Radiative cascade transitions and the 12C(α,γ)16O E2 cross section to the ground state of 16O -: art. no. 022801

The alpha -particle width of the 6.917 MeV, J(pi) = 2(+) state in O-16 is the most uncertain value in the determination of the E2 radiative ground state transition of the astrophysically important C-12(alpha, gamma) O-16 reaction at energies relevant to stellar helium burning. As discussed previously in literature it is possible in principle to derive a precise value of the alpha width of the 6.917 MeV state by measuring the E2 direct capture into this state. Here the theory based on the R-matrix formalism connecting the cascade direct capture and the E2 ground state transition is described in more complete form than hitherto and resulting direct capture cross sections are estimated. In addition, conditional fits are made to existing data and the resulting value of the alpha -particle width is discussed. From these considerations the cascade cross sections are found to be of the order of 10 nb in an extended energy range proposed to be measured. Angular distributions for the primary gamma transition are also given. Conclusions for future experiments are then drawn. The branching ratio of the 7.1-->6.9 MeV gamma transition should also be measured.