Understanding the total width of the 31-state in C 12

Abstract

Background: Recent measurements indicate that the previously established upper limit for the γ-decay branch of the 31-resonance in C12 at Ex=9.641(5) MeV may be incorrect. As a result, the 31-resonance has been suggested as a significant resonance for mediating the triple-α reaction at high temperatures above 2 GK. Accurate estimations of the 31-contribution to the triple-α reaction rate require accurate knowledge of not only the radiative width, but also the total width. Purpose: In anticipation of future measurements to more accurately determine the γ-decay branch of the 31-resonance, the objective of this work is to accurately determine the total width of the 31-resonance. Method: An evaluation was performed on all previous results considered in the current ENSDF average of 46(3) keV for the physical total width (FWHM) of the 31-resonance in C12. A new R-matrix analysis for the 31-resonance was performed with a self-consistent, simultaneous fit of several high-resolution C12 excitation-energy spectra populated with direct reactions. Results: The global analysis performed in this work yields a formal total width of Γ(Er)=46(2) keV and an observed total width of Γobs(Er)=38(2) keV for the 31-resonance. Conclusions: Significant unaccounted-for uncertainties and a misstated result were discovered in the previous results employed in the ENSDF for the physical (or observed) total width of the 31-resonance. These previously reported widths are fundamentally different quantities, leading to an invalid ENSDF average. An observed total width of Γobs(Er)=38(2) keV is recommended for the 31-resonance in C12. This observed total width should be employed for future evaluations of the observed total radiative width for the 31-resonance and its contribution to the high-Temperature triple-α reaction rate.