The boron-to-carbon ratio from the first cosmic ray energetics and mass balloon campaign

The Cosmic Ray Energetics and Mass (CREAM) project consists of a series of balloon campaigns intended to study the composition of high-energy cosmic-ray nuclei near the knee of the all-particle cosmic-ray spectrum. Since cosmic-ray nuclei at these energies are very rare, a large number of flights are required to obtain a statistically meaningful data set. Data from the first CREAM flight, which set a new endurance record of nearly 42 days that has only recently been broken, will be presented here, specifically, the ratio of boron nuclei, which are created by spallation of heavier nuclei en route from cosmic-ray acceleration sites, to carbon nuclei, which are predominantly of primary origin. This secondary-to-primary ratio is important for understanding models of cosmic-ray propagation, which state that the path length traversed by a cosmic ray before escaping the Galaxy is proportional to E -delta, where E is the cosmic-ray energy. Data from the B/C ratio of the first CREAM flight indicate delta ∼ 0.5--0.6; this is consistent with many current propagation models and previous data at lower energies. The differential flux of carbon and oxygen nuclei is observed to obey a power law in energy with spectral index -2.6. The spectral index observed at earth will be a factor delta steeper than that observed at cosmic-ray acceleration sites due to the energy dependence of cosmic-ray escape from the Galaxy. The expected power law index at cosmic ray acceleration sites is therefore ∼ -2.0, which is consistent with the current theoretical understanding of cosmic-ray acceleration in supernova shocks.