The November 1, 1952 thermonuclear explosion ("Mike") produced all of the uranium isotopes U 239 , U 240 , • • • U 255 through multiple neutron capture by U 238 . The long-lived products of successive 0~ decays from these isotopes were measured mass spectrometrically and radiometrically. The logarithms of the abundances decline smoothly with increasing mass number; the even-mass abundances slightly exceed the geometric mean of adjacent odd-mass abundances. Some nuclear properties of neutron-rich heavy nuclides, not subject to ordinary investigation are inferred.

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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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