Stellar neutron capture cross section of the unstables-process branching pointSm151

Abstract

The neutron capture cross sections of the radioactive isotope $^{151}\mathrm{Sm}$ and of natural samarium have been measured in the energy range from 3 keV to 225 keV at the Karlsruhe 3.7 MV Van de Graaff accelerator. Neutrons were produced via the $^{7}\mathrm{Li}$($p,n$)$^{7}\mathrm{Be}$ reaction by bombarding metallic Li targets with a pulsed proton beam and capture events were registered with the Karlsruhe 4\ensuremath{\pi} Barium Fluoride Detector. The cross sections were determined relative to the gold standard using a 206 mg sample of samarium oxide with 90% enrichment in $^{151}\mathrm{Sm}$. Over most of the measured energy range uncertainties of ~2--3% could be achieved for the $^{151}\mathrm{Sm}$/$^{197}\mathrm{Au}$ ratio. Maxwellian averaged neutron capture cross sections of $^{151}\mathrm{Sm}$ were calculated for thermal energies between kT = 8 keV and 100 keV with due consideration of the stellar enhancement factor and were found to be systematically larger than all previous theoretical predictions used in the analysis of the s-process branching at $^{151}\mathrm{Sm}$. In the context of the branching analysis, an experimental determination of the stellar enhancement factor due to captures in thermally excited states is proposed, and the tentative determination of the p-process residual of $^{152}\mathrm{Gd}$ and a few other cases is discussed.