Differential Cross Section for Proton-Helium Scattering near 17.5 Mev

The differential cross section for the elastic scattering of 17.45±0.2-Mev protons by helium has been measured at 33 angles between the center-of-mass angles 6.4 to 168 degrees. An external beam of protons from the Princeton cyclotron was scattered by helium gas in the interior of the Princeton 60-inch precision scattering chamber, scattered particles being detected in a potassium iodide scintillation counter. Values found for the cross section are considered to have an over-all accuracy of 2% except at the smallest angles where the error was larger due to experimental difficulties. The resulting data have been analyzed in terms of phase shifts of the $S$, $P$, $D$, and $F$ waves with the following results: $S$, —100.7°; $P_{\frac{3}{2}}$, 81.2°; $P_{\frac{1}{2}}$, 36.4°; $D_{\frac{5}{2}}$, 157.8°; $D_{\frac{3}{2}}$, 10.9°; and $F$, 1.2°. These values provide a least-squares fit to the experimental curve.