SProcess Isotope Abundance Anomalies in Meteoritic Silicon Carbide: Data for Dysprosium
Abstract
Several trace elements contained in Murchison SiC grains have been found to show clearly the isotopic signature of an overabundance of nuclides from the sprocess of nucleosynthesis. The sprocess isotopic composition of Dy is of interest because it is affected by the branching at ^163Dy. This nucleus is terrestrially stable but it becomes unstable at s process temperatures due to electrondensitydependent boundstate betadecay into ^163Ho. Using assumptions for neutron density and temperature based on analyses of other branchings a measurement of the isotopic composition of s process Dy may be able to provide constraints on the electron density at the site of the sprocess. The figure shows results for ^164Dy and ^163Dy; errors are 2sigma. Corrections have been applied at mass 164 for an interference during the mass spectrometric analysis from ^164Er. The upper data points are calculated using the minimum possible Er correction (terrestrial isotopic composition of Er), the lower data points using the maximum possible Er correction (sprocess isotopic composition of Er, estimated using the "local approximation" sigmaN=const. and the neutron capture cross sections from [1]). Both regression lines indicate a mixture of terrestrial Dy and sDy, the composition of which must plot to the left of that of sample 5a. For comparison the predictions according to the "local approximation" and the classical model from Kaeppeler et al. [2] are shown. Clearly they are not compatible with the experimental data. Extrapolation of our data to the virtual case of delta(^163Dy/^162Dy)x1000 o/oo shows that ^164Dy is to a large extent produced by the detour betadecay of ^163Dy neutron capture by ^163Ho  betadecay of ^164Ho. An estimate of the sprocess composition by extrapolation to vanishing abundances of the ponly isotopes ^156Dy and ^158Dy was not possible because of nondentified molecular interferences at these masses. We therefore give the isotopic composition of sample 5a as a limit for the sprocess composition of Dy: 160/161/162/163/164(corr. using terr. Er)/164(corr. using sEr)=1.222+.036/0.454+ 0.018/=1/0.054 +/0.005/0.314 +/ 0.014/0.200 +/ 0.013. Using the neutron capture cross sections from Bao et al. [1] (kT=30keV) and our data for sample 5a as an upper limit to the ratio (^163Dy/^162Dy)(sub)s a branching analysis shows that at least 87% of the ^163Dy produced in the sprocess makes a boundstate betadecay into ^163Ho. Assuming a neutron density of 3.4 x 10^8 cm^3 as derived by Wisshak [3] we obtain a lower limit for the betadecay rate of ^163Dy of (6.2 +/ 2.8) x 10^7s^1 (2sigmaerror). This value is outside the range of (1.15 x 10^15 2.38x10^7)s^1 derived by Takahashi and Yokoi [4] for the temperature range (0.55) x 10^8 K and for electron densities of (130) x 10^26cm^3, which covers the range expected at the site of the s process. The reason for this might be an overestimate of the sprocess neutron density and some uncertainty in the neutron capture cross sections of Dy. References:[1] Bao Z. Y.and Kaeppeler F. (1987) Atomic Data and Nucl. Data Tables, 36, 411. [2] Kaeppeler F. et al. (1989) Rep. Prog. Phys., 52, 945. [3] Wisshak K. (1993) Phys. Rev. C, 48, 3, 1401. [4] Takahashi K. and Yokoi K.(1987) Atomic Data and Nucl. Tables, 36, 375.
 Publication:

Meteoritics
 Pub Date:
 July 1994
 Bibcode:
 1994Metic..29..522R
 Keywords:

 Abundance;
 Anomalies;
 Dysprosium Isotopes;
 Meteoritic Composition;
 Murchison Meteorite;
 Silicon Carbides;
 Absorption Cross Sections;
 Astronomical Models;
 Electron Density (Concentration);
 Mass Spectroscopy;
 Radioactive Decay;
 Temperature Effects;
 Lunar and Planetary Exploration;
 DYSPROSIUM; ISOTOPIC ANOMALIES; MURCHISON; NUCLEOSYNTHESIS; S PROCESS; SILICON CARBIDE