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Naturally occurring cerium (58 Ce) is composed of 4 stable isotopes: ... with a half-life of 284.893 days; 139 Ce, with a half-life of 137.640 days and 141 Ce, ...
The most stable of them are 144 Ce with a half-life of 284.9 days, 139 Ce with a half-life of 137.6 days, and 141 Ce with a half-life of 32.5 days. All other radioactive cerium isotopes have half-lives under four days, and most of them have half-lives under ten minutes. [21]
Radioactive isotope table "lists ALL radioactive nuclei with a half-life greater than 1000 years", incorporated in the list above. The NUBASE2020 evaluation of nuclear physics properties F.G. Kondev et al. 2021 Chinese Phys. C 45 030001. The PDF of this article lists the half-lives of all known radioactives nuclides.
The longest-lived isotope is 247 Cm, with half-life 15.6 million years – orders of magnitude longer than that of any known isotope beyond curium, and long enough to study as a possible extinct radionuclide that would be produced by the r-process. [2] [3] The longest-lived known isomer is 246m Cm with a half-life of 1.12 seconds.
The isobar forming 132 Te/ 132 I is: Tin-132 (half-life 40 s) decaying to antimony-132 (half-life 2.8 minutes) decaying to tellurium-132 (half-life 3.2 days) decaying to iodine-132 (half-life 2.3 hours) which decays to stable xenon-132. The creation of tellurium-126 is delayed by the long half-life (230 k years) of tin-126.
^^ Bismuth-209 was long believed to be stable, due to its half-life of 2.01×10 19 years, which is more than a billion times the age of the universe. § Europium-151 and samarium-147 are primordial nuclides with very long half-lives of 4.62×10 18 years and 1.066×10 11 years, respectively.
In this situation it is generally uncommon to talk about half-life in the first place, but sometimes people will describe the decay in terms of its "first half-life", "second half-life", etc., where the first half-life is defined as the time required for decay from the initial value to 50%, the second half-life is from 50% to 25%, and so on.
243 Cm with a ~30-year half-life and good energy yield of ~1.6 W/g could be a suitable fuel, but it gives significant amounts of harmful gamma and beta rays from radioactive decay products. As an α-emitter, 244 Cm needs much less radiation shielding, but it has a high spontaneous fission rate, and thus a lot of neutron and gamma radiation.