well beyond 10 14 eV. TRACER identifies individual cosmic ray nuclei with single element resolution, and measures their energy over a wide range, from about 0.5 to 10,000 GeV/amu. This is accomplished with a gas detector system of 1600 single wire proportional tubes and plastic fiber radiators combined with plastic scintillators and acrylic Cerenkov counters. A 2-week flight in Antarctica in December 2003 has led to a measurement of the nuclear species oxygen to iron up to about 3000 GeV/amu. As an example, we shall present the energy spectrum and relative abundance for neon and discuss the implication of this result in the context of current models of acceleration and propagation of galactic cosmic rays. The instrument has been refurbished and flown on a second long duration balloon flight in Summer 2006. For this flight, the dynamic range of TRACER has been extended to permit inclusion of the lighter elements B, C, and N in the measurement.