Iodine

Natural cosmogenic ¹²⁹I enters groundwater and other terrestrial environments from the atmosphere and then decays to ¹²⁹Xe. The isotope-amount ratio n(¹²⁹I)/n(¹²⁷I) can be used as a clock to estimate time since cosmogenic ¹²⁹I entered the system. The amount of product ¹²⁹Xe in such cases is too small to measure; however, excess quantities of ¹²⁹Xe can be found in meteorites and other very old samples that contained extinct primordial ¹²⁹I. Younger water bodies also can be differentiated from older water bodies by determining the amount of anthropogenic ¹²⁹I released since the 1960s from sources such as nuclear bomb tests [393], [394].

¹²⁵I, which has a half-life of about 59 days, is used encapsulated in radiotherapy to target and treat sites of cancerous tumors [395]. 120gI (with a half-life of 1.36 h), where the “g” indicates ground state, and ¹²⁴I (with a half-life of 100 h) are radioactive isotopes that emit positrons and they are used in quantitative, diagnostic imaging of the body using positron emission tomography (PET) [383], [384], [385], [387], [388], [389]. ¹²³I and ¹³¹I (with half-lives of 0.55 day and 8 days, respectively) are used with single-photon emission computed spectroscopy (SPECT) for basic three-dimensional imaging [386], [395]. Radioactive iodine isotopes are produced from radioactive tellurium isotope.