The differential scattering cross section for 20.1-Mev (laboratory energy) protons incident on helium gas has been measured. The following figures give the center-of-mass scattering angle in degrees and minutes, and the corresponding cross section (c.m.) in millibarns per steradian: 8° 42′: 604; 9° 57′: 335; 11° 11′: 262; 12° 25′: 230; 13° 40′: 209; 14° 55′: 204; 16° 8′: 198; 18° 37′: 193; 22° 11′: 198; 24° 51′: 195; 31° 6′: 179; 37° 19′: 167; 43° 31′: 143; 49° 45′: 122; 54° 57′: 104; 87° 6′: 42. At c.m. angles of 111° 48′ and 161° 30′ no scattered particles were detected. The counting statistics varied from about plus or minus one percent at the smallest angles to plus or minus two percent at 54° 57′ (lab angle 45° 0′). The estimated over-all accuracy of the above cross section is ±4%, with the exception of the 87° 6′ figure, which is accurate to about ±10%.

• Received 8 May 1959

DOI:https://doi.org/10.1103/PhysRev.116.674