The independent-electron approximation is used to calculate the single and double ionization cross sections of helium atoms by protons with energies $E$ from 0.01 to 5 MeV. The calculation is carried out for three variants of the one-electron transition probabilities to the continuum. For all energies $E$, the results of the calculation agree qualitatively with experiment, and for $E>$0.1 MeV, the calculated single ionization cross sections coincide with the experimental values. The double ionization cross sections for $E=$0.2-0.7 MeV lie higher, and for $E=$1.5-4 MeV, than the experimental values. This is apparently caused by the neglect of the shock effect on the ionization amplitude.
Research Institute of Nuclear Physics, Moscow State University, Leninskie Gory, Moscow, 119991, Russia
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