Speaker
Description
Neutrons are naturally produced here on earth and in stellar environments through $(\alpha,n)$ reactions. During helium burning in massive stars, the slow neutron capture process ($s$-process) is fueled by $(\alpha,n)$ reactions on light nuclei like $^{13}$C and $^{22}$Ne, and may also be influenced by reactions on $^{17,18}$O and $^{25,26}$Mg. Here on earth, radioactive decays from long-lived actinides produce a constant source of $\alpha$-particles, which then capture on $^{13}$C and $^{17,18}$O producing a constant source of neutrons. The flux is low, but is of a similar level that these neutrons are often significant background sources for ton scale neutrino, dark matter, and double $\beta$-decay experiments. In this talk I will focus on the study of the $^{13}$C$(\alpha,n)^{16}$O reaction, where new experimental measurements have been reported at both the LUNA and JUNA underground facilities, reaching to unprecedentedly low energies. These measurements have been found to be consistent with past $R$-matrix extrapolations, constrained by transfer reaction determinations of the dominant subthreshold resonance strength, where the cross section were predicted to be lower than previous above ground measurements indicated. To further reduce the uncertainty, differential cross sections, spanning the laboratory $\alpha$-particle energy range from 0.8 to 6.5~MeV were measured at the University of Notre Dame Nuclear Science laboratory. The measurements were made in approximately 10~keV energy steps at 18 angles between 0 and 160$^\circ$, resulting in over 700 distinct angular distributions. These measurements are also the first differential measurements to extend below 1~MeV. We use these new data to augment the previous state-of-the-art $R$-matrix fit of the low energy $^{13}$C$(\alpha,n)^{16}$O reaction and use Bayesian uncertainty estimation to demonstrate that the differential data decreases the uncertainty by a factor of two, from $\approx$10\% to $\approx$5\% over the energy region of astrophysical interest.
