First Measurement of the Cascade Transition via the 6.049-MeV State of $^{16}$O in the $^{12}$C($\alpha,\gamma$)$^{16}$O Reaction

The cascade through the 6.049-MeV state of $^{16}$O has rarely been discussed as contributing to the $^{12}$C ($\alpha$,$\gamma$)$^{16}$O cross section at low energies largely due to experimental difficulties in observing this transition. We report here first measurements of this transition in $^{12}$C($\alpha$,$\gamma$)$^{16}$O using the DRAGON separator facility at TRIUMF. The experiment was performed in inverse kinematics with an incident $^{12}$C beam on a windowless $^{4}$He gas target. The coincidence setup included a BGO array around the gas target and a DSSSD detector for the detection of $^{16}$O recoils. The acceptance of DRAGON including the BGO array has been simulated in GEANT as well as measured directly. The transition strength has been derived and analyzed in the R- matrix formalism. We find an extrapolated value of $S_{6.05} (300)=25\pm16$ keVb showing that this transition is likely the most important cascade contribution for $^{12}$C ($\alpha$,$\gamma$)$^{16}$O.