Cross sections for 170.5deg backscattering of 4He from oxygen for 4He energies between 1.8 and 5.0 MeV

Cross sections for 170.5deg backscattering of 4He ions by natural oxygen atoms in target films for 4He laboratory energies in the range 1.8-5.0 MeV have been measured. A thick (≅ 35 keV) Al2O3 target film (≅ 850 A), deposited on a carbon substrate and overcoated with ≅ 200 A of Ta, was used for cross-section measurement in nonresonant regions. A thin (≅ 6 keV) Ta/Nb/Al target film, containing 60% oxygen, was used in strong resonance regions. In both cases, the non-Rutherford 4He-O cross section was determined by comparison with the nearly Rutherford 4He-Ta cross section at the same energy. The cross sections were measured at maximum intervals of 50 keV, with smaller intervals used in the neighborhood of resonances. Results are given in graphical and tabular form; cross sections are given as ratios to the Rutherford cross section, with typical uncertainties of ±2%; resonance energy uncertainties are typically ±5 keV. Discrepancies between these and previous measurements are discussed. (orig.)