Tellurium-128

There are 38 known isotopes and 17 nuclear isomers of tellurium (Te), with atomic masses that range from 105 to 142. These are listed in the table below.

Naturally-occurring tellurium on Earth consists of eight isotopes. Two of these have been found to be radioactive: ¹²⁸Te and ¹³⁰Te undergo double beta decay with half-lives of, respectively, 2.2×10²⁴ (2.2 septillion) years (the longest half-life of all nuclides proven to be radioactive) and 7.9×10²⁰ (790 quintillion) years. The longest-lived artificial radioisotope is ¹²¹Te with a half-life of nearly 19 days. Several nuclear isomers have longer half-lives, the longest being ^121mTe with a half-life of 154 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 the recent measurements of the same team have disproved this. The half-life of ¹²³Te is longer than 9.2 × 10¹⁶ years, and probably much longer.

¹²⁴Te can be used as a starting material in the production of radionuclides by a cyclotron or other particle accelerators. Some common radionuclides that can be produced from tellurium-124 are iodine-123 and iodine-124.

The short-lived isotope ¹³⁵Te (half-life 19 seconds) is produced as a fission product in nuclear reactors. It decays, via two beta decays, to ¹³⁵Xe, the most powerful known neutron absorber, and the cause of the iodine pit phenomenon.

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