Technetium-99m
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Technetium-99m is a metastable nuclear isomer of technetium-99, symbolized as 99mTc. The "m" indicates that this is a metastable nuclear isomer. It is a gamma ray emitting isotope used in radioactive isotope medical tests, for example as a radioactive tracer that medical equipment can detect in the body. It is well suited to the role because it emits readily detectable 140 keV gamma rays (these are about the same wavelength emitted by conventional X-ray diagnostic equipment), and its half-life for gamma emission is 6.01 hours (meaning that about fifteen sixteenths of it decays to 99Tc in 24 hours). The short half life of the isotope allows for scanning procedures which collect data rapidly, but keep total patient radiation exposure low. For a full discussion of its uses in nuclear medicine, see the article on technetium.
Technetium-99m decays to Tc-99 (a less exited state of the same isotope) by rearrangement of nucleons in its nucleus. Technetium-99 is an isotope which emits soft beta rays but no gamma rays.
Due to its short half-life, technetium-99m for nuclear medicine purposes is usually extracted from technetium-99m generators which contain Mo-99, which is the usual parent nuclide for this isotope.
