Physical Review Letters ( IF 8.385 ) Pub Date :
C. Guerrero et al.

The neutron capture cross sections of several unstable isotopes acting as branching points in the s-process are crucial for stellar nucleosynthesis studies. However, measuring them is a challenge due to the difficulties in producing sufficient sample material as well as to the background associated to the activity of the isotopes involved. The unstable isotope ${}^{171}$Tm (t${}_{1/2}$=1.92 years) is part of the branching at A\$\$170 and has never been measured in the full energy range of astrophysical importance. Following the production more than 5 mg of ${}^{171}$Tm at the ILL high flux reactor, two complementary experiments have been carried out at the CERN n_TOF and the SNRC LiLiT facilities by time-of-flight and activation, respectively. The result, the first of its kind, consists of a set of 28 new resonances and a MACS value at 30 keV of 207\$$4${stat}\$\$$25${syst}\$ mb. This value is significantly smaller than all previous predictions and 50% lower than the preliminary result of the only other experiment attempted to date. Our result affects the nucleosynthesis at the A\$\$170 branching, namely, the ${}^{171}$Yb abundance increases by 25% in the material lost by Asymptotic Giant Branch stars, providing a better agreement with the available pre-solar SiC grain measurements.

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