Reactor-Produced Radionuclides. A nuclear reactor (for a description of a nuclear reactor, see Fission-Produced Radionuclides, later) is a source of a large number of thermal neutrons. Thermal neutrons are those neutrons whose kinetic energy is very small (˜0.025 eV, which is the kinetic energy of atoms or molecules at room temperature). At these energies, neutrons can be easily captured by stable nuclides because neutrons are neutral particles and do not experience the repulsive coulomb forces of the positively charged nucleus. The capture reaction of neutrons with a given nuclide is represented by either of the following notations:

In (i), the reactants are to the left of the arrow, and the products of the reaction are to the right. The first equation is written as a chemical reaction; the second equation is a short notation for the same. It can be seen that in the above nuclear reaction, the atomic number (and therefore the chemical nature or element) of the resulting nuclide () does not change; only the mass number A increases by one. Because in this reaction a neutron is being added, the resulting nuclide (if radioactive) quite often decays through β^– emission. We say “if radioactive” because for many neutron capture reactions, the resulting nuclide is stable (e.g., (n, γ) ). In this example, the nuclide is a stable nuclide. Another feature of reactor-produced nuclides is that these, in general, are not carrier free. In a carrier-free sample, only the desired radionuclide is present without contamination from its other isotopes. A sample of can be called carrier free only if no other stable isotope or radioisotope of iodine is present in the sample.

Some reactor-produced radionuclides that have been used in nuclear medicine and the nuclear reactions producing them are given as follows:

⁵¹Cr has been used for labeling red blood cells and spleen scanning.

⁹⁹Mo is the source of ^99mTc, which is so commonly used in nuclear medicine. Presently this is the only method of commercial production of this radionuclide. In North America, there is only one reactor that is the source of all ⁹⁹Mo, and periodic shut down of this reactor causes shortage of this important radionuclide. Currently, other methods for its production are being actively explored. One such method is described in the next section.

¹³³Xe is used for lung ventilation studies.

Accelerator- or Cyclotron-Produced Radionuclides. An accelerator or cyclotron is the source of a large number of high-energy (MeV range) charged particles such as p (protons), (deuterons), (helium 3), and (α). The classification of an accelerator or a cyclotron depends on the way in which these charged particles are accelerated and is not relevant here. However, cyclotrons are much smaller in size than accelerators. The probability of a nuclear reaction occurring with charged particles is highly dependent on the energy of the bombarding particles. For each charged particle and target, there is a threshold energy below which the nuclear reaction does not occur at all. This is due to the repulsive coulomb forces between the positively charged particle and the positively charged target nuclide. Generally, the threshold energy is in the MeV range. The most common reactions for protons are

or

or

The most common reactions for deuterons, (also known as heavy hydrogen), are (in short notations)

Common nuclear reactions for particles are

Common reactions for α () particles are

Most of the above reactions occur in the range of 5 to 30 MeV. As the energy of the bombarding particles further increases, other nuclear reactions occur. Sometimes these additional reactions may also be useful for producing radionuclides. Some radionuclides used routinely in nuclear medicine and produced in an accelerator or cyclotron are given as follows:

¹⁸F is used for labeling radiopharmaceuticals for positron emission tomography (PET).

⁶⁷Ga is used to image inflammation and chronic infections, as well as tumors.

This is currently being explored as an alternative to reactor produced ⁹⁹Mo.

In the above examples, the radionuclide of interest is formed directly as a result of a particular nuclear reaction. Sometimes, the radionuclide of interest may be formed indirectly by the decay of another radionuclide that is formed first with a nuclear reaction. Two examples of these indirect
methods are the production of radionuclides ¹²³I and ²⁰¹Tl:

Because in charged-particle nuclear reactions, the resultant radionuclide generally has an atomic number different from that of the target nuclide, one can chemically separate the two. Therefore, the radionuclides produced by charged-particle reactions are generally carrier free. Also, because in these reactions protons are added to a nuclide, these are generally β⁺– or electron-capturing radionuclides.

Medical cyclotrons that can fit in a small room are now commercially available and have become popular in Nuclear Medicine to produce a number of positron emitting radionuclide. The main drawback of these cyclotrons is the low bombarding energy of the accelerated charged particles, typically about 11 MeV. These are quite efficient though in production of ¹⁸F, a radionuclide that is widely used in PET.

Fission-Produced Radionuclides. Soon after the discovery of radioactivity, naturally occurring radioactive nuclides such as , , or were found to be good sources of a particles. The reactions of these a particles produced neutrons by (α n), . When the reactions of the neutrons thus generated were systematically studied, a surprising discovery was made. For many heavier nuclei (A ˜ 200), it was found that capture of a neutron, instead of producing a heavier radionuclide, resulted in the production of several radionuclides whose mass numbers were about one-half that of the target nuclide. For example, in the case of seldom occurs. Instead, is a much more frequent reaction. A less frequent reaction is the production of ⁹⁹Mo,