Solar abundance of 176Lu and s-process nucleosynthesis

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 Lu+ was examined to ensure the absence of tailing effects and interfering ion beams. The isotope fractionation of the measured Lu-176/Lu-175 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 Yb-171 and Yb-176. 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 Lu-176 is required because of the importance of this isotope in cosmochronometry, cosmothermometry, and s-process branching studies. An accurate abundance of Lu-176 is also required as it is the parent nuclide of the Lu-176/Hf-176 geochronometer. The measured isotopic composition of Lu, corrected for isotope fractionation, is Lu-176/Lu-175 = 0.026680 +/- 0.000013, which gives isotope abundances for Lu-175 of 97.4013 +/- 0.0012% and of Lu-176 of 2.5987 +/- 0.0012%. The isotope abundances and relative atomic masses of the two isotopes give an atomic weight of 174.9668 +/- 0.0001, which is in good agreement with the present Standard Atomic Weight A(r)(Lu) = 174.967 +/- 0.001, but with improved accuracy. An accurate assessment of the Lu-176/Lu-175 ratio is important in order to calculate the Solar System abundances of Lu-175 and Lu-176 for astrophysical evaluations. The experimentally determined Solar System abundances for Lu-175 and Lu-176 of 0.0347918 +/- 0.0000004 and 0.0009282 +/- 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 Lu-176 needs to be reevaluated.