Stellar neutron capture cross sections of the Gd isotopes

Abstract

The neutron capture cross sections of 152 Gd, 154 Gd, 155 Gd, 156 Gd, 157 Gd, and 158 Gd were measured in the energy range from 3 to 225 keV at the Karlsruhe 3.75 MV Van de Graaff accelerator. Neutrons were produced via the 7 Li(p,n)1Be reaction by bombarding metallic Li targets with a pulsed protori beam. Capture events were registered with the Karlsruhe 47r Barium Fluoride Detector, which was improved by replacing crystals with high a background and by introducing a pierced crystal at zero degrees with respect to the beam axis. These changes resulted in a significantly increased efficiency for capture events. The main experimental problern was that the samples of the two s-only isotopes 152 Gd and 154 Gd showed only relatively low enrichment, but the spectroscopic quality of the BaF 2 detector allowed to determine the resulting corrections for isotopic impurities reliably. The cross section ratios could be determined with an overall uncertainty of typically 1%, an improvement by factors of five to ten compared to existing data. Severe discrepancies were found with respect to previous results. Maxwellian averaged neutron capture cross sections were calculated forthermal energies between kT = 10 keV and 100 keV. The new stellar cross sections were used for an updated analysis of the s-process reaction flow in the mass region between samarium and gadolinium, which is characterized by branchings at 151 Sm, 154 Eu, and 155 Eu. With the classical approach, the s-process temperature could be constrained corresponding to a range of thermal energies between kT=28 ke V and 33 keV. The 152 Gd production in low mass starswas found to depend strongly on the neutron freeze-out at the end of the heliumshell burning episodes.

Measurements of keV-neutron capture cross sections and capture gamma-ray spectra for Sn and Gd isotopes

“…As shown in figure 7, the present results for 156 Gd are in general agreement with those of Kononov et al [15] and Wisshak et al [16] and with the evaluated values in ENDF/B-Z-b2 [17] and JENDL-3.3 [18]. The present results for 158 Gd in figure 8 agree with those of Wisshak et al [16] and Bokhovko et al [19] and with the evaluated values in both ENDF/B-VII-b2 [17] and JENDL 3.3 [18]. However, the experimental values of Kononov et al [15] and Stupegia et al [20] are larger than any other values.…”

Thesprocess: Nuclear physics, stellar models, and observations

“…Comprehensive measurements were also performed at Karlsruhe for the long isotope chains of Cd (Wisshak et al, 2002), Sn (Wisshak et al, 1996), Te , and Ba (Voss et al, 1994) to provide detailed information for studying the full mass range of the main s-process component with well-defined MACS data. Reliable cross sections are also instrumental for defining the strength of the branchings in the reaction path, where the specific abundance patterns yield constraints for important parameters of the stellar plasma, i.e.…”

“…This branch has been studied using the solar-system abundance ratios of Gd isotopes (e.g. Wisshak et al (1995)). However, the physical conditions of the s process in an individual site (object) are not obtained from the analysis of solar-system abundances.…”

Stellar neutron capture cross sections of the tin isotopes

“…6, where the TOF spectra before subtraction of the background from isotopic impurities are shown tagether with this background. Here, these corrections are much smaller than for the gadoliniumisotopes [10], where up to 50% of the measured events had to be subtracted. In the 114 Sn and 115 Sn spectra, about 20% of the observed effect is due to isotopic impurities, whereas it is only 2% for 116 Sn, and even less for the other isotopes, which are not shown explicitely.…”