NEUTRON-CAPTURE IN SM-148, SM-150 - A SENSITIVE PROBE OF THE S-PROCESS NEUTRON DENSITY

The neutron capture cross sections of Sm-147, Sm-148, Sm-149, Sm-150, Sm-152 were measured in the energy range from 3 to 225 keV at the Karl sruhe Van de Graaff accelerator using gold as a standard. Neutrons wer e produced via the Li-7(p,n)Be-7 reaction by bombarding metallic Li ta rgets with a pulsed proton beam. Capture events were registered with t he Karlsruhe 4pi barium fluoride detector. Several runs were performed under different experimental conditions to study the systematic uncer tainties in detail. For the first time, data were recorded with an ADC system that allows one to register gamma-ray energy and time of fligh t of the individual detector modules. The cross-section ratios were de termined with an overall uncertainty of, approximately 1%. This is an improvement by about a factor of 5 compared to the existing data. Seve re discrepancies were found with the results of previous measurements. Maxwellian-averaged neutron capture cross sections were calculated fo r thermal energies between kT = 10 and 100 keV by normalizing the cros s-section shape up to 700 keV neutron energy reported in literature to the present data. These stellar cross sections were used in an s-proc ess analysis. The ratio of the values of the s-process current [sigma] N(s) (Maxwellian-averaged neutron capture cross section times s-proces s abundance) for the s-only isotopes Sm-148, Sm-150 is 0.870+/-0.009 r ather than unity as expected by the local approximation. The correspon ding branching in the s-process path is analyzed in the framework of t he classical approach. The resulting mean neutron density, n(n) = (3.8 +/-0.6) X 10(8) cm-3, is the most stringent value obtained so far. Fin ally the new cross sections are used to derive constraints for a stell ar model and to check recently discovered isotopic anomalies in meteor itic samarium.