Isotopes of tellurium
Naturally occurring tellurium on Earth consists of eight isotopes: 120, 122-126, 128, 130. The heaviest two have been found to be radioactive: 128Te and 130Te undergo double beta decay with half-lives of, respectively, 7.7×1024 years and 7.9×1020 years. Artificial radioisotopes of tellurium are known, with atomic masses that range from 104 to 142, of which the most stable is 121Te with a half-life of 19.31 days. Several nuclear isomers have longer half-lives, the longest being 121mTe with a half-life of 164.7 days.
The very long-lived radioisotopes 128Te and 130Te are the two most common isotopes of tellurium. Of elements with at least one stable isotope, only indium and rhenium likewise have a radioisotope in greater abundance than a stable one.
It has been claimed that electron capture of 123Te was observed, but more recent measurements of the same team have disproved this. They have determined the half-life of 123Te to be longer than 9.2 × 1016 years. Its observational stability presents one of only two apparent violations of the Mattauch isobar rule, the other involving 180mTa.
124Te is used as the starting material in the production of certain radionuclides by a cyclotron or other particle accelerator, such as iodine-123 and iodine-124.
With the exception of beryllium, tellurium is the lightest element observed to have isotopes capable of undergoing alpha decay, with isotopes 104Te to 109Te being seen to undergo this mode of decay. Some lighter elements, namely those in the vicinity of 8Be, have isotopes with delayed alpha emission as a rare branch.