The only stable isotopes of thallium (81Tl) are 203Tl and 205Tl, which make up all natural thallium. The five short-lived isotopes 206Tl through 210Tl also occur in nature, but only as part of the natural decay chains of heavier elements. Synthetic radioisotopes are known from 176Tl to 217Tl; the most stable is 204Tl with a half-life of 3.78 years, followed by 202Tl (half-life 12.31 days) and 201Tl (half-life 3.0421 days). The naturally-occurring radioisotopes live minutes only, with the longest being 207Tl, with a half-life of 4.77 minutes. All isotopes of thallium are either radioactive or observationally stable, meaning that they are predicted to be radioactive but no actual decay has been observed. The isotope 204Tl is made by the neutron activation of stable thallium in a nuclear reactor. while 202Tl can be made in a cyclotron as can 201Tl (see section below). In the fully ionized state, the isotope 205Tl81+ becomes unstable, undergoing bound-state β− decay to 205Pb81+ with a half-life of 291+33−27 days, but 203Tl remains stable. 205Tl is the decay product of bismuth-209, an isotope that was once thought to be stable but is now known to undergo alpha decay with an extremely long half-life of 2.01×1019 y. Thus 205Tl is now placed at the end of the neptunium decay chain.
List of isotopes
Thallium-201 Thallium-201 (201Tl) is a synthetic radioisotope of thallium. It has a half-life of 3.0421 days and decays by electron capture, emitting photons consisting mainly of K X-rays (~70–80 keV), and gammas of 135 and 167 keV (the latter stronger, emitted in 10% of decays). Thallium-201 is synthesized by the neutron activation of stable thallium in a nuclear reactor, or by the 203Tl(p, 3n)201Pb nuclear reaction in cyclotrons, as 201Pb then decays to 201Tl. It is a radiopharmaceutical, as it has fair imaging characteristics without excessive patient radiation dose. It was the most popular isotope used for nuclear cardiac stress tests. This nuclide has largely been replaced by technetium-99m, which has a shorter half-life (6 hours instead of 3 days) and a single high-energy photon peak (140 keV), which is better for imaging than the 3 energy peaks of thallium-201. Thallium-201 is now mostly used for myocardial viability studies. It will redistribute in body tissues, whereas Tc will not; Tl is taken up by the cardiac muscle via Na+/K+ pumps. Delayed imaging will show uptake in damaged but still living myocardial cells, which would appear as a scar with Tc or 82Rb.
See also Daughter products other than thallium
Isotopes of lead Isotopes of mercury Isotopes of gold
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