A technetium-99m generator, or colloquially a technetium cow is a device used to extract the metastable isotope 99mTc of technetium from a source of decaying molybdenum-99. 99Mo has a half-life of 66 hours and can be easily transported over long distances to hospitals where its decay product technetium-99m (with an inconvenient half-life of only 6 hours for transport) is extracted and used for a variety of nuclear medicine diagnostic procedures, where its low half-life is very useful.
The half-life of the mother nuclide (99Mo) is much longer than that of the daughter nuclide (99mTc). 50% of equilibrium activity is reached within one daughter half-life, 75% within two daughter half-lives. Hence, removing the daughter nuclide (elution process) from the generator ("milking" the cow) is reasonably done every 6 hours or, at most, twice daily in a 99Mo/99mTc generator. Most commercial 99Mo/99mTc generators use column chromatography, in which 99Mo is adsorbed onto acid alumina (Al2O3). Pulling normal saline solution through the column of immobilized 99Mo elutes the soluble 99mTc, resulting in a saline solution containing the 99mTc which is then added to an appropriate concentration to the organ-specific pharmaceutical to be used. The isotope can also be used without pharmaceutical tagging for specific procedures requiring only the 99mTc as the primary radiopharmaceutical. The useful life of a 99Mo/99mTc generator is about 3 parent half lives, or approximately one week. Hence, any clinical nuclear medicine units purchase at least one such generator per week or order several in a staggered fashion.
99Mo can be obtained by the neutron activation (n,γ reaction) of 98Mo in a high neutron flux reactor. The most used method requires a uranium target with high enriched uranium (up to 90% 235U) or low enriched uranium (less than 20% 235U). The target should be irradiated with neutrons to form 99Mo as a fission product .
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