Elastic Alpha-C-12 Scattering And The C-12(Alpha, Gamma)O-16 E2 S Factor

Angular distributions of C-12(alpha, alpha)C-12 have been measured for E-alpha = 2.6-8.2 MeV, at angles from 24degrees to 166degrees, yielding 12 864 data points. R-matrix analysis of the ratios of elastic scattering yields a reduced width amplitude of gamma(12) = 0.47 +/- 0.06 MeV1/2 for the E-x = 6.917 MeV (2(+)) state in O-16 (a = 5.5 fm). The dependence of the chi(2) surface on the interaction radius a has been investigated and a deep minimum is found at a = 5.42(-0.27)(+0.16) fm. Using this value of gamma(12), radiative alpha capture and N-16 beta-delayed alpha-decay data, the S factor is calculated at E-c.m. = 300 keV to be S-E2(300) = 53(-18)(+13) keV b for destructive interference between the subthreshold resonance tail and the ground state E2 direct capture.