Transient radioactive equilibrium of ^121mTe and ¹²¹Te and determination of their half-lives

[^121mTe]-Tellurium and [¹²¹Te]-tellurium form a genetic mother/daughter pair in a transient radioactive equilibrium. Both nuclides are found as undesired byproducts in [¹²³I]-iodide solution, a radionuclide established in radiopharmacy for synthesis of tracers with application in diagnostic nuclear medicine. Because these Te-isotopes affect the radionuclidic purity of the product and contribute to the overall dose for the patient, their half-lives must be known accurately in order for the dose calculations to be correct. For each of the two isotopes, literature reports several distinct, but inconsistent values for the respective half-life T_1/2: ^121mTe: 125 d / ∼140 d / 154 d / 164 d // ¹²¹Te: ∼17 d / 19 d. ZAG Zyklotron AG is a commercial manufacturer of [¹²³I]-sodium iodide and therefore it was a reasonable goal to add to literature an additional view on the half-lives of ^121mTe and ¹²¹Te from long-time examination of retention samples and of quality control samples drawn from routine production. This work experimentally found T_1/2 of ^121mTe to be 165.1(7) d and T_1/2 of ¹²¹Te to be 19.38(3) d. In addition, the transient radioactive equilibrium was examined closer with these values for the IT internal transition ratio of ^121mTe.