# First Accurate Normalization of the $β$-delayed $α$ Decay of $^{16}$N and Implications for the $^{12}$C$(α,γ)^{16}$O Astrophysical Reaction Rate

Kirsebom, OS, Tengblad, O, Lica, R, Munch, M, Riisager, K, Fynbo, HOU, Borge, MJG, Madurga, M, Marroquin, I, Andreyev, AN
et al (show 34 more authors) (2018) First Accurate Normalization of the $β$-delayed $α$ Decay of $^{16}$N and Implications for the $^{12}$C$(α,γ)^{16}$O Astrophysical Reaction Rate. Physical Review Letters. (Submitted)

The $^{12}$C$(\alpha,\gamma)^{16}$O reaction plays a central role in astrophysics, but its cross section at energies relevant for astrophysical applications is only poorly constrained by laboratory data. The reduced $\alpha$ width, $\gamma_{11}$, of the bound $1^-$ level in $^{16}$O is particularly important to determine the cross section. The magnitude of $\gamma_{11}$ is determined via sub-Coulomb $\alpha$-transfer reactions or the $\beta$-delayed $\alpha$ decay of $^{16}$N, but the latter approach is presently hampered by the lack of sufficiently precise data on the $\beta$-decay branching ratios. Here we report improved branching ratios for the bound $1^-$ level and for $\beta$-delayed $\alpha$ emission. In the case of the $\beta$-delayed $\alpha$ branch, we find a $5\sigma$ deviation from the literature value. With our new branching ratios, the constraints imposed on $\gamma_{11}$ by the $\beta\alpha$-decay and $\alpha$-transfer data are of similar precision and, for the first time, in good agreement. The weighted average of the two gives a robust and precise determination of $\gamma_{11}$, which may permit the $^{12}$C$(\alpha,\gamma)$ cross section to be constrained within $10\%$ in the energy range relevant to hydrostatic He burning.