Proton-Proton Scattering from 1.8 Mev to 4.2 Mev

Measurements of the scattering of protons by protons have been made at six energies in the range from 1.8 Mev to 4.2 Mev. At each energy, the differential cross section was measured as a function of angle from 6° in the laboratory system up to 45° or higher. In the angular range above 20°, the cross sections are determined to within ±0.3 percent. Below 20°, the uncertainties become larger, approaching ±0.6 percent at 6°. Departures from pure $S$-wave scattering are observed at all energies. These deviations are larger than can be explained by the experimental uncertainties. They closely resemble the effect of a negative $P$-wave phase shift of approximately -0.1° at 4 Mev and exhibit a reasonable energy dependence.