Thallium (₈₁Tl) has 41 isotopes with atomic masses that range from 176 to 216. ²⁰³Tl and ²⁰⁵Tl are the only stable isotopes and ²⁰⁴Tl is the most stable radioisotope with a half-life of 3.78 years. ²⁰⁷Tl, with a half-life of 4.77 minutes, has the longest half-life of naturally occurring Tl radioisotopes. 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.

Thallium-202 (half-life 12.23 days) can be made in a cyclotron^[4] while thallium-204 (half-life 3.78 years) is made by the neutron activation of stable thallium in a nuclear reactor.^[5]

In the fully ionized state, the isotope ²⁰⁵Tl becomes beta-radioactive, decaying to ²⁰⁵Pb,^[6] 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.^{[7] 205}Tl is at the end of the neptunium series decay chain.

List of isotopes

Thallium-201

Thallium-201 (²⁰¹Tl) is a synthetic radioisotope of thallium. It has a half-life of 73 hours and decays by electron capture, emitting X-rays (~70–80 keV), and photons of 135 and 167 keV in 10% total abundance.^[17] Thallium-201 is synthesized by the neutron activation of stable thallium in a nuclear reactor,^[17][18] or by the ²⁰³Tl(p, 3n)²⁰¹Pb nuclear reaction in cyclotrons, as ²⁰¹Pb naturally decays to ²⁰¹Tl afterwards.^[19] It is a radiopharmaceutical, as it has good imaging characteristics without excessive patient radiation dose. It is the most popular isotope used for thallium nuclear cardiac stress tests.^[20]