Study of the 6.05 MeV cascade transition in C-12(alpha, gamma)O-16

The radiative capture reaction 12C(α,γ )16O has been investigated in the energy range E = 3.3 to 4.5 MeV. This experiment focused in particular on the cascade transition to the 0+ state at Ex = 6.05 MeV in 16O and was performed by detecting the capture γ -rays with a NaI detector array at the windowless 4He gas target of the recoil mass separator ERNA in coincidence with the 16O ejectiles. The 6.05 MeV transition has been considered recently as a component accounting for up to 15% of the 12C(α,γ )16O total cross section at astrophysical energies. The arrangement of the detector array yielded additional information on the γ -ray multipolarity, i.e. the ratio σE2/σE1, and it was found that the 6.05 MeV transition is entirely E2 in the studied energy range. The results for this transition are analyzed in an R-matrix formalism and extrapolated to the relevant Gamow energy of stellar helium burning, E0 300 keV. In contrast to a previous analysis, the present extrapolation suggests a negligible contribution from this amplitude, S6.05(300) < 1 keV b. Additional data for cascade transitions to excited states at Ex = 6.13, 6.92, and 7.12 MeV, respectively, as well as to the ground state were obtained and the corresponding S factors in the studied energy range are given.