a Measurement of MU, P and he Energy Spectra at the Small Atmospheric Depth.

The cosmic-ray proton, helium, and muon spectra at small atmospheric depths of 4.5 - 28 g/cm² were precisely measured during the slow descending period of the BESS-2001 balloon flight. The variation of atmospheric secondary particle fluxes as a function of atmospheric depth provides fundamental information to study hadronic interactions of the primary cosmic rays with the atmosphere.