Technetium lies between manganese and rhenium in group 7 of the periodic table, and its chemical properties are intermediate between those of these two adjacent elements. In 1937 Segrè and Perrier succeeded in isolating the isotopes technetium-95m and technetium-97. The most common naturally occurring isotope is 99Tc. In 1961, technetium-99 was first identified as a fission product of uranium-238 when it was isolated in very small quantities from African pitchblende. Technetium-99m ("m" means metastable) is used in radioactive isotope medical imaging. It is a gamma emitter with a half-life of 6.01 hours (meaning that about 94% of it decays to technetium-99 in 24 hours). This single isotope can be used for a multitude of diagnostic tests such as thyroid scanning. Technetium-99 is produced by the nuclear fission of both uranium-235 and plutonium-239. It is therefore present in radioactive waste and in the nuclear fallout of fission bomb explosions.
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