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The knowledge of the neutron capture cross sections of s-process branching point isotopes represents a basic requirement for the understanding of star evolution. Since such branching point isotopes are by definition radioactive, the measurement of their cross sections from thermal to stellar energies becomes a challenging task. Considerable amounts of material have to be produced, representing a significant radioactive hazard. We report here on the production and separation of 3.5 mg 171Tm from 240 mg 170Er2O3 and 72 μg 147Pm from 100 mg 146Nd2O3 irradiated at the ILL high flux reactor. Thin targets were prepared with high chemical and radioisotopic purity suitable for neutron capture measurements at n_TOF CERN and the SARAF-LiLiT facility.

The neutron capture cross sections of several unstable nuclides acting as branching points in the s process are crucial for stellar nucleosynthesis studies. The unstable Tm171 (t1/2=1.92 yr) is part of the branching around mass A∼170 but its neutron capture cross section as a function of the neutron energy is not known to date. In this work, following the production for the first time of more than 5 mg of Tm171 at the high-flux reactor Institut Laue-Langevin in France, a sample was produced at the Paul Scherrer Institute in Switzerland. Two complementary experiments were carried out at the neutron time-of-flight facility (n_TOF) at CERN in Switzerland and at the SARAF liquid lithium target facility at Soreq Nuclear Research Center in Israel by time of flight and activation, respectively. The result of the time-of-flight experiment consists of the first ever set of resonance parameters and the corresponding average resonance parameters, allowing us to make an estimation of the Maxwellian-averaged cross sections (MACS) by extrapolation. The activation measurement provides a direct and more precise measurement of the MACS at 30 keV: 384(40) mb, with which the estimation from the n_TOF data agree at the limit of 1 standard deviation. This value is 2.6 times lower than the JEFF-3.3 and ENDF/B-VIII evaluations, 25% lower than that of the Bao et al. compilation, and 1.6 times larger than the value recommended in the KADoNiS (v1) database, based on the only previous experiment. Our result affects the nucleosynthesis at the A∼170 branching, namely, the Yb171 abundance increases in the material lost by asymptotic giant branch stars, providing a better match to the available pre-solar SiC grain measurements compared to the calculations based on the current JEFF-3.3 model-based evaluation. Ministerio de Economía y Competitividad FPA2013-45083-P, FPA2014-53290-C2-2-P, FPA2016-77689-C2-1-R European Community 334315, 605203 Israel Science Foundation 1387/15 Science and Technology Facilities Council UK ST/M006085/1 European Research Council ERC-2015-STG-677497

The nuclide 35Cl can act as a minor “neutron poison” in the stellar slow neutron capture process. Neutron activation combined with accelerator mass spectrometry (AMS) was applied to measure the (n,γ) cross section of 35Cl for neutron spectra simulating Maxwell-Boltzmann distributions of kT,∼,30,keV and 40,keV, respectively. The neutron activations were performed at the Karlsruhe Van de Graaff accelerator and at the superconducting linear accelerator of the Soreq Applied Research Accelerator Facility utilizing the 7Li(p,n)7Be reaction. AMS measurements of the irradiated samples were performed at the 3,MV Vienna Environmental Research Accelerator, the 6,MV tandem accelerator at the Dresden AMS facility, and the 14,UD tandem accelerator of the Australian National University in Canberra. Our method is independent of previous measurements. For an energy of kT,=,30,keV, we report a Maxwellian averaged cross sections of 8.33(32),mb. Using this new value in stellar isotopic abundance calculations, minor changes for the abundances of 35Cl, 36Cl and 36S are derived.

The doubly magic 208Pb nucleus is a bottleneck at the termination of the s-process path due to its very low neutron capture cross section. This cross section is also important for the decomposition ofs,r processes and U/Th radiogenic decay contributions to the Pb-Bi solar abundances. The 208Pb(n,γ ) 209Pb cross section was measured at the Soreq Applied Research Accelerator Facility Phase I using an intense quasi-Maxwellian neutron source produced by irradiation of the liquid-lithium target with a 1.5-mA continuous-wave proton beam at 1.94 MeV. The cross section was measured by counting the β activity from the irradiated lead target. The measurement allowed us to evaluate the Maxwellian averaged cross section (MACS) at 30 keV obtaining a value of 0.33(2) mb. This has been compared with the earlier activation and time-of-flight measurements found in the literature. The MACS cross-sectional value of the 63Cu(n,γ ) 64Cu reaction was determined in the same experiment and is compared to a recent published value. EC NeutAndalus (FP7-PEOPLE-2012-CIG No. 334315)