Absolute isotopic composition of molybdenum and the solar abundances of thep-process nuclidesMo92,94

The isotopic composition of molybdenum has been measured with high precision using a thermal ionization mass spectrometer, the linearity of which has been verified by measuring the isotopically-certified reference material for strontium (NIST 987). The abundance sensitivity of the mass spectrometer in the vicinity of the molybdenum ion beams has been carefully examined to ensure the absence of tailing effects. Particular care was given to ensuring that potential isobaric interferences from zirconium and ruthenium did not affect the measurement of the isotopic composition of molybdenum. Gravimetric mixtures of two isotopically enriched isotopes, ${}^{92}\mathrm{Mo}$ and ${}^{98}\mathrm{Mo}$, were analyzed mass spectrometrically to calibrate the mass spectrometer, in order to establish the isotope fractionation of the spectrometer for the molybdenum isotopes. This enabled the “absolute” isotopic composition of molybdenum to be determined. An accurate determination of the isotopic composition is required in order to calculate the atomic weight of molybdenum, which is one of the least accurately known values of all the elements. The absolute isotope abundances (in atom %) of molybdenum measured in this experiment are as follows: ${}^{92}\mathrm{Mo}$ $=14.5246\pm 0.0015$; ${}^{94}\mathrm{Mo}$ $=9.1514\pm 0.0074$; ${}^{95}\mathrm{Mo}$ $=15.8375\pm 0.0098$; ${}^{96}\mathrm{Mo}$ $=16.672\pm 0.019$; ${}^{97}\mathrm{Mo}$ $=9.5991\pm 0.0073$; ${}^{98}\mathrm{Mo}$ $=24.391\pm 0.018$; and ${}^{100}\mathrm{Mo}$ $=9.824\pm 0.050$, with uncertainties at the 1s level. These values enable an atomic weight $A_r$(Mo) of $95.9602\pm 0.0023$ (1s) to be calculated, which is slightly higher than the current Standard Atomic Weight $A_r$(Mo) $=95.94\pm 0.02$ and with a much improved uncertainty interval. These “absolute” isotope abundances also enable the Solar System abundances of molybdenum to be calculated for astrophysical purposes. Of particular interest are the Solar System abundances of the two $p$-process nuclides—${}^{92}\mathrm{Mo}$ and ${}^{94}\mathrm{Mo}$, which are present in far greater abundance than $p$-process theory suggests. The Solar System abundances for ${}^{92}\mathrm{Mo}$ and ${}^{94}\mathrm{Mo}$ of $0.364\pm 0.012$ and $0.230\pm 0.008$ respectively, (with respect to silicon $={10}^6$ atoms), are the most accurate values measured to date, and should therefore be adopted in future $p$-process calculations, rather than the existing values of $0.378\pm 0.021$ and $0.236\pm 0.013$, respectively.