CROSS-SECTIONS FOR 165-DEGREES BACKSCATTERING OF HE-4 FROM SODIUM AND ALUMINUM FOR A HE-4 ENERGY BETWEEN 2.0 AND 6.0 MEV

Cross sections for 165-degrees backscattering of He-4 ions incident on Na and Al films at energies in the range of 2.0-6.0 MeV have been measured. The NaF (400 angstrom) film was deposited on a self-supporting carbon film (10-mu-g/cm2) and coated with 100 angstrom of Ag. The Al (200 angstrom) film was evaporated on a pyrolytic graphite substrate and also coated with 100 angstrom of Ag. The cross sections were measured at maximum intervals of 50 keV, with typical uncertainties of +/-2.5%. Experimental results show that, for sodium and aluminum, the upper energies at which the backscattering cross section can still be predicted by the Rutherford scattering formula, are 3.0 and 3.5 MeV, respectively. When the energy is higher than the upper value, the backscattering cross section shows a continuous nuclear resonance distribution for both sodium and aluminum.