Isotopes of thallium

The only stable isotopes of thallium 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 and ²⁰¹Tl. 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.
In the fully ionized state, the isotope ²⁰⁵Tl⁸¹⁺ becomes unstable, undergoing bound-state β⁻ decay to ²⁰⁵Pb⁸¹⁺ with a half-life of 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.
Image:Decay Chain.svg|300px|thumb|right|The neptunium decay chain, ending at ²⁰⁵Tl.

List of isotopes

Thallium-201

Thallium-201 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, and gammas of 135 and 167 keV. Thallium-201 is synthesized by the neutron activation of stable thallium in a nuclear reactor, or by the ²⁰³Tl²⁰¹Pb 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This nuclide has largely been replaced by technetium-99m, which has a shorter half-life and a single high-energy photon peak, which is better for imaging than the 3 energy peaks of thallium-201.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 Rb-82.