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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 short 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 [1].

• Brookhaven National Laboratory site on the history of the technetium cow.

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