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Abstract
| Branching points along the reaction path of the slow nucleosynthesis process are very special isotopes for which there is competition between neutron capture and β-decay. The accurate knowledge of the decay properties and capture cross sections in the vicinity of these branching points are of key importance for determining the stellar conditions, namely the neutron density and temperature during the main s-process component in low-mass AGB stars. However, accurate values of these quantities, in particular capture cross sections at the corresponding stellar temperatures, are difficult to measure; thus data are very scarce and, when existing, very limited. For the particular and important case of the branching at A=147/148, the main branching point is $^{147}$Pm; for which there was a very challenging and successful activation measurement in 2003 at the stellar neutron energy of kT=25 keV using just 28 ng of material. In the main s-process, however, 95% of the neutron exposure takes place during H-burning episodes in thermally pulsing AGB stars at temperatures of around 90 MK (kT=8 keV). A comprehensive understanding of the branching at $^{147}$Pm therefore requires measuring the neutron capture cross section over a broad energy range. With the inauguration of the new CERN neutron beam line n_TOF-EAR2 and the availability of a specially produced $^{147}$Pm sample of 300 μg we propose to measure the associated σ(n,γ) in the full energy range of interest for astrophysics, providing the first ever experimental MACS values over the full energy range between 5 and 100 keV. |