ENERGY SPECTRA OF PRIMARY AND SECONDARY COSMIC-RAY NUCLEI MEASURED WITH TRACER

The Transition Radiation Array for Cosmic Energetic Radiation (TRACER) cosmic-ray detector, first flown on long-duration balloon (LDB) in 2003 for observations of the major primary cosmic-ray nuclei from oxygen (Z = 8) to iron (Z = 26), has been upgraded to also measure the energies of the lighter nuclei, including the secondary species boron (Z = 5). The instrument was used in another LDB flight in 2006. The properties and performance of the modified detector system are described, and the analysis of the data from the 2006 flight is discussed. The energy spectra of the primary nuclei carbon (Z = 6), oxygen, and iron over the range from 1 GeV amu–1 to 2 TeV amu–1 are reported. The data for oxygen and iron are found to be in good agreement with the results of the previous TRACER flight. The measurement of the energy spectrum of boron also extends into the TeV amu–1 region. The relative abundances of the primary nuclei, such as carbon, oxygen, and iron, above ∼10 GeV amu–1 are independent of energy, while the boron abundance, i.e., the B/C abundance ratio, decreases with energy as expected. However, there is an indication that the previously reported E –0.6 dependence of the B/C ratio does not continue to the highest energies.