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

TL;DR

This paper remeasures the proton emission partial width of the 16.1 MeV 2+ resonance in 12C using modern detectors, resolving previous inconsistencies and refining reaction rate estimates relevant to nuclear astrophysics.

Contribution

It provides a new, more accurate measurement of the proton partial width of the 12C resonance, clarifying previous data discrepancies and impacting reaction rate evaluations.

Findings

Determined the proton partial width as 21.0(13) eV.

Identified and corrected normalization issues in previous data.

Implications for isospin symmetry and reaction rate scaling.

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.