The production of radionuclides plays a crucial role in medicine, particularly in nuclear medicine for diagnostics and therapy. Radionuclides can be produced using two major methods: nuclear reactors (which utilize neutrons) and particle accelerators (which utilize protons, deuterons, alpha particles, or heavy ions).
In both methods, the target material is typically a stable nuclide. Depending on the method of production, this target material may undergo different reactions, leading to either a neutron-rich or neutron-deficient radionuclide. Reactor-produced radionuclides tend to be neutron-rich, while accelerator-produced radionuclides tend to be neutron-deficient.
A nuclear reactor is a facility that uses nuclear fission to produce energy. The core of a reactor contains fissile materials such as 235U (Uranium-235), 239Pu (Plutonium-239), and 241P (Plutonium-241). These materials undergo fission when they absorb neutrons. The process releases a significant amount of energy, and, importantly for radionuclide production, it generates fast neutrons with energies ranging from about 1 MeV to 10 MeV.
The fast neutrons produced in the reactor are then slowed down (or moderated) by materials such as water, heavy water, or graphite. This process reduces the neutrons’ energy, resulting in thermal neutrons, which are typically at an energy of 0.025 eV. These thermal neutrons are more likely to interact with target nuclei, initiating nuclear reactions.
The typical neutron flux in reactors is on the order of 1014 neutrons · cm-2 · s-1, which represents the number of neutrons passing through a square centimeter of material every second. This flux is crucial for ensuring a consistent production rate of radionuclides. However, the exact flux may vary depending on the reactor's design and the target material's location within the reactor core.
The most common nuclear reaction used in reactor-produced radionuclides is the neutron capture reaction. In this process, a target nucleus absorbs a thermal neutron and becomes a compound nucleus. This compound nucleus is typically in an excited state and may subsequently release energy in the form of gamma radiation (denoted by γ).
A prime example of this reaction is the capture of a neutron by 59Co (Cobalt-59) to produce 60Co (Cobalt-60). This radionuclide is widely used in radiotherapy for the treatment of cancer.
The reaction for this process can be written as:
Radionuclides produced in reactors are particularly important for radiotherapy, especially in the treatment of cancer. The key to therapeutic applications is the emission of high-energy beta radiation (β), which can damage the DNA of cancer cells, causing them to die. Reactors produce a wide range of radionuclides that emit beta radiation, and these are typically used in clinical settings.
Examples of reactor-produced radionuclides used in therapy include:
90Y (Yttrium-90): Used in therapies such as radioembolization for liver cancer.131I (Iodine-131): Used for thyroid cancer treatment.177Lu (Lutetium-177): Used in targeted radiotherapy for cancers such as neuroendocrine tumors.The production of 177Lu (Lutetium-177) is an important process in nuclear medicine. There are two major methods for producing 177Lu:
While thermal neutrons are most commonly used for radionuclide production, some reactions require fast neutrons (neutrons with energy greater than 10 MeV). Fast neutrons typically have lower cross-sections for nuclear reactions, meaning that fewer reactions occur. However, these reactions are still important for producing certain radionuclides that cannot be efficiently produced with thermal neutrons. For example, 32P (Phosphorus-32), which is used in medical applications, must be produced with fast neutrons.
Fast neutron reactions generally have cross-sections measured in millibarns (1 millibarn = 10-3 barns), a unit used to measure the probability of a nuclear reaction occurring. Despite the lower yields compared to thermal neutrons, fast neutrons are essential for producing certain critical radionuclides.