The neutron capture cross section of 209Bi has been measured at the CERN n_TOF facility by employing the pulse-height-weighting technique. Improvements over previous measurements are mainly because of an optimized detection system, which led to a practically negligible neutron sensitivity. Additional experimental sources of systematic error, such as the electronic threshold in the detectors, summing of gamma-rays, internal electron conversion, and the isomeric state in bismuth, have been taken into account. Gamma-Ray absorption effects inside the sample have been corrected by employing a nonpolynomial weighting function. Because 209Bi is the last stable isotope in the reaction path of the stellar s-process, the Maxwellian averaged capture cross section is important far the recycling of the reaction flow by alfa -decays. In the relevant stellar range of thermal energies between kT = 5 and 8 keV our new capture rate is about 16 percent higher than the presently accepted value used for nucleosynthesis calculations. At this low temperature an important part of the heavy Pb-Bi isotopes are supposed to be synthesized by the s-process in the He shells of low mass, thermally pulsing asymptotic giant branch stars. With the improved set of cross sections we obtain an s-process fraction of 19 plus or minus 3 per cent of the solar bismuth abundance, resulting in an r-process residual of 81 plus or minus 3 per cent. The present (n, gamma) cross-section measurement is also of relevance for the design of accelerator driven systems based on a liquid metal Pb/Bi spallation target.

New Measurement of Neutron Capture Resonances in 209Bi

Ventura, A.
2006-08-01

Abstract

The neutron capture cross section of 209Bi has been measured at the CERN n_TOF facility by employing the pulse-height-weighting technique. Improvements over previous measurements are mainly because of an optimized detection system, which led to a practically negligible neutron sensitivity. Additional experimental sources of systematic error, such as the electronic threshold in the detectors, summing of gamma-rays, internal electron conversion, and the isomeric state in bismuth, have been taken into account. Gamma-Ray absorption effects inside the sample have been corrected by employing a nonpolynomial weighting function. Because 209Bi is the last stable isotope in the reaction path of the stellar s-process, the Maxwellian averaged capture cross section is important far the recycling of the reaction flow by alfa -decays. In the relevant stellar range of thermal energies between kT = 5 and 8 keV our new capture rate is about 16 percent higher than the presently accepted value used for nucleosynthesis calculations. At this low temperature an important part of the heavy Pb-Bi isotopes are supposed to be synthesized by the s-process in the He shells of low mass, thermally pulsing asymptotic giant branch stars. With the improved set of cross sections we obtain an s-process fraction of 19 plus or minus 3 per cent of the solar bismuth abundance, resulting in an r-process residual of 81 plus or minus 3 per cent. The present (n, gamma) cross-section measurement is also of relevance for the design of accelerator driven systems based on a liquid metal Pb/Bi spallation target.
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Utilizza questo identificativo per citare o creare un link a questo documento: http://hdl.handle.net/20.500.12079/1189
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