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 (the longest half-life of all nuclides proven to be radioactive) 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 (or 2 × 10¹⁵ years without any theoretical assumptions). 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 (following proton or beta emission) as a rare branch.