Cyclotron Info

What Cyclotrons Are Used For

The energy frontier moved on to synchrotrons decades ago, but more cyclotrons are running today than at any time in history — most of them in hospitals. Here is what the machines actually do, with sources for the specific numbers.

Medical isotope production

The single biggest cyclotron application is manufacturing short-lived radioactive isotopes for diagnostic imaging. A compact 10–20 MeV proton machine in a hospital basement bombards a small target for an hour or two and produces the day's supply.

PET (positron emission tomography) runs almost entirely on cyclotron output. Its workhorse tracer, fluorine-18 (half-life 109.8 minutes), is made by the 18O(p,n)18F reaction on enriched water, typically with 9–19 MeV protons — the energy class the IAEA's guidance on medical cyclotrons is written around. Because two hours of half-life doesn't survive long-distance shipping well, PET forced a distributed model: well over a thousand small medical cyclotrons now operate worldwide (IAEA cyclotron database), each serving nearby scanners. The other classic PET nuclides — carbon-11 (20.4 min), nitrogen-13 (10 min), oxygen-15 (2 min) — decay so fast the cyclotron must be effectively next door to the patient.

SPECT imaging uses longer-lived cyclotron products — thallium-201, gallium-67, indium-111, iodine-123 — made in regional commercial facilities at somewhat higher energies (roughly 20–30 MeV).

The Tc-99m story shows what's at stake. Technetium-99m (half-life 6.0 h) is the most-used medical isotope on Earth — on the order of 30 million procedures a year, roughly 80% of all nuclear-medicine imaging (National Academies, Molybdenum-99 for Medical Imaging, 2016). It has traditionally come not from cyclotrons but from a handful of aging research reactors, which irradiate uranium to make molybdenum-99 (66 h) — shipped worldwide in "moly generators" that hospitals milk for Tc-99m. When two of those reactors went down together in 2009–2010, clinics worldwide postponed scans for months. One response: direct cyclotron production via 100Mo(p,2n)99mTc on ~16–24 MeV machines, demonstrated at scale in Canada and since approved for clinical use there — a rare case of cyclotrons substituting for nuclear reactors.

Proton and particle therapy

A proton slowing in tissue deposits most of its energy in a sharp spike at the end of its range — the Bragg peak — then stops. Tune the energy so the peak lands in the tumor and the tissue behind it receives essentially nothing, while the tissue in front receives far less than an X-ray beam would deliver. Robert Wilson pointed out the medical potential in 1946 (Radiology 47, 487); the first patients were treated at Berkeley's 184-inch in 1954; the first hospital-based center opened at Loma Linda in 1990. Treating a deep tumor takes roughly 200–250 MeV protons, and cyclotrons — isochronous and, lately, compact superconducting synchrocyclotrons small enough for a rotating gantry — power a large share of the more than one hundred particle-therapy centers now operating worldwide (PTCOG facility registry).

PIXE: analyzing anything without destroying it

Fire a few-MeV proton beam at a sample and every element in it emits characteristic X-rays — particle-induced X-ray emission, introduced by Johansson and colleagues in 1970. PIXE detects trace elements down to parts-per-million from micrograms of material, without dissolving or damaging the sample, which makes it beloved of art historians and archaeologists: the AGLAE accelerator under the Louvre has spent decades quietly analyzing pigments, alloys, and gemstones. Small cyclotrons and electrostatic machines in the 2–4 MeV range both do this work. It is also, incidentally, the class of experiment most within reach of an ambitious amateur machine.

Ion implantation and radiation testing

Routine semiconductor doping uses electrostatic implanters at keV energies, not cyclotrons — but when MeV-class ions are needed, cyclotrons take over: deep implantation, and above all radiation-effects testing, where heavy-ion beams from machines like the Texas A&M K500 simulate years of cosmic-ray strikes on candidate spacecraft and satellite electronics in an afternoon. Wafer-scale neutron transmutation and isotope tracing for materials science live in the same niche.

Physics and chemistry research

Cyclotrons discovered technetium, astatine, neptunium, plutonium, and most of the early transuranics, and they never left the lab bench. Today they serve as drivers for rare isotope science — RIKEN's and (until its LINAC successor) Michigan State's beams fragmenting heavy nuclei to make isotopes that exist nowhere on Earth — as intense neutron and muon sources (the PSI and TRIUMF "meson factories" run muon-based materials science daily), and as general-purpose nuclear-structure and cross-section workhorses at dozens of universities. Much of the nuclear data underpinning reactor design, medicine, and astrophysics comes from cyclotron measurements.

Accelerator mass spectrometry

AMS counts individual atoms of a rare isotope — carbon-14 at one part in 1012 — by accelerating them and separating them by mass and charge, letting radiocarbon dating work on milligram samples instead of grams. Richard Muller proposed doing this with cyclotrons in 1977 (Science 196, 489), and Berkeley built a small "cyclotrino" to try it; in practice the field standardized on tandem electrostatic accelerators, whose stripping stage kills the pernicious molecular backgrounds. Cyclotron AMS remains a periodically revisited idea for compact instruments, but an honest summary is: AMS is an accelerator application, and only marginally a cyclotron one.

Sources