The chronicles of the $3_{1}^{-}$ total width in $\mathrm{^{12}C}$: the formal, the observed and the FWHM
arXiv (Cornell University)(2023)
摘要
Recent measurements indicate that the previously established upper limit for the $\gamma$-decay branch of the $3_{1}^{-}$ resonance in $^{12}\textrm{C}$ at $E_{x} = 9.641(5)$ MeV may be incorrect. The $3_{1}^{-}$ resonance has thus been touted 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. In anticipation of future measurements to more accurately determine the $\gamma$-decay branch of the $3_{1}^{-}$ state, the objective of this work is to accurately determine the total width of the $3_{1}^{-}$ state. An evaluation was performed on all previous results considered in the current ENSDF average for the $3_{1}^{-}$ total width of 46(3) keV. A new R-matrix analysis for the $3_{1}^{-}$ state was performed with a self-consistent, simultaneous fit of several high-resolution $\mathrm{^{12}C}$ excitation-energy spectra populated with direct reactions. The global analysis performed in this work yields a formal total width of $\Gamma(E_{r}) = 46(2)$ keV and an observed total width of $\Gamma_{\textrm{obs}}(E_{r}) = 38(2)$ keV for the $3_{1}^{-}$ state. Significant unaccounted-for uncertainties and/or formalism errors were discovered in the previous results employed in the ENSDF for the observed total width of the $3_{1}^{-}$ state. These previously reported widths are fundamentally different quantities, leading to an invalid ENSDF average. An observed total width of $\Gamma_{\textrm{obs}}(E_{r}) = 38(2)$ keV is recommended for the $3_{1}^{-}$ state in $\mathrm{^{12}C}$. This total width should be employed for future evaluations of the radiative width for the $3_{1}^{-}$ state and its contribution to the high-temperature triple-$\alpha$ reaction rate.
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total width
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