Speaker
Description
The neutron-capture cross section of $^{140}\mathrm{Ce}$ plays an important role in s-process nucleosynthesis due to its neutron-magic character and correspondingly low capture probability. It has been suggested as a possible origin of the discrepancy between predicted and observed stellar cerium abundances, which has motivated recent experimental efforts to re-evaluate the $^{140}\mathrm{Ce}$(n,$\gamma$) cross section. In particular, Amaducci et al. reported energy-dependent cross sections from n_TOF data, while Sahoo et al. performed an activation measurement using the $^{7}\mathrm{Li}$(p,n) reaction at $kT \approx 34~\mathrm{keV}$ and extrapolated Maxwellian-averaged cross sections (MACSs) to lower energies by scaling evaluated cross-section libraries. The two results show a discrepancy of about 30% between their reported MACS values at 10 keV.
To address this discrepancy, we performed an activation measurement of the $^{140}\mathrm{Ce}$(n,$\gamma$) cross section at $kT \approx 11~\mathrm{keV}$ using the $^{18}\mathrm{O}$(p,n) reaction operated near threshold. A natural Ce sample was irradiated at the PTB Ion Accelerator Facility, and the induced activities were measured with an ultra-low-background HPGe detector at the Felsenkeller underground laboratory. The cross section was determined relative to the $^{197}\mathrm{Au}$(n,$\gamma$) standard and used to derive MACS values.
We obtain a MACS of $\langle \sigma \rangle_{kT=10~\mathrm{keV}} = 22.2 \pm 1.1~\mathrm{mb}$. This result is higher than the value previously adopted in stellar models, indicating that the observed $^{140}\mathrm{Ce}$ abundance discrepancy is unlikely to originate from an overestimated neutron-capture cross section. Beyond the specific case of $^{140}\mathrm{Ce}$, the present result provides a direct test of the widely used MACS extrapolation from $\sim 30~\mathrm{keV}$ to lower thermal energies and highlights its potential limitations.
We will present the experimental methodology and results, discuss their implications for s-process nucleosynthesis, and the unique insight they provide into the reliability of MACS extrapolation procedures.