The Absolute Flux of Protons and Helium at the Top of the Atmosphere Using IMAX

The cosmic-ray proton and helium spectra from 0.2 GeV nucleon^-1 to about 200 GeV nucleon^-1 have been measured with the balloon-borne experiment Isotope Matter-Antimatter Experiment (IMAX) launched from Lynn Lake, Manitoba, Canada, in 1992. IMAX was designed to search for antiprotons and light isotopes using a superconducting magnet spectrometer together with scintillators, a time-of-flight system, and Cherenkov detectors. Using redundant detectors, an extensive examination of the instrument efficiency was carried out. We present here the absolute spectra of protons and helium corrected to the top of the atmosphere and to interstellar space. If demodulated with a solar modulation parameter of φ=750 MV, the measured interstellar spectra between 20 and 200 GV can be represented by a power law in rigidity, with (1.42+/-0.21)×10⁴R^-2.71+/-0.04 (m² GV s sr)^-1 for protons and (3.15+/-1.03)×10³R^-2.79+/-0.08 (m² GV s sr)^-1 for helium.