CROSS-SECTIONS FOR 170-DEGREES BACKSCATTERING OF HE-4 FROM OXYGEN IN THE ENERGY-RANGE OF 2.0-9.0 MEV

We report the 170-degrees backscattering cross sections of He-4 from 0 in the energy range of 2.0-9.0 MeV. Our measurements show that the alpha-oxygen scattering remains Rutherfordlike up to E(alpha) = 2.35 MeV, and there exist two energy regions, 5.55-5.85 MeV and 8.50-8.80 MeV, where the cross sections present an enhanced and smooth variation. The measured cross sections in these two regions exhibit enhancement factors of 9.7 and 34, respectively, compared to the Rutherford values. In the 8.50-8.80 MeV region, the cross sections at scattering angles of 160-degrees, 165-degrees, 170-degrees and 175-degrees were measured and found to be strongly angle-dependent. The strongest resonance was seen at 7.60 MeV, where the cross section is 200 times its Rutherford value. We report the use of this resonance to measure the O areal density on a Cu surface, with a sensitivity of 1 X 10(15) oxygen atoms/cm2. We have also studied the scattering angle dependence of the threshold energy at which the cross section begins to depart from its Rutherford value. Our experimental results are not in agreement with the prediction from the analytical formula developed by Bozoian et al. [Nucl. Instr. and Meth. B 51 (1990) 311].