sup 3 He and sup 4 He production by 800 MeV protons from sup 12 C, Ti, and Pb at forward angles

The doubly differential cross section for the production of {sup 3}He and {sup 4}He by 800 MeV protons from {sup 12}C, Ti, and Pb has been measured at laboratory angles of 6{degree} and 15{degree}. The momentum of the detected helium nuclei varied from 1 to 2 GeV/{ital c}, the maximum being well above the incident proton momentum of 1.46 GeV/{ital c}. The cross sections were found to increase with increasing target mass and decrease with increasing momentum and scattering angle. In our momentum region, the {sup 3}He production cross section is 1.5--10 times larger than {sup 4}He depending on the target and the momentum. The data are consistent with the hypothesis that the dominant reaction mechanism is a direct process where the initial nucleon-nucleon scattering is followed by a sequential pickup of neutrons.