Plutonium-244
Plutonium-244 is an isotope of plutonium that has a half-life of 80 million years. This is longer than any of the other isotopes of plutonium and longer than any other actinide isotope except for the three naturally abundant ones: uranium-235, uranium-238, and thorium-232.
Accurate measurements, beginning in the early 1970s, have detected primordial plutonium-244, making it the shortest-lived primordial nuclide. The amount of 244Pu in the pre-Solar nebula was estimated as 0.8% the amount of 238U. As the age of the Earth is about 57 half-lives of 244Pu, the amount of plutonium-244 left should be very small; Hoffman et al. estimated its content in the rare-earth mineral bastnasite as = 1.0×10−18 g/g, which corresponded to the content in the Earth crust as low as 3×10−25 g/g. Since plutonium-244 cannot be easily produced by natural neutron capture in the low neutron activity environment of uranium ores, its presence cannot plausibly be explained by any other means than creation by r-process nucleosynthesis in supernovas. Plutonium-244 thus should be the second shortest-lived and the heaviest primordial isotope yet detected or theoretically predicted.
However, the detection of primordial 244Pu in 1971 is not confirmed by recent, more sensitive measurements using the method of accelerator mass spectrometry. In this study, no traces of plutonium-244 in the samples of bastnasite were observed, so only an upper limit on the 244Pu content was obtained: < 0.15×10−18 g/g, which is 370 atoms per gram of the sample, at least 7 times lower than the abundance measured by Hoffman et al.
Live interstellar plutonium-244 has been detected in meteorite dust in marine sediments, although the levels detected are much lower than would be expected from current modelling of the in-fall from the interstellar medium.
Unlike plutonium-238, plutonium-239, plutonium-240, plutonium-241, and plutonium-242, plutonium-244 is not produced in quantity by the nuclear fuel cycle, because further neutron capture on plutonium-242 produces plutonium-243 which has a short half-life and quickly beta decays to americium-243 before having much opportunity to further capture neutrons in any but very high neutron flux environments. However, a nuclear weapon explosion can produce some plutonium-244 by rapid successive neutron capture.
