Synchrocyclotron

The invention: 



A powerful particle accelerator that performed

better than its predecessor, the cyclotron.



The people behind the invention:



Edwin Mattison McMillan (1907-1991), an American physicist

who won the Nobel Prize in Chemistry in 1951

Vladimir Iosifovich Veksler (1907-1966), a Soviet physicist

Ernest Orlando Lawrence (1901-1958), an American physicist

Hans Albrecht Bethe (1906- ), a German American physicist









The First Cyclotron



The synchrocyclotron is a large electromagnetic apparatus designed

to accelerate atomic and subatomic particles at high energies.

Therefore, it falls under the broad class of scientific devices

known as “particle accelerators.” By the early 1920’s, the experimental

work of physicists such as Ernest Rutherford and George

Gamow demanded that an artificial means be developed to generate

streams of atomic and subatomic particles at energies much

greater than those occurring naturally. This requirement led Ernest

Orlando Lawrence to develop the cyclotron, the prototype for most

modern accelerators. The synchrocyclotron was developed in response

to the limitations of the early cyclotron.

In September, 1930, Lawrence announced the basic principles behind

the cyclotron. Ionized—that is, electrically charged—particles

are admitted into the central section of a circular metal drum. Once

inside the drum, the particles are exposed to an electric field alternating

within a constant magnetic field. The combined action of the

electric and magnetic fields accelerates the particles into a circular

path, or orbit. This increases the particles’ energy and orbital radii.

This process continues until the particles reach the desired energy

and velocity and are extracted from the machine for use in experiments

ranging from particle-to-particle collisions to the synthesis of

radioactive elements.

Although Lawrence was interested in the practical applications

of his invention in medicine and biology, the cyclotron also was applied

to a variety of experiments in a subfield of physics called

“high-energy physics.” Among the earliest applications were studies

of the subatomic, or nuclear, structure of matter. The energetic

particles generated by the cyclotron made possible the very type of

experiment that Rutherford and Gamow had attempted earlier.

These experiments, which bombarded lithium targets with streams

of highly energetic accelerated protons, attempted to probe the inner

structure of matter.

Although funding for scientific research on a large scale was

scarce beforeWorldWar II (1939-1945), Lawrence nevertheless conceived

of a 467-centimeter cyclotron that would generate particles

with energies approaching 100 million electronvolts. By the end of

the war, increases in the public and private funding of scientific research

and a demand for higher-energy particles created a situation

in which this plan looked as if it would become reality, were it not

for an inherent limit in the physics of cyclotron operation.





Overcoming the Problem of Mass





In 1937, Hans Albrecht Bethe discovered a severe theoretical limitation

to the energies that could be produced in a cyclotron. Physicist

Albert Einstein’s special theory of relativity had demonstrated

that as any mass particle gains velocity relative to the speed of light,

its mass increases. Bethe showed that this increase in mass would

eventually slow the rotation of each particle. Therefore, as the rotation

of each particle slows and the frequency of the alternating electric

field remains constant, particle velocity will decrease eventually.

This factor set an upper limit on the energies that any cyclotron

could produce.

Edwin Mattison McMillan, a colleague of Lawrence at Berkeley,

proposed a solution to Bethe’s problem in 1945. Simultaneously and

independently, Vladimir Iosifovich Veksler of the Soviet Union proposed

the same solution. They suggested that the frequency of the

alternating electric field be slowed to meet the decreasing rotational

frequencies of the accelerating particles—in essence, “synchroniz-

ing” the electric field with the moving particles. The result was the

synchrocyclotron.

Prior toWorldWar II, Lawrence and his colleagues had obtained

the massive electromagnet for the new 100-million-electronvolt cyclotron.

This 467-centimeter magnet would become the heart of the

new Berkeley synchrocyclotron. After initial tests proved successful,

the Berkeley team decided that it would be reasonable to convert

the cyclotron magnet for use in a new synchrocyclotron. The

apparatus was operational in November of 1946.

These high energies combined with economic factors to make the

synchrocyclotron a major achievement for the Berkeley Radiation

Laboratory. The synchrocyclotron required less voltage to produce

higher energies than the cyclotron because the obstacles cited by

Bethe were virtually nonexistent. In essence, the energies produced

by synchrocyclotrons are limited only by the economics of building

them. These factors led to the planning and construction of other

synchrocyclotrons in the United States and Europe. In 1957, the

Berkeley apparatus was redesigned in order to achieve energies of

720 million electronvolts, at that time the record for cyclotrons of

any kind.





Impact



Previously, scientists had had to rely on natural sources for highly

energetic subatomic and atomic particles with which to experiment.

In the mid-1920’s, the American physicist Robert Andrews Millikan

began his experimental work in cosmic rays, which are one natural

source of energetic particles called “mesons.” Mesons are charged

particles that have a mass more than two hundred times that of the

electron and are therefore of great benefit in high-energy physics experiments.

In February of 1949, McMillan announced the first synthetically

produced mesons using the synchrocyclotron.

McMillan’s theoretical development led not only to the development

of the synchrocyclotron but also to the development of the

electron synchrotron, the proton synchrotron, the microtron, and

the linear accelerator. Both proton and electron synchrotrons have

been used successfully to produce precise beams of muons and pimesons,

or pions (a type of meson).

The increased use of accelerator apparatus ushered in a new era

of physics research, which has become dominated increasingly by

large accelerators and, subsequently, larger teams of scientists and

engineers required to run individual experiments. More sophisticated

machines have generated energies in excess of 2 trillion

electronvolts at the United States’ Fermi National Accelerator Laboratory,

or Fermilab, in Illinois. Part of the huge Tevatron apparatus

at Fermilab, which generates these particles, is a proton synchrotron,

a direct descendant of McMillan and Lawrence’s early

efforts.



See also: Atomic bomb; Cyclotron; Electron microscope;

Field ionmicroscope; Geiger counter; Hydrogen bomb;

Mass spectrograph;Neutrino detector; Scanning tunneling microscope;

Synchrocyclotron



Further Reading :