text: Technetium-99m is a metastable nuclear isomer, as indicated by the "m" after its mass number 99. This means it is a nuclide in an excited (metastable) state that lasts much longer than is typical. The nucleus will eventually relax (i.e., de-excite) to its ground state through the emission of gamma rays or internal conversion electrons. Both of these decay modes rearrange the nucleons without transmuting the technetium into another element.
  Tc decays mainly by gamma emission, slightly less than 88% of the time. (Tc → Tc + γ) About 98.6% of these gamma decays result in 140.5 keV gamma rays and the remaining 1.4% are to gammas of a slightly higher energy at 142.6 keV. These are the radiations that are picked up by a gamma camera when Tc is used as a radioactive tracer for medical imaging. The remaining approximately 12% of Tc decays are by means of internal conversion, resulting in ejection of high speed internal conversion electrons in several sharp peaks (as is typical of electrons from this type of decay) also at about 140 keV (Tc → Tc + e). These conversion electrons will ionize the surrounding matter like beta radiation electrons would do, contributing along with the 140.5 keV and 142.6 keV gammas to the total deposited dose.
  Pure gamma emission is the desirable decay mode for medical imaging because other particles deposit more energy in the patient body (radiation dose) than in the camera. Metastable isomeric transition is the only nuclear decay mode that approaches pure gamma emission.
  Tc's half-life of 6.0058 hours is considerably longer (by 14 orders of magnitude, at least) than most nuclear isomers, though not unique. This is still a short half-life relative to many other known modes of radioactive decay and it is in the middle of the range of half lives for radiopharmaceuticals used for medical imaging.
  After gamma emission or internal conversion, the resulting ground-state technetium-99 then decays with a half-life of 211,000 years to stable ruthenium-99. This process emits soft beta radiation without a gamma. Such low radioactivity from the daughter product(s) is a desirable feature for radiopharmaceuticals.