Understanding the Total Width of the 3-1 State in 12C

Background: Recent measurements indicate that the previously established upper limit for the gamma -decay branch of the 3-1 resonance in 12C at Ex = 9.641(5) MeV may be incorrect. As a result, the 3-1 resonance has been suggested as a significant resonance for mediating the triple-alpha reaction at high temperatures above 2 GK. Accurate estimations of the 3-1 contribution to the triple-alpha 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 gamma -decay branch of the 3- 1 resonance, the objective of this work is to accurately determine the total width of the 3-1 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 3-1 resonance in 12C. A new R -matrix analysis for the 3- 1 resonance was performed with a self-consistent, simultaneous fit of several high -resolution 12C excitation-energy spectra populated with direct reactions. Results: The global analysis performed in this work yields a formal total width of P(Er) = 46(2) keV and an observed total width of robs(Er) = 38(2) keV for the 3-1 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 3-1 resonance. These previously reported widths are fundamentally different quantities, leading to an invalid ENSDF average. An observed total width of robs(Er) = 38(2) keV is recommended for the 3-1 resonance in 12C. This observed total width should be employed for future evaluations of the observed total radiative width for the 3-1 resonance and its contribution to the high -temperature triple-alpha reaction rate.