The Isotopic Constitution of Lanthanum and Cerium

Abstract
The isotopic abundances of lanthanum and cerium have been determined by means of a mass spectrometer. In addition to the known lanthanum isotope of mass 139, a new isotope of mass 138 and abundance 0.089 percent has been discovered. Since this isotope is isobaric with two neighboring stable isotopes it should be radioactive. No activity was observed. Upper limits for the non-existence of other lanthanum isotopes were determined. The abundances of cerium isotopes of masses 136, 138, 140, and 142 were shown to be 0.193, 0.250, 88.48, and 11.07 percent, respectively. Upper limits for the non-existence of other cerium isotopes were determined.

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