What Are Cyclotrons?

Cyclotrons are circular particle accelerators that use a combination of static magnetic fields and oscillating electric fields to accelerate charged particles, such as protons, deuterons, or alpha particles, in a spiral path to high kinetic energies. Invented in 1930 by Ernest O. Lawrence and M. Stanley Livingston at the University of California, Berkeley, the cyclotron was the first device capable of accelerating particles to energies useful for nuclear physics experiments without requiring a linear machine of impractical length. Lawrence received the Nobel Prize in Physics in 1939 for this invention.

Cyclotrons occupy a distinct position among accelerator types. Unlike linear accelerators, which require a separate accelerating gap for each energy increment, a cyclotron re-uses the same pair of accelerating gaps by bending the particle path into a spiral with a magnet. Unlike synchrotrons, which vary the magnetic field to keep particles on a fixed orbit, classical cyclotrons operate with a constant magnetic field, which limits their energy range due to relativistic effects. Modern isochronous cyclotrons address this limitation by shaping the magnetic field to compensate for the increasing relativistic mass of particles as they gain speed.

Operating Principles

A cyclotron consists of two hollow D-shaped electrodes, called dees, separated by a small gap. Charged particles are injected near the center and accelerated across the gap by an alternating voltage tuned to the cyclotron resonance frequency. The magnetic field bends the particle path into a semicircle within each dee, and the particle crosses the gap again after each half-turn, gaining energy with each crossing. Because the radius of each semicircle increases with speed, the overall path spirals outward until the particles reach the edge of the dees and are extracted. The IAEA overview of cyclotron technology summarizes the core operating principles and the conditions under which relativistic mass gain limits the achievable energy.

Magnet and RF Systems

The main magnet in a cyclotron establishes the uniform or shaped dipole field that bends particle trajectories. Conventional cyclotrons use room-temperature electromagnets with iron poles; compact superconducting cyclotrons use superconducting coils to achieve much higher fields in a smaller footprint, reducing the machine's size and cost. The radio-frequency (RF) system drives the alternating voltage across the dee gap at frequencies typically in the range of 10 to 100 MHz, synchronized to the cyclotron resonance condition. Beam current, energy uniformity, and transmission efficiency depend strongly on the design of both the magnet and RF subsystems. Detailed engineering parameters for a high-current 15-MeV per nucleon cyclotron design are reported in work archived through OSTI, the Department of Energy's scientific database.

Radioisotope Production

The largest application of cyclotrons today is the production of short-lived radioisotopes for nuclear medicine. Protons or deuterons are directed at stable target materials to produce radionuclides through nuclear reactions; the most common medical targets include oxygen-18 water to produce fluorine-18 for positron emission tomography (PET) imaging. Because PET isotopes decay quickly (fluorine-18 has a half-life of about 110 minutes), cyclotrons must be located close to clinical facilities. The IAEA inventory of cyclotron facilities worldwide records approximately 1,300 operational cyclotrons globally, the majority dedicated to medical isotope production.

Applications

Cyclotrons have applications in a wide range of scientific and clinical fields, including:

• Positron emission tomography (PET) radiotracer production for oncology and neurology
• Proton therapy and boron neutron capture therapy for cancer treatment
• Basic nuclear physics and nuclear reaction cross-section measurements
• Production of radioisotopes for industrial imaging and non-destructive testing
• Materials irradiation studies for space and nuclear reactor component qualification