The isotopic composition and atomic weight of lanthanum

The isotopic composition of lanthanum has been measured with high precision using a thermal ionization mass spectrometer, equipped with a Daly collector, whose linearity was verified by measuring an isotopically certified reference material for potassium (NIST 985), whose isotopes span a wide range of isotope ratios. The abundance sensitivity of the mass spectrometer in the vicinity of the measured LaO+ ion beams was examined to ensure the absence of tailing effects and interfering isotopes. The isotope fractionation of the lanthanum, isotopes was estimated by reference to the isotopically certified reference material for lead (NIST 981) and the fractionation of oxygen isotopes determined from LaO+ measurements. These procedures are essential because of the extremely low isotope abundance of La-138. An accurate determination of the abundance of La-138 is required in order to calculate the atomic weight, and because it is the parent nuclide of two geochronometers, La-138-Ba-138 and La-138-Ce-138. The magnitude of the rare odd-odd neutron-deficient isotope La-138 is a key nuclide in p-process nucleosynthetic calculations. The measured isotopic composition has been corrected for isotope fractionation to give La-139/La-138 = 1125 +/- 3, which gives isotope abundances for La-138 of 0.000888 +/- 0.000002 and La-139 of 0.999112 +/- 0.000002. The isotope abundances and relative atomic masses of the two isotopes give an atomic weight of La of 138.905461 +/- 0.000003. (c) 2005 Elsevier B.V. All rights reserved.