Molybdenum-99 (Mo) is a radioactive isotope of molybdenum with a relatively short half-life of approximately 66 hours ( hours). Its primary significance stems from its decay product, Technetium-99m (Tc), which is by far the most widely utilised medical radioisotope in diagnostic imaging. The reliable supply of Mo is therefore paramount for global healthcare services employing nuclear medicine.
Mo itself is not directly used for medical imaging. Instead, it serves as a 'generator' for Tc. Nuclear medicine departments commonly receive molybdenum-technetium generators, which contain Mo adsorbed onto an alumina column. As the Mo decays, the resulting Tc accumulates and can be eluted (milked) daily. This allows medical facilities to have a ready supply of Tc, which has a half-life of 6 hours, without requiring immediate proximity to a nuclear reactor or processing facility.
Technetium-99m is invaluable due to its ideal physical characteristics for diagnostic imaging:
Common applications of Tc include bone scans, cardiac stress tests, kidney function studies, and brain imaging.
There are two principal methods for the production of Mo:
The most prevalent method, accounting for the vast majority of global supply, involves the neutron-induced fission of highly enriched uranium (HEU) or low enriched uranium (LEU) targets in research reactors. Uranium-235 (U) targets are irradiated, and Mo is one of the numerous fission products. The relevant nuclear reaction is:
After irradiation, the targets undergo complex chemical processing to separate and purify the Mo from other fission products, some of which are highly radioactive. This method yields a high specific activity product, which is crucial for efficient generators.
A less common method involves the neutron activation of naturally occurring Molybdenum-98 (Mo). This process, known as the (n,γ) reaction, transmutes Mo into Mo:
This method typically produces Mo with a lower specific activity compared to fission-derived Mo, as the activated molybdenum is mixed with unactivated Mo. It also requires the use of enriched Mo targets to achieve practical yields and reduce impurity levels. The lower specific activity means that larger quantities of molybdenum material are required in generators to achieve the same amount of Tc, which can be a practical limitation.
Mo undergoes beta-minus decay to Tc (87%) and directly to the ground state Tc (13%). The Tc then decays via isomeric transition to Tc. The decay chain is represented as:
The gamma emission from the Tc isomeric transition is what makes it so useful for medical imaging.
The global supply of Mo has historically faced challenges due to its production being concentrated in a few ageing research reactors. Disruptions at any of these facilities can lead to severe shortages of Tc, impacting patient care worldwide. Efforts are underway to diversify production methods, including encouraging the use of LEU targets instead of HEU to reduce nuclear proliferation risks, and exploring alternative non-reactor-based production routes like accelerator production of Tc or direct production of Mo. These initiatives aim to enhance the reliability and resilience of the Mo supply chain, a critical aspect of nuclear safety and public health.