There are 19 known
radioisotopes ranging from 233Cm to 251Cm. There are also ten known
nuclear isomers. The longest-lived isotope is 247Cm, 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 isomer is 246mCm with a half-life of 1.12 seconds.
^( ) – Uncertainty (1σ) is given in concise form in parentheses after the corresponding last digits.
^# – Atomic mass marked #: value and uncertainty derived not from purely experimental data, but at least partly from trends from the Mass Surface (TMS).
^
abc# – Values marked # are not purely derived from experimental data, but at least partly from trends of neighboring nuclides (TNN).
^Côté, Benoit; Eichler, Marius; Yagüe López, Andrés; Vassh, Nicole; Mumpower, Matthew R.; Világos, Blanka; Soós, Benjámin;
Arcones, Almudena; Sprouse, Trevor M.; Surman, Rebecca; Pignatari, Marco; Pető, Mária K.; Wehmeyer, Benjamin; Rauscher, Thomas; Lugaro, Maria (26 February 2021). "129 I and 247 Cm in meteorites constrain the last astrophysical source of solar r-process elements". Science. 371 (6532): 945–948.
arXiv:2006.04833.
Bibcode:
2021Sci...371..945C.
doi:
10.1126/science.aba1111.
PMID33632846.
S2CID232050526.
^Plus radium (element 88). While actually a sub-actinide, it immediately precedes actinium (89) and follows a three-element gap of instability after
polonium (84) where no nuclides have half-lives of at least four years (the longest-lived nuclide in the gap is
radon-222 with a half life of less than four days). Radium's longest lived isotope, at 1,600 years, thus merits the element's inclusion here.
^Milsted, J.; Friedman, A. M.; Stevens, C. M. (1965). "The alpha half-life of berkelium-247; a new long-lived isomer of berkelium-248". Nuclear Physics. 71 (2): 299.
Bibcode:
1965NucPh..71..299M.
doi:
10.1016/0029-5582(65)90719-4. "The isotopic analyses disclosed a species of mass 248 in constant abundance in three samples analysed over a period of about 10 months. This was ascribed to an isomer of Bk248 with a half-life greater than 9 [years]. No growth of Cf248 was detected, and a lower limit for the β− half-life can be set at about 104 [years]. No alpha activity attributable to the new isomer has been detected; the alpha half-life is probably greater than 300 [years]."
^This is the heaviest nuclide with a half-life of at least four years before the "
sea of instability".
^Excluding those "
classically stable" nuclides with half-lives significantly in excess of 232Th; e.g., while 113mCd has a half-life of only fourteen years, that of 113Cd is eight
quadrillion years.