Naturally occurring zirconium (Zr) is composed of four stable isotopes (of which one may in the future be found radioactive), and one very long-lived radioisotope (96Zr), a primordial nuclide that decays via double beta decay with an observed half-life of 2.0×1019 years; it can also undergo single beta decay, which is not yet observed, but the theoretically predicted value of t1/2 is 2.4×1020 years. The second most stable radioisotope is 93Zr, which has a half-life of 1.53 million years. Twenty-seven other radioisotopes have been observed. All have half-lives less than a day except for 95Zr (64.02 days), 88Zr (63.4 days), and 89Zr (78.41 hours). The primary decay mode is electron capture for isotopes lighter than 92Zr, and the primary mode for heavier isotopes is beta decay.
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Zirconium is the heaviest element that can be formed from symmetric fusion, from either 45Sc, or 46Ca producing 90Zr (after two beta-plus decays from 90Mo) and 92Zr respectively. All heavier elements are formed either through asymmetric fusion or during the collapse of supernovae. As most of these are energy-absorbing processes, most nuclides of elements heavier than zirconium are theoretically unstable to spontaneous fission, although in many cases, the half-life for this is too long to have been observed. See list of nuclides for a tabulation.
Relative atomic mass: 91.224(2).
Zirconium-89
89Zr is a radioisotope of zirconium with a half-life of 78.41 hours. It is produced by proton irradiation of natural yttrium-89. Its most prominent gamma photon has an energy of 909 keV.
Zirconium-89 is employed in specialized diagnostic applications using positron emission tomography imaging, for example, with zirconium-89 labeled antibodies (immuno-PET). For a decay table, see Maria Vosjan. "Zirconium-89 (89Zr)". Cyclotron.nl.
Zirconium-93
93Zr is a radioisotope of zirconium with a half-life of 1.53 million years, decaying with a low-energy beta particle to niobium-93, which decays with a halflife of 14 years and a low-energy gamma ray to ordinary 93Nb. It is one of only 7 long-lived fission products. The low specific activity and low energy of its radiations limit the radioactive hazards of this isotope.
Nuclear fission produces it at a fission yield of 6.3% (thermal neutron fission of 235U), on a par with the other most abundant fission products. Nuclear reactors usually contain large amounts of zirconium as fuel rod cladding (see zircaloy), and neutron irradiation of 92Zr also produces some 93Zr, though this is limited by 92Zr's low neutron capture cross section of 0.22 barns.
93Zr also has a low neutron capture cross section of 0.7 barns. Most fission zirconium consists of other isotopes; the other isotope with a significant neutron absorption cross section is 91Zr with a cross section of 1.24 barns. 93Zr is a less attractive candidate for disposal by nuclear transmutation than are Tc-99 and I-129. Mobility in soil is relatively low, so that geological disposal may be an adequate solution.