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 :