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The decay of radioactive elements occurs at a fixed rate. The half-life of a radioisotope is the time required for one half of the amount of unstable material to degrade into a more stable material. For example, a source will have an intensity of 100% when new. At one half-life, its intensity will be cut to 50% of the original intensity.
203 Tl and 205 Tl are the only stable isotopes and 204 Tl is the most stable radioisotope with a half-life of 3.78 years. Thallium-202 (half life 12.23 days) can be made in a cyclotron while thallium-204 (half life 3.78 years) is made by the neutron activation of stable thallium in a nuclear reactor. The lead-201 can be produced in a cyclotron by the bombardment of thallium with protons or deuterons by the (p,3n) and (d,4n) reactions. 2  combined with sulfur or selenium and arsenic, thallium has been used in the production of high-density glasses that have low melting points in the range of 125 and 150 Â°C.