How are radionuclides produced in a cyclotron?
Radionuclides are produced in a cyclotron by accelerating charged particles, usually protons, to high energies and directing them at a stable target nucleus. The collision induces a nuclear reaction that transforms the target into a radioactive isotope.
Cyclotrons produce radionuclides by accelerating protons into target nuclei, inducing nuclear reactions that generate PET isotopes such as fluorine-18.
Cyclotron production is the primary method for producing positron-emitting radionuclides used in PET imaging, such as fluorine-18, carbon-11, nitrogen-13, and oxygen-15.
Unlike reactor production, which uses neutrons, cyclotron production relies on charged-particle bombardment.
Understanding the physics
A cyclotron is a particle accelerator that uses a combination of a magnetic field and an alternating electric field to accelerate charged particles, usually protons.
Inside the cyclotron are two hollow, semicircular metal electrodes called “dees” because of their D-shape. These dees sit between the poles of a strong magnet.
The magnetic field forces the charged particles to travel in a circular path. Each time the particles cross the gap between the two dees, they experience an alternating electric field that accelerates them. As their speed increases, the radius of their circular path increases, causing them to spiral outward in larger and larger arcs.
Importantly, the electric field alternates at a frequency matched to the particle’s circular motion. This ensures that each time the particle crosses the gap between the dees, it is accelerated rather than slowed.
When the particles reach sufficient energy, they are extracted from the outer edge of the spiral and directed toward a target material containing a stable isotope.
When a high-energy proton strikes a target nucleus, it can enter the nucleus and displace one of its neutrons. This changes the composition of the nucleus and transforms it into a different isotope.
For example, fluorine-18 is commonly produced by bombarding oxygen-18 with protons. In this reaction:
18O(p,n)18F
This notation means:
A proton (p) enters the nucleus.
A neutron (n) leaves.
The target nucleus becomes a new element.
Because cyclotron reactions involve charged particles, they can only occur in specialised accelerator facilities and cannot occur in reactors.
Cyclotron production generally yields radionuclides with high specific activity, because the produced isotope can be chemically separated from the target material.
Cyclotrons are particularly suited to producing radionuclides with short half-lives, which are used quickly after production, often within hours.
Where this matters clinically
PET imaging depends heavily on cyclotron-produced isotopes. Fluorine-18 has a half-life of approximately 110 minutes, allowing regional distribution from a central production facility. Shorter-lived isotopes such as carbon-11 require an on-site cyclotron.
The need for cyclotron infrastructure directly influences tracer availability and cost.