# Isotopes of tellurium
Tellurium (atomic number 52) has eight naturally occurring isotopes, mass numbers 120, 122, 123, 124, 125, 126, 128, and 130, of which three are radioactive with half-lives so long that the element behaves as if it were stable in most settings. The two heaviest, \(^{128}\)Te and \(^{130}\)Te, together make up about 66 percent of natural tellurium and decay by double beta decay to xenon, a property exploited both in geochronology and in the search for neutrinoless double beta decay.[1](https://physics.nist.gov/cgi-bin/Compositions/stand_alone.pl?ascii=ascii&ele=Te) • [2](https://ciaaw.org/tellurium.htm)
| Key fact | Detail |
|---|---|
| Natural isotopes | Eight: \(^{120}\)Te, \(^{122}\)Te, \(^{123}\)Te, \(^{124}\)Te, \(^{125}\)Te, \(^{126}\)Te, \(^{128}\)Te, \(^{130}\)Te[1](https://physics.nist.gov/cgi-bin/Compositions/stand_alone.pl?ascii=ascii&ele=Te) |
| Most abundant | \(^{130}\)Te at mole fraction 0.3408 and \(^{128}\)Te at 0.3174[1](https://physics.nist.gov/cgi-bin/Compositions/stand_alone.pl?ascii=ascii&ele=Te) |
| Rarest | \(^{120}\)Te at mole fraction 0.0009 (about 0.09 atom percent)[1](https://physics.nist.gov/cgi-bin/Compositions/stand_alone.pl?ascii=ascii&ele=Te) • [3](https://www.isotopes.gov/products/tellurium) |
| Radioactive isotopes | \(^{123}\)Te (half-life 1.3(4) × 10\(^{13}\) a, to \(^{123}\)Sb), \(^{128}\)Te and \(^{130}\)Te (double beta decay)[2](https://ciaaw.org/tellurium.htm) |
| Longest half-lives | \(^{128}\)Te 2.5 × 10\(^{24}\) years and \(^{130}\)Te 7 × 10\(^{20}\) years (IUPAC IPTEI values)[4](https://iupac.org/wp-content/uploads/2015/02/IPTEI_postprint_20190301.pdf) |
| Standard atomic weight | \( A_{\mathrm{r}}(\mathrm{Te}) = 127.60(3) \), retained since 1969[2](https://ciaaw.org/tellurium.htm) |
| Decay modes of interest | Two-neutrino and neutrinoless double beta decay of \(^{130}\)Te and \(^{128}\)Te; electron capture of \(^{123}\)Te[5](https://www.mdpi.com/2218-1997/7/7/212) |
## Natural isotopic composition
The eight isotopes span mole fractions from 0.0009 for \(^{120}\)Te to 0.3408 for \(^{130}\)Te, with \(^{126}\)Te at 0.1884, \(^{125}\)Te at 0.0707, \(^{124}\)Te at 0.0474, \(^{122}\)Te at 0.0255, and \(^{123}\)Te at 0.0089.[1](https://physics.nist.gov/cgi-bin/Compositions/stand_alone.pl?ascii=ascii&ele=Te) The standard atomic weight \( A_{\mathrm{r}}(\mathrm{Te}) = 127.60(3) \) was adopted in 1961 from chemical-ratio determinations and retained in 1969 in the absence of calibrated mass-spectrometric measurements.[2](https://ciaaw.org/tellurium.htm) The atomic weight carries the "g" annotation, indicating geological isotopic anomalies: naturally occurring fission products of tellurium have been found in the Oklo fossil reactors at Gabon, south-west Africa, where fission product Te isotopes raise the local atomic weight.[2](https://ciaaw.org/tellurium.htm) • [6](https://publications.iupac.org/publications/pac/2002/pdf/7410x1987.pdf)
All eight isotopes are commercially available in enriched form from the US National Isotope Development Center, with enrichments ranging from 41 to 56 atom percent for \(^{120}\)Te to over 98 percent for \(^{128}\)Te and \(^{130}\)Te.[3](https://www.isotopes.gov/products/tellurium)
## Radioactive isotopes and their half-lives
Three of the natural isotopes are radioactive. The minor isotope \(^{123}\)Te has a half-life of 1.3(4) × 10\(^{13}\) years and transforms into \(^{123}\)Sb without significantly affecting the atomic weight of either element even over geological time.[2](https://ciaaw.org/tellurium.htm) A dedicated underground search for its K-shell electron capture with an array of 340 g of TeO\(_2\) thermal detectors found no evidence for the decay, setting a 90 percent confidence lower limit of \( t_{1/2}^{K} > 5 \times 10^{19} \) years on the K-shell half-life, and \( t_{1/2} > 9.2 \times 10^{16} \) years on the total electron-capture half-life after accounting for the predicted branching ratio.[7](https://arxiv.org/abs/hep-ex/0211015)
