A cyclotron is a type of particle accelerator that uses a uniform magnetic field to accelerate particles in a spiral path. The accelerated particles are then used to bombard target nuclei to produce a wide variety of radionuclides 1.
Cyclotron is an improvement on the linac (linear particle accelerator) as a cyclotron not only saves space but can also use the same electrode system to accelerate particles to much higher velocities 1.
A synchrotron is an upgraded version of the cyclotron, in which the magnetic field increases with time to accelerate particles within a circular path with a fixed diameter to much higher energies than a cyclotron. Synchrotrons are used in large-scale facilities for particle physics research.
On this page:
Mechanism
To produce a radionuclide, a particle should have sufficient energy for nuclear reactions and sufficient beam current to produce reasonable yields 2.
The cyclotron is constructed within a uniform magnetic field. The ion source is placed within the center of the magnetic field. Once an ion is emitted from the source, the magnetic field bends the particles into circular orbits. The ions are accelerated through high-frequency electric fields emanating from two hollow electrodes called the "Dees" (due to their shapes resembling the letter "D"). The particles are also accelerated when passing through the gaps between the Dees 2.
According to the principles of electrodynamics, the rotational frequency of a particle orbiting a center is proportional to the charge of the particle and external magnetic field applied, while inversely proportional to its own mass. The rotational frequency is also independent of the energy of the particle and the radius of orbit 2. Meanwhile, the velocity of the particle increases when the higher energy is applied to the particle. Since the rotating frequency of the particle is constant, the diameter of the orbit has to increase in order to accommodate the higher velocity of the particle. Therefore, the greater the diameter of the cyclotron, the higher the energy of the particle that can be achieved. However, the maximum proton energy achieved within the cyclotron is less than 70 MeV. The accelerated particle is then extracted at the outer edge of the machine for various applications 2.
Types
Cyclotrons primarily use protons to bombard target nuclei. Occasionally, deuterons (a nucleus containing one proton and one neutron) or helium nuclei have been employed as the projectiles 1.
Low energy (<20 MeV)
These types of cyclotrons are used to produce positron-emitting radionuclides for PET scans. These cyclotrons are located in regional centers and hospitals because of the short half-lives of these radionuclides 1.
Medium energy (20 to 35 MeV)
These types of cyclotrons are used to produce gamma-emitting radioisotopes for SPECT scans. These cyclotrons are located in dedicated facilities due to longer half-lives 1.
High energy (>35 MeV)
These cyclotrons are used in the production of radioisotopes for radiotherapy. High electric current (1 mA) is needed to produce these types of isotopes 1.
History and etymology
In 1928, Rolf Wideröe built a linear accelerator that was capable of bringing sodium and potassium ions up to 50 keV 3. In 1930, Ernest O Lawrence developed the first cyclotron at the University of California Berkeley. His cyclotron was capable of accelerating hydrogen ions to 80 keV 4. His graduate student, M Stanley Livingston provided proof of the particle acceleration. Lawrence received the Nobel Prize for Physics in 1939 for his work in cyclotrons. Initially, cyclotrons were mainly used for physics research. It was only in 1941, when the first cyclotron dedicated for medical applications was installed at Washington University in St Louis where radioactive isotopes of phosphorus, iron, arsenic, and sulfur were produced 2.
Unable to process the form. Check for errors and try again.