Fission or (n,f) Reaction
When a target of heavy elements is inserted in the reactor core, heavy nuclei
absorb thermal neutrons and undergo fission. Fissionable heavy elements
are^235 U,^239 Pu,^237 Np,^233 U,^232 Th, and many others having atomic numbers
greater than 92. Fission of heavy elements may also be induced in a
cyclotron by irradiation with high-energy charged particles, but fission prob-
ability depends on the type and energy of the irradiating particle. Nuclides
produced by fission may range in atomic number from about 28 to nearly
- These isotopes of different elements are separated by appropriate chem-
ical procedures that involve precipitation, solvent extraction, ion exchange,
chromatography, and distillation. The fission radionuclides are normally
carrier-free or NCA, and therefore radionuclides of high specific activity
are available from fission. The fission products are usually neutron rich and
decay by b−-emission.
Many clinically useful radionuclides such as^131 I,^99 Mo,^133 Xe, and^137 Cs are
produced by fission of^235 U. An example of thermal fission of^235 U follows,
showing a few representative radionuclides:
(^23592) U + (^10) n → (^23692) U → (^13153) I + (^10239) Y + 310 n
→^9942 Mo +^13550 Sn + 210 n
→^11746 Pd +^11746 Pd + 210 n
→^13354 Xe +^10138 Sr + 210 n
→^13755 Cs + 3797 Rb + 210 n
→^15562 Sm +^7830 Zn + 310 n
→^15662 Sm +^7730 Zn + 310 n
Many other nuclides besides those mentioned in the example are also
produced.
Neutron Capture or (n,g) Reaction
In neutron capture reaction, the target nucleus captures one thermal
neutron and emits g-rays to produce an isotope of the same element. The
radionuclide so produced is therefore not carrier-free, and its specific activ-
ity is relatively low. This reaction takes place in almost all elements with
varying probability. Some examples of neutron capture reactions are
(^98) Mo(n,g) (^99) Mo, (^196) Hg(n,g) (^197) Hg, and (^50) Cr(n,g) (^51) Cr. Molybdenum-99 so pro-
duced is called the irradiated molybdenum as opposed to the fission molyb-
denum described earlier. This method is commonly used in the analysis of
trace elements in various samples.
The method of production and various characteristics of radionuclides
commonly used in nuclear medicine are presented in Table 5.1.
Reactor-Produced Radionuclides 47