Page "Decay chain" Paragraph 7

Due to the quite short half-life of its starting isotope neptunium-237 ( 2. 14 million years ), the fourth chain, the neptunium series with A = 4n + 1, is already extinct in nature, except for the final rate-limiting step, decay of bismuth-209.

The ending isotope of this chain is now known to be thallium-205.

Some older sources give the final isotope as bismuth-209, but it was recently discovered that it is radioactive, with a half-life of.