I is produced by proton irradiation of Xe. The caesium isotope produced is unstable and decays to I. The isotope is usually supplied as the iodide and hypoiodate in dilute sodium hydroxide solution, at high isotopic purity. I has also been produced at Oak Ridge National Laboratories by proton bombardment of Te.
I decays by electron capture with a half-life of 13.22 hours. The emitted 159 keV gamma ray is used in single-photon emission computed tomography (SPECT). A 127 keV gamma ray is also emitted.
I is frequently used in radioimmunoassays because of its relatively long half-life (59 days) and ability to be detected with high sensitivity by gamma counters.
I is present in the environment as a result of the testing of nuclear weapons in the atmosphere. It was also produced in the Chernobyl and Fukushima disasters. I decays with a half-life of 15.7 million years, with low-energy beta and gamma emissions. It is not used as a tracer, though its presence in living organisms, including human beings, can be characterized by measurement of the gamma rays.