6.1 percent results in the production of Mo-99 or about 37 barns. The production cross section for the 98Mo(n,γ)99Mo reaction is about 0.13 barn for thermal neutrons, a factor of almost 300 less than the fission process even accounting for the 6.1 percent fission yield for Mo-99.

There are 6 stable isotopes (92, 93, 94, 95, 96, 97) of Mo and two very long-lived isotopes (98 is >1012 years and 100 is >1018 years). Both Mo-98 and Mo-100 have long enough half-lives that they exist in nature and can be used as target material. Thus the ability to produce large amounts of Mo-99 from the direct reaction route would depend upon the availability of a high flux reactor that could compensate for the lower cross section. For example, typical fluxes from the National Research Universal (NRU) reactor are around 1.5 × 1014 neutrons per cm2 per second while the High Flux Isotope Reactor (HFIR) at Oak Ridge has a flux of 1015 neutrons per cm2 per second, more than enough to be competitive in producing large amounts of Mo-99 via the (n,γ) approach.2 However, these additional neutrons are not free and would add to the costs of producing Mo-99 by this method.

However, the Mo-99 produced by this process has a very low specific activity3,4 because most of the Mo in the product is Mo-98. The specific activity for fission-produced Mo-99 is two to four orders of magnitude higher than from the neutron capture process (Ottinger and Collins, 1996). This has practical implications for using neutron capture Mo-99 in medical isotope procedures: First, the technetium generators that are used for fission-produced Mo-99 would have to be redesigned to use neutron capture-produced Mo-99. A larger technetium generator column would be needed, which would increase the size of the generator and the size and weight of its shield. A larger volume of liquid would be required to elute Tc-99m from the column, which would require all of the current Tc-99m kits (e.g., see Table 2.1) to be reformulated. In addition, the useful lifetime of the generator would be reduced due to the potential for higher breakthrough5 of the Mo-99. This would require users to purchase additional generators.


If desired, the isotope could also be enriched in Mo-98 using mass separation processes.


Specific activity is defined as the amount of radioactivity per unit mass as is usually expressed in terms of Becquerel’s per gram or curies per gram.


Delft University researchers are examining the feasibility of using Szilard Chalmers reactions to increase specific activities. However, the yields from this process are likely to be small, and a great deal of development work would likely be required to get to a useful, practical process, if indeed it is possible at all. See http://www.tudelft.nl/live/pagina.jsp?id=29b23a65-485b-44ee-9210-f460e363c2c6&lang=en. Accessed October 23, 2008.


When the generator is eluted to obtain Tc-99m a very small amount of Mo-99 is released. The generator can no longer be used when the amount of Mo-99 in the eluted solution exceeds a certain level. The amount of breakthrough is roughly proportional to the amount of molybdenum present, both radioactive Mo-99 and nonradioactive Mo-98.

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