Naturally occurring niobium (Nb), element 41, is composed of one stable isotope (93Nb). 93Nb is the lightest nuclide theoretically susceptible to spontaneous fission, and although this has never been observed, it makes niobium theoretically the lightest element with no stable isotope. The first 40 elements (to zirconium) all have at least one stable nuclide susceptible in theory only to proton decay.
The most stable radioisotope is 92Nb with a half-life of 34.7 million years. This nuclide is the longest-lived radionuclide of all elements that has not yet been detected in nature as a primordial isotope. (The nuclide with the next longest half-life, 146Sm with half-life 68 million years, has been detected, and is thus primordial).
The next longest-lived niobium nuclides are 94Nb (half-life: 20,300 years), and 91Nb with a half-life of 680 years. There is also a meta state at 31 keV whose half-life is 16.13 years. Twenty three other radioisotopes have been characterized. Most of these have half-lives that are less than two hours except 95Nb (35 days), 96Nb (23.4 hours) and 90Nb (14.6 hours). The primary decay mode before the stable 93Nb is electron capture and the primary mode after is beta emission with some neutron emission occurring in 104–110Nb.
Only 95Nb (35 days) and 97Nb (72 minutes) and heavier isotopes (half-lives in seconds) are fission products in significant quantity, as the other isotopes are shadowed by stable or very long-lived (93Zr) isotopes of the preceding element zirconium from production via beta decay of neutron-rich fission fragments. 95Nb is the decay product of 95Zr (64 days), so disappearance of 95Nb in used nuclear fuel is slower than would be expected from its own 35-day halflife alone. Tiny amounts of the other isotopes may be produced as direct fission products.
Niobium's relative atomic mass is 92.90638(2).