A measurement of the proton and helium components in the atmosphere

We report on the measurements of the proton and helium components in the atmosphere from the data collected by the MASS apparatus during the ascent of the balloon-borne experiment of 1991. The detector configuration was such that proton identification was possible both below 1 GeV/ and in a higher momentum range, 5-15 GeV/ . The helium component was studied above the vertical geomagnetic cutoff rigidity of 4.5 GV. We point out the different dependences on atmospheric depth for low energy secondary protons with respect to primary particles. In addition, we have measured sigificantly different attenuation lengths for protons and helium nuclei, consistently with the expectations.