The only stable isotopes of thallium (₈₁Tl) are²⁰³Tl and²⁰⁵Tl, which make up all natural thallium. The five short-lived isotopes²⁰⁶Tl through²¹⁰Tl also occur in nature, but only as part of the naturaldecay chains of heavier elements. Synthetic radioisotopes are known from¹⁷⁶Tl to²¹⁷Tl; the most stable is²⁰⁴Tl with ahalf-life of 3.78 years, followed by²⁰²Tl (half-life 12.31 days) and²⁰¹Tl (half-life 3.0421 days). The naturally-occurring radioisotopes live minutes only, with the longest being²⁰⁷Tl, with a half-life of 4.77 minutes. All isotopes of thallium are either radioactive orobservationally stable, meaning that they are predicted to be radioactive but no actual decay has been observed.

The isotope²⁰⁴Tl is made by theneutron activation of stable thallium in anuclear reactor.^[4] while²⁰²Tl can be made in a cyclotron^[5] as can²⁰¹Tl (see section below).

In the fully ionized state, the isotope²⁰⁵Tl⁸¹⁺ becomes unstable, undergoingbound-state β⁻ decay to²⁰⁵Pb⁸¹⁺ with a half-life of291+33
−27 days,^[6][7] but²⁰³Tl remains stable.

²⁰⁵Tl is the decay product ofbismuth-209, an isotope that was once thought to be stable but is now known to undergoalpha decay with an extremely long half-life of 2.01×10¹⁹ y.^[8] Thus²⁰⁵Tl is now placed at the end of theneptunium decay chain.

Thallium-201 (²⁰¹Tl) is asynthetic 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).^[11] Thallium-201 is synthesized by theneutron activation of stable thallium in anuclear reactor,^[12] or by the²⁰³Tl(p, 3n)²⁰¹Pb nuclear reaction incyclotrons, as²⁰¹Pb then decays to²⁰¹Tl.^[13] It is aradiopharmaceutical, as it has fair imaging characteristics without excessive patient radiation dose. It was the most popular isotope used for nuclearcardiac stress tests.^[14]

This nuclide has largely been replaced bytechnetium-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 orRb-82.

Daughter products other than thallium

• Isotopes of lead
• Isotopes of mercury
• Isotopes of gold

• Isotopes of thallium
• Thallium
• Lists of isotopes by element

• Articles with short description
• Short description is different from Wikidata