The ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}(\alpha ,\gamma ){}_{}{}^{16}{}_{}{}^{}\mathrm{O}$ capture cross section was measured for $\alpha$-particle energies between 1.86 MeV ($\sigma =1.1\mathrm{}\pm 0.4$ nb) and 3.11 MeV ($\sigma =29\mathrm{}\pm 4$ nb) using a pulsed ${}_{}{}^4{}_{}{}^{}\mathrm{He}_{}^{+}{}_{}{}^{}$ beam from the ORNL 6-MV Van de Graaff accelerator and a 23- by 30-cm NaI(Tl) crystal viewed by six matched, bialkali photomultiplier tubes. An upper limit was obtained for the capture cross section at $E_{\alpha }=1.6\mathrm{}$ MeV. The over-all time resolution (full width at half maximum) of the system for 8-10-MeV pulses due to $\gamma$ rays is 2.7 nsec. Enriched (99.94%) ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$ targets ranging in thickness from 98 to 178 μg/${\mathrm{cm}}^2$ were used. Pulses resulting from fast neutrons [from the ${}_{}{}^{13}{}_{}{}^{}\mathrm{C}(\alpha ,n){}_{}{}^{16}{}_{}{}^{}\mathrm{O}$ reaction] interacting with the NaI(Tl) crystal were further separated from true $\gamma$ pulses through the use of a new technique based on rise-time distribution differences of the respective neutron- and $\gamma$-ray-produced pulses. The face of the crystal (shielded with a 10.2 cm thickness of ${}_{}{}^6{}_{}{}^{}\mathrm{LiH}$) was 12.2 cm from the target. The astrophysical significance of this reaction in the helium-burning sequence of stellar nucleosynthesis is also discussed.

• Received 11 February 1970

DOI:https://doi.org/10.1103/PhysRevC.2.63