Measurement of the 209Bi(n,γ)210Big cross section and updated s-process analysis of the Pb/Bi region

The neutron capture cross section of 209 Bi(n,γ) 210 Bi has been measured using the 3.7MV Van de Graaff accelerator at Forschungszentrum Karlsruhe. The measurement was carried out by activation of a high purity Bi sample in a quasi-stellar neutron spectrum with kT = 25keV by employing the 7 Li(p,n) 7 Be reaction at Ep = 1912keV. We performed four activations: two to count the α-activity of 210 Po and two more for Accelerator Mass Spectrometry (AMS) analysis at the VERA facility in Vienna. The aim of our study is to measure the partial cross sections to the ground and the isomeric state with improved accuracy, which is necessary to understand the recycling via α-decay of 210,211 Po and the corresponding accumulation of the s-process abundances at the end of the s-path. The Maxwellian averaged cross section for populating the ground state of 210 Bi at a thermal energy of kT = 30keV was determined to be � σv� /vT = 2.16 ± 0.07mb. For the isomeric state the measurements are in progress. Based on the present results and on the new data for the stable Pb isotopes measured at n TOF, we performed a theoretical analysis using stellar models of Asymptotic Giant Branch (AGB) stars. The effect of the new cross section values in the Pb/Bi region was studied by averaging the results of stellar models for M = 1. 5a nd 3Mand a half-solar metallicity for the main component, and at (Fe/H)= −1.3 for the strong component. At this lower metallicity the AGB stars have been shown to produce the maximum amount of s-process lead, thus providing a good approximation of the strong component. A study of these aspect is in preparation.