Cyclotron

The invention: The first successful magnetic resonance accelerator
for protons, the cyclotron gave rise to the modern era of particle
accelerators, which are used by physicists to study the structure
of atoms.
The people behind the invention:
Ernest Orlando Lawrence (1901-1958), an American nuclear
physicist who was awarded the 1939 Nobel Prize in Physics
M. Stanley Livingston (1905-1986), an American nuclear
physicist
Niels Edlefsen (1893-1971), an American physicist
David Sloan (1905- ), an American physicist and electrical
engineer
The Beginning of an Era
The invention of the cyclotron by Ernest Orlando Lawrence
marks the beginning of the modern era of high-energy physics. Although
the energies of newer accelerators have increased steadily,
the principles incorporated in the cyclotron have been fundamental
to succeeding generations of accelerators, many of which were also
developed in Lawrence’s laboratory. The care and support for such
machines have also given rise to “big science”: the massing of scientists,
money, and machines in support of experiments to discover
the nature of the atom and its constituents.
At the University of California, Lawrence took an interest in the
new physics of the atomic nucleus, which had been developed by
the British physicist Ernest Rutherford and his followers in England,
and which was attracting more attention as the development
of quantum mechanics seemed to offer solutions to problems that
had long preoccupied physicists. In order to explore the nucleus of
the atom, however, suitable probes were required. An artificial
means of accelerating ions to high energies was also needed.
During the late 1920’s, various means of accelerating alpha particles,
protons (hydrogen ions), and electrons had been tried, but none had been successful in causing a nuclear transformation when
Lawrence entered the field. The high voltages required exceeded
the resources available to physicists. It was believed that more than
a million volts would be required to accelerate an ion to sufficient
energies to penetrate even the lightest atomic nuclei. At such voltages,
insulators broke down, releasing sparks across great distances.
European researchers even attempted to harness lightning to accomplish
the task, with fatal results.
Early in April, 1929, Lawrence discovered an article by a German
electrical engineer that described a linear accelerator of ions that
worked by passing an ion through two sets of electrodes, each of
which carried the same voltage and increased the energy of the ions
correspondingly. By spacing the electrodes appropriately and using
an alternating electrical field, this “resonance acceleration” of ions
could speed subatomic particles to many times the energy applied
in each step, overcoming the problems presented when one tried to
apply a single charge to an ion all at once. Unfortunately, the spacing
of the electrodes would have to be increased as the ions were accelerated,
since they would travel farther between each alternation
of the phase of the accelerating charge, making an accelerator impractically
long in those days of small-scale physics.
Fast-Moving Streams of Ions
Lawrence knew that a magnetic field would cause the ions to be
deflected and form a curved path. If the electrodes were placed
across the diameter of the circle formed by the ions’ path, they
should spiral out as they were accelerated, staying in phase with the
accelerating charge until they reached the periphery of the magnetic
field. This, it seemed to him, afforded a means of producing indefinitely
high voltages without using high voltages by recycling the accelerated
ions through the same electrodes. Many scientists doubted
that such a method would be effective. No mechanism was known
that would keep the circulating ions in sufficiently tight orbits to
avoid collisions with the walls of the accelerating chamber. Others
tried unsuccessfully to use resonance acceleration.
Agraduate student, M. Stanley Livingston, continued Lawrence’s
work. For his dissertation project, he used a brass cylinder 10 centimeters in diameter sealed with wax to hold a vacuum, a half-pillbox
of copper mounted on an insulated stem to serve as the electrode,
and a Hartley radio frequency oscillator producing 10 watts. The
hydrogen molecular ions were produced by a thermionic cathode (mounted near the center of the apparatus) from hydrogen gas admitted
through an aperture in the side of the cylinder after a vacuum
had been produced by a pump. Once formed, the oscillating
electrical field drew out the ions and accelerated them as they
passed through the cylinder. The accelerated ions spiraled out in a
magnetic field produced by a 10-centimeter electromagnet to a collector.
By November, 1930, Livingston had observed peaks in the
collector current as he tuned the magnetic field through the value
calculated to produce acceleration.
Borrowing a stronger magnet and tuning his radio frequency oscillator
appropriately, Livingston produced 80,000-electronvolt ions
at his collector on January 2, 1931, thus demonstrating the principle
of magnetic resonance acceleration.Impact
Demonstration of the principle led to the construction of a succession
of large cyclotrons, beginning with a 25-centimeter cyclotron
developed in the spring and summer of 1931 that produced
one-million-electronvolt protons. With the support of the Research
Corporation, Lawrence secured a large electromagnet that had been
developed for radio transmission and an unused laboratory to
house it: the Radiation Laboratory.
The 69-centimeter cyclotron built with the magnet was used to
explore nuclear physics. It accelerated deuterons, ions of heavy
water or deuterium that contain, in addition to the proton, the neutron,
which was discovered by Sir James Chadwick in 1932. The accelerated
deuteron, which injected neutrons into target atoms, was
used to produce a wide variety of artificial radioisotopes. Many of
these, such as technetium and carbon 14, were discovered with the
cyclotron and were later used in medicine.
By 1939, Lawrence had built a 152-centimeter cyclotron for medical
applications, including therapy with neutron beams. In that
year, he won the Nobel Prize in Physics for the invention of the cyclotron
and the production of radioisotopes. During World War II,
Lawrence and the members of his Radiation Laboratory developed
electromagnetic separation of uranium ions to produce the uranium
235 required for the atomic bomb. After the war, the 467-centimeter cyclotron was completed as a synchrocyclotron, which modulated
the frequency of the accelerating fields to compensate for the increasing
mass of ions as they approached the speed of light. The
principle of synchronous acceleration, invented by Lawrence’s associate,
the American physicist Edwin Mattison McMillan, became
fundamental to proton and electron synchrotrons.
The cyclotron and the Radiation Laboratory were the center of
accelerator physics throughout the 1930’s and well into the postwar
era. The invention of the cyclotron not only provided a new tool for
probing the nucleus but also gave rise to new forms of organizing
scientific work and to applications in nuclear medicine and nuclear
chemistry. Cyclotrons were built in many laboratories in the United
States, Europe, and Japan, and they became a standard tool of nuclear
physics.