A cyclotron combines a uniform magnetic field with an alternating radio-frequency electric field, accelerating charged particles in a circular path while keeping their orbital period constant (isochronous condition). Starting in the 1930s, Lawrence refined the D-shaped electrodes (Dee electrodes) and boosted ion energies to tens of MeV in machines only tens of centimeters wide. Bombarding targets with these particles enabled large-scale production of artificial radioisotopes such as ^32P and ^14C, opening new frontiers in nuclear physics. The technique broadened studies of neutron–proton interactions and set the stage for medical tracers and cancer radiotherapy. Modern PET scans and isotope therapies trace their roots back to this invention.

In an isochronous cyclotron, the RF frequency is fixed at qB/2πm so that the particle enters the accelerating gap in phase during each revolution. Up to energies where relativistic mass increase becomes noticeable, the orbit period remains nearly constant while the radius grows as r∝v/B. Lawrence placed copper Dee electrodes inside a vacuum tank, applying tens of kilovolts across the gap to incrementally raise the kinetic energy each half-turn. The resulting high-energy ions enabled (p,n) and (d,n) reactions that produced many β-emitting nuclides, helping to map nuclear reaction chains before the era of nuclear fission. Modern isochronous AVF cyclotrons, synchrocyclotrons, and hospital-scale compact cyclotrons directly extend this principle with RF modulation and azimuthally varying fields.

Lawrence’s 27-inch and 60-inch cyclotrons operated around 37 MHz, incorporating a single-gap simplicity cavity with duty factor ~1/2. Beam currents reached the μA regime, while phase stability was maintained using Galileo modulation and high-Q tank circuits that limited RF losses to a few hundred watts. Cast-iron pole pieces at B≈1.2 T were trimmed with correction coils, giving ΔE/E<10⁻³. Neutron sources from the ^7Li(d,n)^8Be reaction enhanced Laue-Bragg scattering flux by two orders of magnitude, enabling precise σ_tot measurements. Artificial nuclides such as ^59Fe and ^24Na became indispensable for early nuclear medicine uptake studies. The design philosophy underlies strong-focusing cyclotrons and modern superconducting AVF machines, drastically lowering the cost of radio-isotope production.