Synchrocyclotron

Synchrocyclotron. A synchrocyclotron is a type of particle accelerator, a modification of the classical cyclotron invented independently by Edwin McMillan and Vladimir Veksler in 1946. It was developed to overcome the relativistic limitations of earlier cyclotrons, enabling the acceleration of protons and other particles to higher energies for research in nuclear physics and particle physics. This innovation was a critical step in the evolution of high-energy physics laboratories worldwide.

History and development

The development of the synchrocyclotron was driven by the limitations of the standard cyclotron, as described by the Lawrence Berkeley National Laboratory team led by Ernest O. Lawrence. As particles approach relativistic speeds, their increasing mass causes them to fall out of synchronization with the fixed-frequency oscillator of a conventional cyclotron, a problem known as the relativistic mass increase. In 1945, working independently, Edwin McMillan at the University of California, Berkeley and Vladimir Veksler at the Lebedev Physical Institute in the Soviet Union proposed the principle of phase stability. This breakthrough, published in the Physical Review, demonstrated that by modulating the frequency of the accelerating voltage, particles could remain in phase and be accelerated to much higher energies. The first operational synchrocyclotron was built at the University of California, Berkeley in 1946, achieving energies of up to 350 MeV for protons. This success was quickly replicated at institutions like the University of Chicago and the University of Rochester, marking a significant era in post-war nuclear physics research.

Operating principle

The operating principle of a synchrocyclotron is based on the concept of phase stability, also known as the Veksler-McMillan principle. Unlike a classical cyclotron which uses a constant radio frequency, a synchrocyclotron employs a frequency-modulated oscillator. The frequency is carefully decreased as the particles gain energy and their relativistic mass increases, ensuring they arrive at the accelerating dees at the correct phase to receive a boost. The particles are guided by a strong, constant magnetic field produced by large electromagnets, which forces them into a spiral path. This modulation compensates for the relativistic mass increase, allowing acceleration to energies where the mass of a proton may increase by several tens of percent. The beam is typically extracted after reaching the maximum radius of the machine and directed onto a target for experiments.

Design and components

The primary components of a synchrocyclotron include a large, flat cylindrical vacuum chamber placed between the poles of a powerful electromagnet. Inside the chamber are hollow, D-shaped electrodes called dees, which are connected to a radio-frequency system. A key distinguishing feature is the frequency-modulation system, often a rotating capacitor or a variable-frequency oscillator, which adjusts the accelerating voltage's frequency. An ion source, such as a duoplasmatron, injects particles like protons or deuterons into the center of the chamber. The entire assembly is housed within a massive magnet yoke, often weighing thousands of tons, to provide the required uniform magnetic field. Shielding, typically made of concrete and sometimes lead, surrounds the device to protect personnel from secondary radiation.

Applications and limitations

Synchrocyclotrons were instrumental in mid-20th century research, enabling pioneering studies in meson physics, nuclear structure, and spallation reactions. They were used to produce intense beams of pions and muons for investigating fundamental forces and were crucial at facilities like the University of Chicago and the CERN 600 MeV Synchrocyclotron, which operated from 1957 to 1990. However, their major limitation is a low average beam intensity due to the pulsed, frequency-modulated operation, which limits the data collection rate for experiments. They also have a practical energy ceiling of about 1 GeV for protons, beyond which the magnet size and cost become prohibitive. These limitations led to their eventual replacement by more efficient machines like the synchrotron.

Notable examples

Several historically significant synchrocyclotrons were constructed. The first was the 184-inch synchrocyclotron at the University of California, Berkeley, which began operation in 1946. The University of Chicago built a 450 MeV machine that contributed to early cosmic ray and particle physics research. In Europe, the 600 MeV Synchrocyclotron at CERN was a workhorse for physics from 1957 until its decommissioning, supporting experiments by scientists like Carlo Rubbia. The University of Liverpool and the Joint Institute for Nuclear Research in Dubna also operated major facilities. While most have been decommissioned, some, like the 250 MeV machine at the Paul Scherrer Institute, were later adapted for specialized applications such as proton therapy for cancer treatment.

Category:Particle accelerators Category:Nuclear physics Category:Physics experiments