The two major isotopes \(^{128}\)Te and \(^{130}\)Te undergo double beta decay, in which two neutrons convert to two protons simultaneously, producing xenon. The IUPAC Periodic Table of the Elements and Isotopes gives their half-lives as 2.5 × 10\(^{24}\) and 7 × 10\(^{20}\) years, respectively.[4](https://iupac.org/wp-content/uploads/2015/02/IPTEI_postprint_20190301.pdf) These values rest on geochemical measurements: ion-counting mass spectrometry of xenon trapped in ancient tellurium ores, combined with common-lead dating of the ores, yielded a \(^{130}\)Te half-life of (2.7 ± 0.1) × 10\(^{21}\) years and a \(^{128}\)Te half-life of (7.7 ± 0.4) × 10\(^{24}\) years, described in the 1993 Physical Review C paper as the longest radioactive decay lifetime measured to date.[8](https://doi.org/10.1103/PhysRevC.47.806) The same work fixed the half-life ratio \( T_{1/2}^{130} / T_{1/2}^{128} = (3.52 \pm 0.11) \times 10^{-4} \).[8](https://doi.org/10.1103/PhysRevC.47.806) Geochemical half-life determinations for \(^{130}\)Te have historically fallen into two distinct groups differing by about a factor of three, a disagreement that persisted for more than 30 years without resolution, and the \(^{128}\)Xe half-life derived from the ratio inherits that uncertainty.[9](https://www.sciencedirect.com/science/article/pii/S0375947408006027)
## Double beta decay and the CUORE experiment
Because \(^{130}\)Te has a high natural abundance of about 34 percent and a high decay energy \( Q_{\beta\beta} \) of 2,527.5 keV, which places the signal region above most natural gamma-ray backgrounds, it is a leading isotope for the search for neutrinoless double beta decay, a hypothetical process that would establish that neutrinos are their own antiparticles (Majorana particles).[10](https://doi.org/10.1038/s41586-022-04497-4) • [5](https://www.mdpi.com/2218-1997/7/7/212)
The [CUORE](https://www.edgepedia.org/cuore) experiment (Cryogenic Underground Observatory for Rare Events) at the [Laboratori Nazionali del Gran Sasso](https://www.edgepedia.org/laboratori-nazionali-del-gran-sasso) in Italy uses an array of 988 TeO\(_2\) cryogenic bolometers cooled to about 10 mK, containing about 206 kg of \(^{130}\)Te at natural isotopic abundance.[10](https://doi.org/10.1038/s41586-022-04497-4) • [11](https://cuoreexperiment.org/results/) With an exposure of 372.5 kg·yr, CUORE found no evidence for neutrinoless decay and set a 90 percent credibility interval Bayesian lower limit of 3.2 × 10\(^{25}\) years on the \(^{130}\)Te half-life for that process, corresponding to an effective Majorana mass limit of 75 to 350 meV depending on the nuclear matrix elements used.[11](https://cuoreexperiment.org/results/)
The ordinary two-neutrino mode has been measured directly. CUORE's 2021 analysis of 300.7 kg·yr gave \( T_{1/2}^{2\nu} = 7.71 \times 10^{20} \) years, and a 2025 reanalysis of the full 1038 kg·yr dataset with optimized event selection improved the precision by a factor of two, obtaining \( T_{1/2}^{2\nu} = (9.32^{+0.05}_{-0.04}\,\mathrm{stat}^{+0.07}_{-0.07}\,\mathrm{syst}) \times 10^{20} \) years.[12](https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.126.171801) • [13](https://pubmed.ncbi.nlm.nih.gov/40929320/) The 2025 measurement also enabled the first application of the improved two-neutrino formalism to \(^{130}\)Te, extracting higher-order nuclear matrix element ratios that test nuclear models used in neutrinoless decay predictions.[13](https://pubmed.ncbi.nlm.nih.gov/40929320/) Searches have also been made for decay of \(^{130}\)Te to excited states of \(^{130}\)Xe; combining CUORICINO and CUORE-0 data gives \( T_{1/2}^{0\nu} > 1.4 \times 10^{24} \) years for the neutrinoless mode to the first 0\(^+\) excited state.[11](https://cuoreexperiment.org/results/)
