Synchrocyclotron
A synchrocyclotron is a type of particle accelerator that emerged in the mid-20th century, designed to overcome limitations faced by its predecessor, the cyclotron. Developed by Edwin McMillan and Vladimir Veksler, the synchrocyclotron addresses the issue of particle mass increasing as they approach the speed of light, which can cause them to lose synchronization with the accelerating voltage. By adjusting the frequency of this voltage, the synchrocyclotron allows particles to continue gaining energy, achieving energies up to 200 million electronvolts for deuterons and 400 million electronvolts for alpha particles.
Operational from November 1946, the synchrocyclotron played a crucial role in advancing nuclear physics and medical applications. It enabled significant discoveries within atomic nuclei by producing intense beams of pions and muons, facilitating the study of their properties. The synchrocyclotron also became a vital tool in medical proton therapy, effectively targeting tumors with minimal damage to surrounding tissues. Notably, it contributed to the development of nuclear medicine, treating conditions such as acromegaly and Cushing's disease. Despite its advantages, the synchrocyclotron's heavy and costly magnets led to the evolution of lighter and more efficient circular accelerators known as synchrotrons.
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Synchrocyclotron
Form of particle accelerator with a higher maximum energy than a standard cyclotron, created by changing the frequency of the driving electric field
The maximum energy that could be obtained with a cyclotron (a type of particle accelerator) was limited because, as the particles approached the speed of light while rotating in a constant magnetic field, they also gained mass and slowed, in accordance with Alfred Einstein’s theory of relativity. A synchrocyclotron has only one accelerating electrode rather than the two in a standard cyclotron, and the drive frequency is modulated to keep the pulses in phase with the accelerating voltage.
Ernest Orlando Lawrence won the Nobel Prize in Physics for his work on the cyclotron and built a series of the machines, but they were unable to reach energies of 100 million electronvolts because of the relativistic increase of mass of the particles as they approached the speed of light. This increase in mass caused them to get out of phase with the voltage that drove the acceleration, falling behind the peak of the voltage and therefore not gaining any more energy as they circled through the accelerator. Edwin McMillan at the University of California and Berkeley, and Vladimir Veksler in Russia explained this phenomenon and suggested that if the frequency of the accelerating voltage was lowered as the particles were slowed, they would remain in phase and the particles would continue to gain energy. It was possible to obtain nearly 200 million electronvolt deuterons and 400 million electronvolt alpha particles with the first synchrocyclotron, which became operational in November of 1946.
The main disadvantage of the synchrocyclotrons was the huge weight and cost of their magnets. This led to the next innovation in particle accelerators, keeping the particles focused in a single orbit in a circular magnet, eliminating the need for the heavy central part of the magnet. All later circular accelerators were synchrotrons of this sort.
Impact
The synchrocyclotron was the highest energy particle accelerator of its time. It made possible the beginning studies of the interior of the nucleus. Mesons had been discovered in cosmic rays, but the intensities available were too small to determine their properties. The synchrocyclotron made it possible for the first time to obtain beams of pions and muons and to study their properties and interaction with atomic nuclei.
The synchrocyclotron quickly became the dominant accelerator for use in medical proton therapy. The protons were energetic enough to penetrate within the body to a tumor site, and at the end of their range they became very highly ionizing, destroying tumor cells while having only minor effects on intermediate cells.
The new field of nuclear medicine was initiated by Lawrence’s 184-inch synchrocyclotron at the University of California at Berkeley. The new accelerator successfully treated acromegaly and Cushing’s disease.
Bibliography
Heilbron, J. L., and Robert W. Seidel. Lawrence and His Laboratory: The History of the Lawrence Berkeley Laboratory. Berkeley: University of California Press, 1989.
Livingston, M. Stanley. Particle Accelerators: A Brief History. Cambridge, Mass.: Harvard University Press, 1969.
Sessler, Andrew, and Edmund Wilson. Engines of Discovery. Hackensack, N.J.: World Scientific, 2007.