The cosmic-ray proton and helium spectra between 0.4 and 200 GV

We report on the hydrogen nuclei (protons and deuterons) spectrum from 0.15 to 200 GeV and on the helium nuclei spectrum over the energy range from 0. 2 to 100 GeV nucleon(-1) at the top of the atmosphere measured by the ballo on-borne experiment Cosmic Antiparticle Ring-Imaging Cerenkov Experiment (C APRICE), which was flown from Lynn Lake, Manitoba, Canada, on 1994 August 8 -9. We also report on the proton spectrum over the energy range from 0.15 t o 4.2 GeV. The experiment used the NMSU-WiZard/CAPRICE balloon-borne magnet spectrometer equipped with a solid radiator Ring-Imaging Cerenkov (RICH) d etector and a silicon-tungsten calorimeter for particle identification. Thi s was the first time a RICH was used together with an imaging calorimeter i n a balloon-borne experiment. These detectors allowed for clear particle id entification, as well as excellent control of the detector efficiencies. Th e data were collected during 18 hr at a residual mean atmospheric depth of 3.9 g cm(-2). With this apparatus 516,463 hydrogen and 32,457 helium nuclei were identified in the rigidity range 0.4 to 200 GV and 1.2 to 200 GV, res pectively. The observed energy spectrum at the top of the atmosphere can be represented by (1.1 +/- 0.1) x 10(4) E-2.73+/-0.06 particles (m(2) GeV sr s)(-1) for hydrogen (E in GeV) between 20 and 200 GeV and (4.3 +/- 0.9) x 1 0(2) E-2.65+/-0.07 particles (m2 GeV nucleon(-1) sr s)(-1) for helium nucle i (E in GeV nucleon-l) between 10 and 100 GeV nucleon(-1). These spectra ar e in good agreement with other recent measurements above 10 GeV. The observ ed spectra flatten below 10 GeV due to solar modulation and are consistent with earlier measurements when solar modulation is taken into account. Betw een 5 and 200 GV the hydrogen to helium ratio as a function of rigidity was found to be approximately constant at 6.1 +/- 0.1.