Solar abundance ofLu176ands-process nucleosynthesis

Abstract

The isotopic composition of lutetium has been measured with high precision using a thermal ionization mass spectrometer whose linearity was verified by measuring an isotopically certified reference material for potassium prepared by the National Institute of Standards and Technology (NIST 985). The abundance sensitivity of the mass spectrometer for the measured ion beams of ${\mathrm{Lu}}^{+}$ was examined to ensure the absence of tailing effects and interfering ion beams. The isotope fractionation of the measured $^{176}\mathrm{Lu}$/$^{175}\mathrm{Lu}$ ratio was estimated with reference to the isotope fractionation of ytterbium (whose isotopes are in the same mass region as lutetium), which was recently measured in this laboratory using gravimetrically prepared solutions of the enriched isotopes $^{171}\mathrm{Yb}$ and $^{176}\mathrm{Yb}$. This is the first reported publication in which the measured isotope ratio of Lu has been corrected for isotope fractionation. An accurate determination of the abundance of $^{176}\mathrm{Lu}$ is required because of the importance of this isotope in cosmochronometry, cosmothermometry, and s-process branching studies. An accurate abundance of $^{176}\mathrm{Lu}$ is also required as it is the parent nuclide of the $^{176}\mathrm{Lu}$/$^{176}\mathrm{Hf}$ geochronometer. The measured isotopic composition of Lu, corrected for isotope fractionation, is $^{176}\mathrm{Lu}$/$^{175}\mathrm{Lu}$ = 0.026680 \ifmmode\pm\else\textpm\fi{} 0.000013, which gives isotope abundances for $^{175}\mathrm{Lu}$ of 97.4013 \ifmmode\pm\else\textpm\fi{} 0.0012% and of $^{176}\mathrm{Lu}$ of 2.5987 \ifmmode\pm\else\textpm\fi{} 0.0012%. The isotope abundances and relative atomic masses of the two isotopes give an atomic weight of 174.9668 \ifmmode\pm\else\textpm\fi{} 0.0001, which is in good agreement with the present Standard Atomic Weight ${A}_{r}$(Lu) = 174.967 \ifmmode\pm\else\textpm\fi{} 0.001, but with improved accuracy. An accurate assessment of the $^{176}\mathrm{Lu}$/$^{175}\mathrm{Lu}$ ratio is important in order to calculate the Solar System abundances of $^{175}\mathrm{Lu}$ and $^{176}\mathrm{Lu}$ for astrophysical evaluations. The experimentally determined Solar System abundances for $^{175}\mathrm{Lu}$ and $^{176}\mathrm{Lu}$ of 0.0347918 \ifmmode\pm\else\textpm\fi{} 0.0000004 and 0.0009282 \ifmmode\pm\else\textpm\fi{} 0.0000004, respectively (as compared to silicon equals ${10}^{6}$ atoms), should now be used for these purposes. This determination of the isotopic composition of Lu also demonstrates that the presently accepted half-life of $^{176}\mathrm{Lu}$ needs to be reevaluated.