IPST, University of Maryland
Measurement of Galactic Cosmic-Ray Hydrogen and Helium Isotopes with BESS-Polar II
The Balloon-Borne Experiment with a Superconducting Spectrometer (BESS-Polar II) flew successfully over Antarctica during 24.5 days in December 2007 through January 2008 during a period of minimum solar activity. BESS-Polar II is configured with a solenoidal superconducting magnet and a suite of precision particle detectors. It can accurately identify hydrogen and helium isotopes among the incoming cosmic-ray nuclei with energies from 0.2 up to about 1.5 GeV/n. The long duration of the flight, and the good stability of the detectors, improved by a factor of 5 the number of cosmic-ray events previously recorded with BESS-Polar I, reaching about 4.7 billion collected particles. Energy spectrum of cosmic-ray hydrogen and helium isotopes have been measured with the BESS-Polar II instrument with unprecedented accuracy. The isotope flux and secondary-to-primary ratio measurements provide important information to better understand the propagation history of cosmic rays in the Galaxy. The results will be presented and compared to various propagation models.