The partial widths of the 16.1 MeV 2+ resonance in 12C

Abstract

The 16.1MeV 2+ resonance in 12C situated slightly above the proton threshold can decay by proton-, $\alpha$-, and $\gamma$ emission. The partial width for proton emission cannot be directly measured due to the low proton energy and the small branching ratio. Instead it must be indirectly derived from other observables. However, due to several inconsistent data the derived partial width varies by almost a factor 2 dependent on the data used. Here we trace the majority of this inconsistency to different measurements of the $(p,\alpha)$ cross sections. We have remeasured this cross section using modern large area silicon strip detectors allowing to measure all final state particles, which circumvents a normalization issue affecting some of the previous measurements. Based on this we determine $\Gamma_{p}$ = 21.0(13)eV. We discuss the implications for other observables related to the 16.1 MeV $2^{+}$ resonance and for isospin symmetry in the $A=12$ system. In addition, we conclude that the dataset currently used for the NACRE and NACRE II evaluation of the $^{11}\mathrm{B}(p,3\alpha)$ reaction should be scaled by a factor of 2/3. This impacts the reaction rate accordingly.