Decay
The detailed decay mechanism is electron capture to form an excited state of the nearly-stable nuclide tellurium-123 (half life so long that it is considered stable for all practical purposes). This excited state of Te-123 produced is not the metastable nuclear isomer Te-123m (the decay of I-123 does not involve enough energy to produce Te-123m), but rather is a lower-energy nuclear isomer of Te-123 that immediately gamma decays to ground state Te-123 at the energies noted, or else (13% of the time) decays by internal conversion electron emission (127 keV), followed by an average of 11 Auger electrons emitted at very low energies (50-500 eV). The latter decay channel also produces ground-state Te-123. Especially because of the internal conversion decay channel, I-123 is not an absolutely pure gamma-emitter, although it is sometimes clinically assumed to be one.
The Auger electrons from the radioisotope have been found in one study to do little cellular damage, unless the radionuclide is incorporated chemically directly into cellular DNA, which is not the case for present radiopharmaceuticals which use I-123 as the radioactive label nuclide. The damage from the more penetrating gamma radiation and 127 keV internal conversion electron radiation from the initial decay of Te-123 is moderated by the relatively short half-life of the isotope.
Read more about this topic: Iodine-123
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