Isotope abundance ofTa180mandp-process nucleosynthesis

Abstract

The p-process of stellar nucleosynthesis produces the stable neutron-deficient nuclides heavier than the iron peak elements. An accurate determination of the isotopic composition of tantalum is required to enable p-process nucleosynthetic calculations to be evaluated in terms of an accurate isotope abundance for $^{180}\mathrm{Ta}$. This odd-odd nuclide has the remarkable property of having a long-lived isomeric state and a short-lived ground state, so that in reality one is measuring the isotope abundance of $^{180}\mathrm{Ta}{}^{m}$, which is a unique situation in nature. $^{180}\mathrm{Ta}{}^{m}$ is the rarest isotope of nature's rarest element and is therefore an important isotope in deciphering the origin of the p-process. Because the isotopic composition of tantalum has only been measured on two occasions with relatively large uncertainties, an accurate determination is required to provide a better basis for p-process production calculations. A thermal ionization mass spectrometer was used to measure the isotope abundance of $^{180}\mathrm{Ta}{}^{m}$ with high precision. The linearity of this instrument was verified by measuring the isotopically certified reference material for potassium (NIST 985), whose isotopes span a wide range of isotope ratios. The abundance sensitivity of the mass spectrometer for the measured ion beams has been examined to ensure the absence of tailing effects and interfering isotopes. These procedures are essential because of the extremely low isotope abundance of $^{180}\mathrm{Ta}{}^{m}$. The isotope fractionation of the tantalum isotopes was estimated by reference to the isotope fractionation of the isotopically certified reference material for rhenium (NIST 989). The isotopic composition of tantalum has been determined to be $^{181}\mathrm{Ta}/^{180}\mathrm{Ta}{}^{m}$=8325 \ifmmode\pm\else\textpm\fi{} 43, which gives isotope abundances for $^{180}\mathrm{Ta}{}^{m}$=0.0001201 \ifmmode\pm\else\textpm\fi{} 0.0000008 and $^{181}\mathrm{Ta}$=0.9998799 \ifmmode\pm\else\textpm\fi{} 0.0000008. This gives a Solar System abundance of $^{180}\mathrm{Ta}{}^{m}$ of 2.49 \ifmmode\times\else\texttimes\fi{} 10${}^{\ensuremath{-}6}$ with reference to silicon=10${}^{6}$. These isotope abundances, together with the relative atomic masses, give an atomic weight for tantalum of 180.947878 \ifmmode\pm\else\textpm\fi{} 0.000002.