cyclotrons

A cyclotron is a type of particle accelerator in which charged particles are accelerated outward in a spiral by a constant magnetic field and an alternating electric field applied between two hollow, D-shaped electrodes known as “dees.” The particles start at the center and gain energy as they cross the gap between the dees, following a circular path that gradually increases its radius as momentum grows.

In the simplest, nonrelativistic case, the particle’s revolution frequency is approximately independent of its speed and is given by f = qB/(2πm), where q is the particle’s charge, B the magnetic field, and m its mass. The alternating voltage is synchronized so that each crossing of the gap boosts the particle’s energy. As speeds rise, relativistic effects alter the motion, reducing synchronization. To address this, variants such as the synchrocyclotron modulate the RF frequency, while isochronous cyclotrons adjust the field or ring geometry to keep the turn frequency nearly constant.

History and name: Cyclotrons were invented in 1931 by Ernest O. Lawrence and M. Stanley Livingston at the University of California, Berkeley. The device is named for the circular path the particles follow, driven by the magnetic field in the gaps between the dees.

Applications: Cyclotrons are widely used to produce medical radioisotopes for imaging and therapy, including PET tracers such as fluorine-18. They are also used in physics and materials research and, in some configurations, to generate protons or light ions for cancer therapy. Larger accelerators based on related concepts are employed for high-energy physics research, where much higher energies are required.