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Jai Ganesh

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Cyclotron

Cyclotron

Gist

A cyclotron is a compact particle accelerator that uses a constant magnetic field to bend charged particles into a spiral path and a rapidly varying electric field to accelerate them to high speeds. Invented by Ernest Lawrence in 1929–1930, it is primarily used in medicine to produce short-lived radioisotopes for cancer diagnosis (PET scans) and treatment. (PET : Positron Emission Tomography).

Cyclotrons are particle accelerators used in medicine (producing radioisotopes for imaging/therapy, cancer treatment via proton beams) and nuclear physics research (bombarding nuclei for experiments, studying atomic properties, creating new elements/isotopes). Their compact size makes them practical for generating high-energy particle beams for these scientific and medical applications, offering advantages over linear accelerators in certain scenarios.

Summary

A cyclotron is a particle accelerator that uses magnetic and electric fields to speed up charged particles to very high speeds and powers many of the tools, treatments, and discoveries that improve our daily lives.

If you have ever had a PET scan at a hospital or heard about radiation treatment for cancer or brain tumours, there's a good chance a cyclotron was involved.

But what is a cyclotron and how is it used?

Let’s break it down.

A cyclotron is a type of particle accelerator. It uses magnetic and electric fields to speed up charged particles like protons or ions to very high speeds. This allows the particles to collide with target materials to produce radioisotopes through nuclear reactions.

Radioisotopes have several uses, including in life saving medical treatments, scientific research, and even clean energy technologies.

The cyclotron was invented in 1931 by American physicist Ernest O. Lawrence and his student M. Stanley Livingston at the University of California, Berkeley. Their early prototype - just about 10 cm wide - was already capable of accelerating particles to high energy levels. Lawrence’s groundbreaking work earned him the Nobel Prize in Physics in 1939.

How does a Cyclotron Work?

The process begins when charged particles like positive or negative ions are injected into the centre of the cyclotron, where they start to move outward in a spiral path.

Inside the cyclotron, are two hollow, D-shaped metal electrodes called ‘dees’, placed between the poles of a large magnet. The magnetic field forces the particles into a circular path, while an alternating electric field boosts the particle’s energy every time it crosses the gap between two dees. As the particles gain speed and energy, they continue to spiral outward.

Once the particles reach the outer edge of the cyclotron, they are directed toward a target. When the accelerated particles collide with the target, they can cause nuclear reactions, producing radioactive isotopes.

Nearly a century after their invention, cyclotrons remain in high demand because of their reliability, efficiency, and versatility.

What’s the Difference Between Cyclotrons and other Particle Accelerators?

Particle accelerators have many applications in medicine, industry and research. These machines accelerate charged particles, such as electrons and protons, to high speeds, sometimes even close to the speed of light.

While all particle accelerators share a common goal - boosting the energy of particles - they achieve this in different ways.

Cyclotrons accelerate particles in a spiral path using a constant magnetic field and an alternating electric field. The spiral design is one of the cyclotron’s main advantages. It allows for continuous acceleration in a relatively small space. As a result, cyclotrons are typically smaller, often room-sized, and more affordable than other accelerators. They can be installed in hospitals or university labs without needing massive facilities. Cyclotrons are also well-suited for producing specific types of radioactive isotopes needed in medical imaging and cancer treatment, and for other localized applications in research or industry.

In contrast, linear accelerators, or linacs, propel particles in a straight line using a series of electric fields. While linacs can be simpler in design, they often require much more space to achieve the same energy levels as a cyclotron. They are commonly used in radiotherapy, where precise targeted beams of radiation are used to treat tumours.

Another type of accelerator is the synchrotron - a much larger and more complex machine found in national research centres. Like cyclotrons, they guide particles in a circular path, but with variable magnetic fields and radiofrequency acceleration. These machines can reach extremely high energies, making them suitable for research in particle physics, materials science, and even drug development. However, due to their size and cost, they are typically used by national or international research centres, not hospitals or small labs.

Details

A cyclotron is a type of particle accelerator invented by Ernest Lawrence in 1929–1930 at the University of California, Berkeley, and patented in 1932. A cyclotron accelerates charged particles outwards from the center of a flat cylindrical vacuum chamber along a spiral path. The particles are held to a spiral trajectory by a static magnetic field and accelerated by a rapidly varying electric field. Lawrence was awarded the 1939 Nobel Prize in Physics for this invention.

