Study of the 6.05 MeV cascade transition in C12(α,γ)O16

Abstract The radiative capture reaction C 12 ( α , γ ) O 16 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 E x = 6.05 MeV in 16 O and was performed by detecting the capture γ -rays with a NaI detector array at the windowless 4 He gas target of the recoil mass separator ERNA in coincidence with the 16 O ejectiles. The 6.05 MeV transition has been considered recently as a component accounting for up to 15% of the C 12 ( α , γ ) O 16 total cross section at astrophysical energies. The arrangement of the detector array yielded additional information on the γ -ray multipolarity, i.e. the ratio σ E 2 / σ E 1 , 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, E 0 ≃ 300 keV . In contrast to a previous analysis, the present extrapolation suggests a negligible contribution from this amplitude, S 6.05 ( 300 ) 1 keV b . Additional data for cascade transitions to excited states at E x = 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.