Cyclotron: Design, History, and Modern Applications in Medicine and Research

Background

The contemporary cyclotron employs two hollow, D‑shaped electrodes housed in a high‑vacuum chamber between the poles of a powerful electromagnet. A high‑frequency AC voltage drives each electrode, while an ion source generates either positive or negative ions depending on the configuration. Ions are accelerated toward one electrode by electrostatic attraction; when the AC polarity reverses, they are drawn toward the other electrode. This alternating process, combined with a strong magnetic field, forces the ions to follow a circular path. Each transition between electrodes adds energy, expanding the orbit into a spiral until the ions reach the outer edge of the acceleration region. At that point, the accelerated particles are extracted and form a beam that can bombard target materials to produce radioactive isotopes.

Isotopes produced by cyclotrons are indispensable in medicine: as tracers for diagnostic imaging and as therapeutic agents for cancer treatment. The cyclotron’s beam is also used for positron emission tomography (PET), a powerful imaging modality that measures concentrations of positron‑emitting radioisotopes in living tissue, enabling early detection of neurological disorders, cerebral vascular disease, epilepsy, and tumors.

History

In 1929, Ernest O. Lawrence and his graduate students at the University of California, Berkeley, achieved the first successful cyclotron. The initial machine was modest—a 10‑watt radio‑frequency oscillator, a 4‑inch electromagnet, and a 5‑inch diameter vacuum chamber that accelerated hydrogen ions to 5–45 MeV. (1 MeV = 1.602 × 10⁻¹³ J.) Over the ensuing decades, the cyclotron grew in size and complexity, involving physicists, engineers, and chemists. Lawrence’s work laid the groundwork for modern nuclear physics and chemistry, earning him the Nobel Prize in Physics (1939) and the National Medal of Science (1958).

Raw Materials

The cyclotron’s core magnet consists of 25 tons of low‑carbon steel with nickel‑plated poles, creating a 55‑ton apparatus. It is housed within a vault lined with 2‑meter‑thick concrete walls to shield the surrounding area from the brief‑lived radiation generated during operation. The cyclotron’s dimensions are approximately 30.5 m × 30.6 m × 11.9 m. Coils are fabricated from annealed copper, insulated with fiberglass, and coated in epoxy resin. The aluminum vacuum chamber is sealed with polyurethane O‑rings, while a tungsten filament ionizes the hydrogen gas. Borated polyethylene packing mitigates thermal neutron buildup, and the target changer—primarily aluminum with minimal stainless steel—reduces neutron activation.

Design

Custom cyclotrons are engineered to meet client specifications. Ebco Technologies Inc. offers two negative‑ion models: the TR19 (max 19 MeV) and the TR32 (max 32 MeV). The TR19 can accommodate two external beamlines and up to eight targets, with optional scalability. It is available in self‑shielded or unshielded configurations; the self‑shielded version eliminates the need for a dedicated vault. The RF system comprises an amplifier, coaxial transmission line, power supply, and diagnostic instrumentation, all integrated into a computerized control framework. Ion injection is achieved through a 10‑cm diameter magnetic cylinder, with a helium‑cooled target assembly made from high‑purity silver, aluminum, or titanium.

Ernest Orlando Lawrence

Born in South Dakota on August 8, 1901, Lawrence earned his B.S. in physics at the University of South Dakota (1922), an M.S. at the University of Minnesota (1923), and a Ph.D. from Yale (1925). He became an associate professor at UC Berkeley in 1928 and, two years later, the university’s youngest full professor. Lawrence’s 1929 cyclotron invention revolutionized nuclear science. He was elected to the National Academy of Sciences (1934), received the Nobel Prize in Physics (1939), the Medal of Merit (1946), and the Fermi Award (1957). Lawrence remained at Berkeley until his death on August 27, 1958.

The Manufacturing Process

1. Project teams coordinate conduit, cable tray, floor duct, and related equipment before shipping, rigging, and installing the cyclotron and its subsystems.
2. The 25‑ton steel magnet is machined from 10‑inch slabs and positioned between the poles of the electromagnet, ensuring precise magnetic field measurement.
3. Nickel‑plated magnetic poles are forged from low‑carbon steel.
4. Coil assemblies, made from annealed hollow copper, are bent, hardened, mounted in the yoke, water‑cooled, insulated with fiberglass, and coated in epoxy.
5. The aluminum vacuum tank, placed between the poles, is bolted into place and equipped with cryopumps that cool the chamber to −459 °F (−273 °C) to freeze out residual gases.
6. Electrodes are machined from 0.06‑inch low‑resistivity copper, cut, and etched to optimize RF energy transfer.
7. After mounting electrodes inside the tank, polyurethane O‑rings seal the assembly; nylon screws and spacers secure the electrodes in an industrial Lisex nylon housing with dedicated ports for oscillator wiring, vacuum pump, and gauge.
8. A PVC ring surrounds the electrodes, containing a detector storage tube and channels for voltage supply, set screws, and attachment points for a brass hook.
9. Clear industrial plastic covers the PVC for visibility and structural reinforcement.
10. Silicon gel maintains a vacuum‑tight seal around the main chamber, essential for preventing alpha‑particle attenuation.
11. Nylon screws hold the copper deflector plate in place, electrically isolated from the rest of the system; set screws adjust its position.
12. The RF system is assembled in a 19‑inch square, 6‑ft high metal chassis, with resistors, transmitters, switches, tuning circuits, inductors, and capacitors hand‑wired.
13. Power supply cabinets are assembled for water‑cooled targets, magnets, ion sources, cryopumps, and circuitry.
14. The ion source—a 10‑inch diameter, 4.7‑inch long magnetic cylinder—injects hydrogen gas via a capillary tube.
15. The tilted spiral inflector is mounted on a grounded, helical electrode, machined on a fixed‑axis milling machine.
16. Target bodies (high‑purity silver, aluminum, titanium) feature helium‑cooled thin‑foil windows to separate target material from the cyclotron vacuum.
17. A recirculating closed‑loop helium cooling system chills the foil windows.
18. Tubing, solenoid valves, water‑cooled beam stops, and electrically isolated collimators are integrated into the target assembly.
19. The target assembly includes a solid aluminum plug with a 10‑cm hole serving as the collimator.
20. External grooves on the plug and an O‑ring create a vacuum seal between the target body and the four‑position target changer.
21. A collimating disc with windows on both sides sits between the plug and target body.
22. The complete system is integrated with supervisory software to control and monitor PLC hardware.

Quality Control

Each manufacturing step is rigorously inspected to ensure component integrity. Defects such as cracks or leaks are identified promptly to prevent radiation leakage. The magnet steel’s properties are verified, and magnetic fields are continuously monitored using Nuclear Magnetic Resonance (NMR) techniques.

Byproducts/Waste

Production generates 2–3 tons of metal waste, which is recycled whenever possible. Excess materials are salvaged or scrapped based on their condition.

The Future

Advances in sealing technologies allow newer cyclotrons to require less concrete shielding, yielding safer and more compact units. State‑of‑the‑art, four‑sector, negative‑ion cyclotrons feature external ion sources, cryopumps, high‑precision power and control systems, and modular design, enabling scalability across various sizes and applications.

Where to Learn More

Books

Lawrence, E. O., & Langmuir, I. (1942). Molecular Films: The Cyclotron & the New Biology. New Brunswick: Rutgers University Press.

Periodicals

Burgerjon, J. J., & Strathdee, A. (eds.). (1972). Cyclotrons. New York: American Institute of Physics.