Precise measurements of cosmic-ray hydrogen and helium spectra with BESS Original Research Article

A series of Balloon-borne Experiments with a Superconducting Spectrometer, BESS, has been made since 1993. We report here the absolute cosmic-ray proton and helium spectra in energy ranges of 1 to 120 GeV and 1 to 54 GeV / nucleon, respectively, as measured by the 1998 balloon flight of the BESS spectrometer. Those spectra were determined within overall uncertainties of ±5 % for protons and ±10 % for helium nuclei including statistical and systematic errors. The covered energy range is relevant to the atmospheric neutrinos observed as “fully contained events” in Super-Kamiokande. In order to extend the energy range up to around 1 TeV, we have started to design and develop new tracking detectors. Those energy spectra in a higher energy region will help to estimate the interstellar spectra of primary cosmic rays and the absolute fluxes of “up-going muons.”