Neutron capture in Sm148,150: A sensitive probe of the s-process neutron density

The neutron capture cross sections of Sm147,148,149,150,152 were measured in the energy range from 3 to 225 keV at the Karlsruhe Van de Graaff accelerator using gold as a standard. Neutrons were produced via the Li7(p,n)7Be reaction by bombarding metallic Li targets with a pulsed proton beam. Capture events were registered with the Karlsruhe 4π barium fluoride detector. Several runs were performed under different experimental conditions to study the systematic uncertainties in detail. For the first time, data were recorded with an ADC system that allows one to register gamma-ray energy and time of flight of the individual detector modules. The cross-section ratios were determined with an overall uncertainty of ∼1%. This is an improvement by about a factor of 5 compared to the existing data. Severe discrepancies were found with the results of previous measurements. Maxwellian-averaged neutron capture cross sections were calculated for thermal energies between kT=10 and 100 keV by normalizing the cross-section shape up to 700 keV neutron energy reported in literature to the present data. These stellar cross sections were used in an s-process analysis. The ratio of the values of the s-process current σNs (Maxwellian-averaged neutron capture cross section times s-process abundance) for the s-only isotopes Sm148,150 is 0.870±0.009 rather than unity as expected by the local approximation. The corresponding branching in the s-process path is analyzed in the framework of the classical approach. The resulting mean neutron density, nn=(3.8±0.6)×108 cm-3, is the most stringent value obtained so far. Finally the new cross sections are used to derive constraints for a stellar model and to check recently discovered isotopic anomalies in meteoritic samarium.