## Applications in Earth science and medicine
The double beta decay of \(^{130}\)Te has been used to determine gas-retention ages of tellurium minerals, since the accumulated radiogenic xenon records the time since the mineral cooled.[4](https://iupac.org/wp-content/uploads/2015/02/IPTEI_postprint_20190301.pdf) More broadly, tellurium isotopes are a mixture of r-process, s-process, and p-process nucleosynthesis products, the three main stellar neutron- and photon-driven production mechanisms, which makes them useful for studying the contribution of stellar products to the molecular cloud from which the Sun and planets formed.[4](https://iupac.org/wp-content/uploads/2015/02/IPTEI_postprint_20190301.pdf)
The rarest natural isotope has a medical use: \(^{120}\)Te is used to produce \(^{120g}\)I, the ground-state iodine-120, via the \(^{120}\)Te(p,n)\(^{120g}\)I reaction. Iodine-120 has a half-life of 1.36 hours and serves as a positron emission tomography (PET) and beta-emitting isotope.[14](https://pubchem.ncbi.nlm.nih.gov/element/52)
## References
1. [Atomic Weights and Isotopic Compositions for Tellurium, NIST Physical Measurement Laboratory](https://physics.nist.gov/cgi-bin/Compositions/stand_alone.pl?ascii=ascii&ele=Te)
2. [Tellurium, Commission on Isotopic Abundances and Atomic Weights (CIAAW)](https://ciaaw.org/tellurium.htm)
3. [Tellurium, National Isotope Development Center](https://www.isotopes.gov/products/tellurium)
4. [N. E. Holden et al., IUPAC Periodic Table of the Elements and Isotopes (IPTEI), section 4.52 Tellurium](https://iupac.org/wp-content/uploads/2015/02/IPTEI_postprint_20190301.pdf)
5. [Status and Perspectives on Rare Decay Searches in Tellurium Isotopes, Universe 7, 212 (2021)](https://www.mdpi.com/2218-1997/7/7/212)
6. [Isotopic compositions of the elements (IUPAC, Pure Appl. Chem. 74, 1987 (2002))](https://publications.iupac.org/publications/pac/2002/pdf/7410x1987.pdf)
7. [A. Alessandrello et al., New Limits on Naturally Occurring Electron Capture of 123Te, arXiv:hep-ex/0211015](https://arxiv.org/abs/hep-ex/0211015)
8. [T. Bernatowicz et al., Precise determination of relative and absolute ββ-decay rates of 128Te and 130Te, Phys. Rev. C 47, 806 (1993)](https://doi.org/10.1103/PhysRevC.47.806)
9. [A. P. Meshik et al., 130Te and 128Te double beta decay half-lives, Nuclear Physics A 809, 275 (2008)](https://www.sciencedirect.com/science/article/pii/S0375947408006027)
10. [Search for Majorana neutrinos exploiting millikelvin cryogenics with CUORE, Nature (2022)](https://doi.org/10.1038/s41586-022-04497-4)
11. [CUORE Experiment results page](https://cuoreexperiment.org/results/)
12. [Measurement of the Decay Half-Life of 130Te with CUORE, Phys. Rev. Lett. 126, 171801 (2021)](https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.126.171801)
13. [Half-Life and Precision Shape Measurement of the 2νββ Decay of 130Te, Phys. Rev. Lett. 135, 082501 (2025)](https://pubmed.ncbi.nlm.nih.gov/40929320/)
14. [Tellurium, Te, PubChem Element page](https://pubchem.ncbi.nlm.nih.gov/element/52)
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*Topic: Encyclopedia › Physical world and mathematics › Physics › Particles and nuclei › Nuclear physics › Radioactivity and nuclear decay › Decay modes › Double beta decay*
*Initially written Sep 17, 2026 · Reviewed: Sep 30, 2026; Oct 15, 2026 · Edited: Sep 30, 2026; Oct 15, 2026 · Last review: Oct 15, 2026*
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