CROSS-SECTIONS FOR NON-RUTHERFORD BACKSCATTERING OF HE-4 FROM 5 LIGHT-ELEMENTS

We report the results of measured cross sections for backscattering of He-4 from Al and Si at a scattering angle of 170 +/- 1.5-degrees (lab) for He-4 laboratory energies between 2.0 and 9.0 MeV; and the cross sections for backscattering of He-4 from F, Mg and Cl at a scattering angle of 165 +/- 1.5-degrees (lab) for He-4 laboratory energies between 1.6 and 5.0 MeV, 2.0 and 8.4 MeV and 2.0 and 8.7 MeV, respectively. Experimental results show that, for F, Mg, Al, Si and Cl, the upper energies at which the backscattering cross section can still be predicted by the Rutherford scattering formula, are 2.60, 3.20, 3.60, 3.80 and 4.50 MeV, respectively. When the energy is higher than the upper value, the backscattering cross section shows a continuous nuclear resonance distribution for F, Al and Cl. For silicon, a strong narrow isolated resonance is found at 4.370 MeV (+/- 10 keV). The cross section is 2.90 times Rutherford at 4.370 MeV and the resonance width GAMMA(lambda) is 20 keV. A much stronger isolated narrow resonance is at 5.375 MeV (+/- 10 keV). The cross section is 9.50 times Rutherford at 5.375 MeV and the resonance with GAMMA(lambda) is 10 keV. These two resonances may be used for backscattering analysis.