γ-Ray Decay of the 15.11-MeVT=1State ofC12

The reaction ${}_{}{}^{10}{}_{}{}^{}\mathrm{B}({}_{}{}^3{}_{}{}^{}\mathrm{He},p){}_{}{}^{12}{}_{}{}^{}\mathrm{C}$ at $E_{{}_{}{}^3{}_{}{}^{}\mathrm{He}}=2.2\mathrm{}$ MeV has been used to investigate the $\gamma$ decay of states in ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$. The $\gamma$-ray spectrum from a NaI(Tl) detector in coincidence with protons leading to the 15.11-MeV state of ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$ exhibits transitions of 2.38 ± 0.02, 4.44, 7.48 ± 0.03, 10.71 ± 0.05, and 15.11 MeV. Relative $\gamma$-ray branches from the 15.11-MeV state to the 12.71-, 7.65-, 4.44-MeV and ground states are (1.4 ± 0.4), (2.6 ± 0.7), (2.3 ± 0.9), and (92 ± 2)%, respectively, where an allowance of 1.6% has been made for a possible but unobserved 15.11 → 10.3 transition. Also observed were $\gamma$-ray branches of (15 ± 4) and (85 ± 4)% from the 12.71-MeV state of ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$ to the 4.44-MeV and ground states, respectively. $\gamma$-branching intensities from the 15.11-MeV state are compared with the corresponding $\beta$-transition rates of ${}_{}{}^{12}{}_{}{}^{}\mathrm{N}$ and ${}_{}{}^{12}{}_{}{}^{}\mathrm{B}$ to these same states of ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$, and the conclusions about the orbital contribution to the $M1\mathrm{}$ $\gamma$-ray transition strength are compared with shell-model expectations. Previously reported evidence for an isospin-forbidden $\alpha$-decay branch from the 15.11-MeV level is discussed in the light of the present results.