The triple-α-particle breakup of states in the triple-$\alpha$ continuum of ${}^{12}\mathrm{C}$ has been investigated by way of coincident detection of all three $\alpha$ particles of the breakup. The states have been fed in the $\beta$ decay of ${}^{12}\mathrm{N}$ and ${}^{12}\mathrm{B}$, and the $\alpha$ particles measured using a setup that covers all of the triple-$\alpha$ phase space. Contributions from the breakup through the ${}^8\mathrm{Be}$$(0^{+})$ ground state as well as other channels—interpreted as breakup through excited energies in ${}^8\mathrm{Be}$—have been identified. Spins and parities of ${}^{12}\mathrm{C}$ triple-$\alpha$ continuum states are deduced from the measured phase-space distributions for breakup through ${}^8\mathrm{Be}$ above the ground state by comparison to a fully symmetrized sequential $R$-matrix description of the breakup. At around $10$ MeV in ${}^{12}\mathrm{C}$, the breakup is found to be dominated by $0^{+}$ strength breaking up through the ghost of the ${}^8\mathrm{Be}$$(0^{+})$ ground state with $L=0$ angular momentum between the first emitted $\alpha$ particle and the intermediate ${}^8\mathrm{Be}$ nucleus. For ${}^{12}\mathrm{C}$ energies above the $12.7$ MeV $1^{+}$ state, however, $L=2$ breakup of a ${}^{12}\mathrm{C}$ $2^{+}$ state through the ${}^8\mathrm{Be}$$(2^{+})$ excited state dominates. Furthermore, the possibility of a $2^{+}$ excited state in the $9\text{−}12$ MeV region of ${}^{12}\mathrm{C}$ is investigated.

• Received 9 March 2009

DOI:https://doi.org/10.1103/PhysRevC.80.034316