Measurement of the ${}^{140}\mathrm{Ce}(\mathrm{n},\gamma )$ Cross Section at n_TOF and Its Astrophysical Implications for the Chemical Evolution of the Universe

${}^{140}\mathrm{Ce}(\mathrm{n},\gamma )$ is a key reaction for slow neutron-capture ($s$-process) nucleosynthesis due to being a bottleneck in the reaction flow. For this reason, it was measured with high accuracy (uncertainty $\approx 5\%$) at the n_TOF facility, with an unprecedented combination of a high purity sample and low neutron-sensitivity detectors. The measured Maxwellian averaged cross section is up to 40% higher than previously accepted values. Stellar model calculations indicate a reduction around 20% of the $s$-process contribution to the Galactic cerium abundance and smaller sizeable differences for most of the heavier elements. No variations are found in the nucleosynthesis from massive stars.

Figure 1

Comparison between theoretical predictions and average observed abundances in $s$-process enhanced second-generation stars of the globular cluster M22. The red squares represent the average $s$-process overabundance with respect to iron (we adopt the standard spectroscopic notation: $\{[A/B]=\log [N(A{)}_{\text{Star}}/N(A{)}_{\text{Sun}}]-\log [N(B{)}_{\text{Star}}/N(B{)}_{\text{Sun}}]\}$. The solid and the dashed lines show the results of the nucleosynthesis models obtained by adopting the kadonis0.3 and the new n_TOF ${}^{140}\mathrm{Ce}(\mathrm{n},\gamma )$ reaction rate, respectively. See Straniero et al. [7] for more details.

Figure 2

Fit of the transmission data [22] with the three different spin parities. The resulting ${\chi }^2/\mathrm{NDF}$ for the $1/2^{+}$, $1/2^{-}$, and $3/2^{-}$ are, respectively, 16.8, 1.6, and 3.8.

Figure 3

Top: MACS calculated by n_TOF compared with the values available in KADoNiS0.3 and the MACS calculated on the basis of JENDL-5 library. Bottom: ratio n_TOF/KADoNiS0.3 and n_TOF/JENDL-5. The values of KADoNiS0.3 have been scaled by a factor 1.0785, corresponding to the recent increase in the standard ${}^{197}\mathrm{Au}(\mathrm{n},\gamma )$ cross section [25].

Figure 4

Percentage differences between models computed with the new ${}^{140}\mathrm{Ce}$ MACS and the kadonis0.3 one. Computed models are representative of disk (upper panel) and halo (lower panel) AGB stars. As expected, the abundance of the stable $p$-only nuclei is unchanged. We adopted the iron abundance relative to hydrogen, [$\mathrm{Fe}/\mathrm{H}$], as a tracer of stellar metallicity.