CYCLOTRON

CYCLOTRON, a machine for accelerating charged nuclear particles, commonly protons, so that they may be used to probe the nuclei of target atoms. Such "atom smashers" are considered the microscopes of nuclear physics.

In the nineteenth century, some physicists still labored under the theory—really, the dream of alchemists for centuries—that elements could be made to transmute into other elements through chemical processes. In 1902, Ernest Rutherford and Frederick Soddy explained the new phenomenon of radioactivity as a "transformation" of one element into another, occurring spontaneously in nature; and in 1919, Rutherford succeeded in deliberately causing transmutations by bombarding light elements with the alpha particles emitted from naturally decaying radio-elements. Since very few of the projectile alpha particles collided with nuclei of the target atoms, the number of

transmutations was relatively small. Therefore, scientists sought new ways to increase the number of projectile particles and to accelerate them to higher energies. The copious production of charged particles was the easier task; the high-voltage engineering required for acceleration proved far more difficult.

Scientists tried a number of different approaches to the acceleration problem, including a voltage multiplier circuit (Sir John Douglas Cockcroft and Ernest Walton) and an electrostatic generator (Robert J. Van de Graaff), both linear accelerators. In 1930, University of California at Berkeley physicist Ernest O. Lawrence, with the help of one of his students, M. Stanley Livingston, designed and constructed the first of many magnetic resonance accelerators. Lawrence's accelerator operated at voltages much lower than other machines, yet imparted as much or more energy to its projectiles. Lawrence won the 1939 Nobel Prize for Physics for his work on the cyclotron. During WORLD WAR II he headed a unit of the MANHATTAN PROJECT that worked to perfect the process of separating uranium-235 for the atomic bomb.

These cyclotrons, destined to be the chief tool of nuclear physics, worked on the principle that charged particles, accelerated across a voltage gap, travel in a circular path under the influence of a magnetic field. If confined to a hollow disk-shaped chamber built in two D-shaped halves (called "D's") and if subjected to a radio-frequency voltage alternation as the particle passes from one half to the other, the particle receives two accelerations per cycle and travels at higher velocities in ever-larger circles. The beam of rapidly moving particles may then be deflected onto a target, producing observable nuclear reactions.

The D's of Lawrence's first cyclotron were only about 4 inches in diameter. Subsequent models of 9, 11, 27, 37, and 60 inches followed, with a new model built almost every other year. These larger machines surpassed an early goal of one million electron volts projectile energy; many different types of atoms were split; and scores of new radioisotopes were identified, including the first trans-uranium elements.

Higher energies, suitable for the production of mesons, were impossible with the fixed-frequency cyclotrons, because the projectiles would experience a relativistic mass increase at the required velocities, destroying the resonant operating condition. After World War II scientists overcame this handicap with a new generation of accelerators that use a variable-frequency voltage alternation that exactly balances the mass-velocity change. The synchrocyclotron was the largest machine to use a single magnet.

This postwar synchrocyclotron became the foundation for a government-funded national accelerator. Work on a four-mile-long circular machine in Weston, Illinois, thirty miles west of Chicago, was completed in 1971. Project leader Robert O. Wilson envisioned a series of magnets to boost particle speeds, and he insisted on allowing for space in the tunnel of the main ring for the addition of a second magnet system. When the main ring was about to operate in 1971 he described his idea of a "doubler" that would take the protons from the magnetic ring and inject them into a new ring of super-conducting magnets and double their energy. Physicists working at the laboratory, which in 1974 was named the Fermi National Laboratory for physicist Enrico Fermi, solved the technical problems of building the doubler. The principal Fermilab accelerator subsequently became known as the Tevatron (one TeV is a trillion electron volts). In 1994 the Tevatron revealed the existence of the so-called top quark, the last of twelve subatomic building blocks of all matter.

CYCLOTRON

BIBLIOGRAPHY