Antiprotons and Positrons in Cosmic Rays

The antiproton flux in primary cosmic rays is treated by calculating the ratio of antiproton to positron production in nuclear collisions. It is shown that in the few-BeV energy range at earth, the ratio of the antiproton flux to the positron flux is given directly by their production-cross-section ratio and is independent of astronomical uncertainties. At higher energy, the equilibrium intensity ratio deviates from the production ratio due to modulation in interstellar space. The antiproton production cross section is deduced from the pion production cross section on the basis of a simple physical argument. By comparison with the observed positron intensity, we obtain an antiproton flux which is approximately ${10}^{-4\mathrm{}}$ of the proton flux for $E\ge 5$ BeV and falls off rapidly below 2 BeV. It is suggested that a detection of cosmic-ray antiprotons in the lower energy range would test the validity of the nuclear-interaction model used in the calculation, while extension of the measurements to higher energies, which is perhaps difficult experimentally at present, would serve as a useful probe in the study of the origin and propagation of secondary cosmic-ray particles.