The cyclotron was the first "cyclical" accelerator. The primary accelerators before the development of the cyclotron were electrostatic accelerators, such as the math–Walton generator and the Van de Graaff generator. In these accelerators, particles would cross an accelerating electric field only once. Thus, the energy gained by the particles was limited by the maximum electrical potential that could be achieved across the accelerating region. This potential was in turn limited by electrostatic breakdown to a few million volts. In a cyclotron, by contrast, the particles encounter the accelerating region many times by following a spiral path, so the output energy can be many times the energy gained in a single accelerating step.

Cyclotrons were the most powerful particle accelerator technology until the 1950s, when they were surpassed by the synchrotron. Nonetheless, they are still widely used to produce particle beams for nuclear medicine and basic research. As of 2020, close to 1,500 cyclotrons were in use worldwide for the production of radionuclides for nuclear medicine and ultimately, for the production of radiopharmaceuticals. In addition, cyclotrons can be used for particle therapy, where particle beams are directly applied to patients.

Usage:

Basic research

For several decades, cyclotrons were the best source of high-energy beams for nuclear physics experiments. With the advent of strong focusing synchrotrons, cyclotrons were supplanted as the accelerators capable of producing the highest energies. However, due to their compactness, and therefore lower expense compared to high-energy synchrotrons, cyclotrons are still used to create beams for research where the primary consideration is not achieving the maximum possible energy. Cyclotron-based nuclear physics experiments are used to measure basic properties of isotopes (particularly short lived radioactive isotopes) including half-life, mass, interaction cross sections, and decay schemes.

Medical uses:

Radioisotope production

Cyclotron beams can be used to bombard other atoms to produce short-lived isotopes with a variety of medical uses, including medical imaging and radiotherapy. Positron and gamma emitting isotopes, such as fluorine-18, carbon-11, and technetium-99m are used for PET and SPECT imaging. While cyclotron produced radioisotopes are widely used for diagnostic purposes, therapeutic uses are still largely in development. Proposed isotopes include astatine-211, palladium-103, rhenium-186, and bromine-77, among others.

Beam therapy

The first suggestion that energetic protons could be an effective treatment method was made by Robert R. Wilson in a paper published in 1946 while he was involved in the design of the Harvard Cyclotron Laboratory.

Beams from cyclotrons can be used in particle therapy to treat cancer. Ion beams from cyclotrons can be used, as in proton therapy, to penetrate the body and kill tumors by radiation damage, while minimizing damage to healthy tissue along their path.

As of 2020, there were approximately 80 facilities worldwide for radiotherapy using beams of protons and heavy ions, consisting of a mixture of cyclotrons and synchrotrons. Cyclotrons are primarily used for proton beams, while synchrotrons are used to produce heavier ions.

PET : Positron Emission Tomography.
SPECT : Single Photon Emission Computed Tomography.

Advantages and limitations

Livingston and Lawrence with the 69 cm (27 in) cyclotron at the Lawrence Radiation Laboratory. The metal arch supports the magnet's core, and the large cylindrical boxes contain the coils of wire that generate the magnetic field. The vacuum chamber containing the "dee" electrodes is in the center between the magnet's poles.
The most obvious advantage of a cyclotron over a linear accelerator is that because the same accelerating gap is used many times, it is both more space efficient and more cost efficient; particles can be brought to higher energies in less space, and with less equipment. The compactness of the cyclotron reduces other costs as well, such as foundations, radiation shielding, and the enclosing building. Cyclotrons have a single electrical driver, which saves both equipment and power costs. Furthermore, cyclotrons are able to produce a continuous beam of particles at the target, so the average power passed from a particle beam into a target is relatively high compared to the pulsed beam of a synchrotron.

However, as discussed above, a constant frequency acceleration method is only possible when the accelerated particles are approximately obeying Newton's laws of motion. If the particles become fast enough that relativistic effects become important, the beam slips out of phase with the oscillating electric field, and cannot receive any additional acceleration. The classical cyclotron (constant field and frequency) is therefore only capable of accelerating particles up to a few percent of the speed of light. Synchro-, isochronous, and other types of cyclotrons can overcome this limitation, with the tradeoff of increased complexity and cost.

An additional limitation of cyclotrons is due to space charge effects – the mutual repulsion of the particles in the beam. As the amount of particles (beam current) in a cyclotron beam is increased, the effects of electrostatic repulsion grow stronger until they disrupt the orbits of neighboring particles. This puts a functional limit on the beam intensity, or the number of particles which can be accelerated at one time, as distinct from their energy.

Additional Information

The cyclotron was one of the earliest types of particle accelerators, and is still used as the first stage of some large multi-stage particle accelerators. It makes use of the magnetic force on a moving charge to bend moving charges into a semicircular path between accelerations by an applied electric field. The applied electric field accelerates electrons between the "dees" of the magnetic field region. The field is reversed at the cyclotron frequency to accelerate the electrons back across the gap.

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