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.

2

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

3

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.

4

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.

5

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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