Ernest Lourens Physicist Date of Birth: 08.08.1901 Country: USA

1. Biography of Ernest Lawrence
2. Contributions to Nuclear Physics
3. Later Career and Legacy

Biography of Ernest Lawrence

Ernest Orlando Lawrence, an American physicist, was born in Canton, South Dakota. He was the eldest son of Carl Gustav and Gunda (Jacobson) Lawrence. His parents immigrated to the United States from Norway. His father was a superintendent of local schools and later of the entire state's education system, as well as the president of several teacher colleges. His mother also worked in the education system. Lawrence attended schools in Canton and Pierre. In his free time, he and his best friend and neighbor, Merle Tuve, who also became a prominent physicist, built gliders and created their own wireless telegraph system. When one of his cousins died of leukemia, Lawrence decided to become a medical doctor. He received a scholarship and enrolled at St. Olaf College in Northfield, Minnesota in 1918, but transferred to the University of South Dakota after a year. There, he was drawn to advanced physics by Professor Lewis E. Akeley. After earning his Bachelor of Science degree with honors in 1922, Lawrence entered the graduate program at the University of Minnesota under the supervision of U.F. G. Swann. In graduate school, he conducted experimental research on electrical induction and earned his Master of Science degree in 1923. A year later, Lawrence transferred to the University of Chicago with his mentor Swann. There, his interest in physics grew even more after meeting notable physicists such as Niels Bohr, Arthur Compton, Albert A. Michelson, and X.A. Wilson. In the fall of 1924, Lawrence moved to Yale University and received his doctoral degree. His dissertation on the photoelectric effect in potassium vapor became the first of his significant works in this field of physics. He worked at Yale as a National Research Council fellow for the next two years and was appointed as an assistant professor of physics in 1927. However, in 1928, Lawrence left Yale University and became an adjunct professor at the University of California, Berkeley. In California, Lawrence continued his research in areas such as photoelectricity and the measurement of very short time intervals. Among his other achievements during this time was the experimental demonstration of Werner Heisenberg's uncertainty principle. This principle predicts that the measurement of energy, for example, of a photon of light (which represents a portion, or particle, of electromagnetic energy), becomes more uncertain the shorter the measurement time. Since the energy of a photon is proportional to the frequency of the light, the uncertainty in energy translates to an uncertainty in frequency. In reality, an optical spectrum line represents a narrow (i.e., well-defined) range of light frequencies. By rapidly turning on and off the light during the measurement of a spectral line, Lawrence and his colleague showed that the line broadens. The light source did not undergo any changes, yet its frequency became less well-defined, as predicted by Heisenberg's uncertainty principle.

Contributions to Nuclear Physics

Lawrence then turned his attention to nuclear physics, which was rapidly developing at the time. In 1919, Ernest Rutherford split the atomic nucleus by bombarding it with alpha particles emitted by radium. Rutherford discovered that among the fragments produced after the collisions, there were atoms with lower atomic weight than the original. Some of these fragments were isotopes of known elements, meaning they had the same chemical properties and the same nuclear charge but different weights. Rutherford's methods had serious limitations: radium was a rare element, alpha particles flew out of the source in all directions, the number of observed collisions was extremely small, and the entire observation procedure was laborious. Nuclear physics was in dire need of an abundant source of controlled high-energy particles. Since both the bombarding particles and the target nuclei were positively charged (electrons played a minor role in the collisions), the bombarding particles had to have enough energy to overcome not only the electrical repulsion but also the binding energy that holds the nucleus together. John Cockcroft and Ernest Walton built linear particle accelerators that worked at very high voltages. In these devices, positively charged particles were accelerated in a straight line towards a negatively charged electrode and gained energy proportional to the applied voltage.

Lawrence was not fond of linear accelerators because insulation breakdowns occasionally occurred, resulting in a high-voltage discharge resembling lightning. In 1929, Lawrence came across a German article by Rolf Widerøe, a Norwegian engineer, proposing a particle accelerator scheme previously suggested by the Swedish physicist Gustav A. Ising. Although Lawrence did not have enough command of the German language to understand all the nuances, he grasped the main idea from the illustrations in the article: particles could be accelerated by gradually increasing the voltage instead of creating one big "bump." Lawrence realized that a straight path could be bent into a circle. After performing the necessary calculations, he and a few colleagues began designing and building the first cyclotron. It is with this invention that Lawrence's name is usually associated. The main idea behind Lawrence's cyclotron was that charged particles move in constant magnetic fields along circular paths. This happens because a moving charge represents an electric current, which, like the current in the coils of an electromagnet, creates a magnetic field. Similar to two magnets held close together, the particle and the external magnet exert certain forces on each other, but only the particle can move (in the case of two approaching magnets, one is fixed while the other can move). The direction of the force always forms right angles with the direction of the magnetic field and the direction of particle motion. Since the direction of the particle constantly changes, it moves in a circle. An important feature of particle motion is that it always completes a full circle in the same amount of time regardless of the particle's speed (kinetic energy). However, the diameter of the circle is larger the faster the particle's speed. Lawrence used these characteristics of particle motion when designing his cyclotron.

At the heart of the cyclotron is a huge circular hollow disk divided in half along its diameter, resembling the letter "D" (these halves are called dees). The disk is placed between the flat poles of a large magnet. An electric generator is connected between the dees, creating an alternating voltage in the gap between them. When a charged particle, such as a proton, enters the gap, it is attracted to the dee that currently has a negative voltage, gaining speed. Once inside the dee, the particle follows a semicircular path and exits at the point diametrically opposite the entrance. By the time the proton reaches this point, the voltage has changed sign, and the proton is then attracted to the other dee, which has become negative, accelerating it with the voltage applied to the gap. The proton enters the second dee with a higher speed and therefore moves inside it along a circular arc of a larger radius than before. By the time it exits the second dee, the voltage changes sign again, and the proton is accelerated again, entering the first dee with an even higher speed and moving inside it along a circular arc of an even larger radius. This way, the proton receives a "push" every time it passes through the gap between the dees and moves with an increasingly higher speed along circular arcs of larger and larger radii until it reaches the perimeter of the disk. The proton then exits the cyclotron and is directed toward the chosen target. Large-diameter disks allow particles to be accelerated to high speeds but require larger and therefore more expensive magnets. The dees must be made of non-magnetic material that does not shield the magnetic field, and to prevent particles from losing energy due to collisions with gas molecules, the chamber must have a deep vacuum.

After building his first, rather imperfect cyclotron in 1930, Lawrence and his colleagues at Berkeley quickly created larger models one after another. Using an 80-ton magnet provided by the Federal Telegraph Company, Lawrence accelerated particles to record-breaking energies in the millions of electron volts. Cyclotrons proved to be ideal experimental devices. Unlike particles emitted by atomic nuclei during radioactive decay, the particle beam produced by the cyclotron was unidirectional, the energy could be controlled, and the intensity of the beam was far greater than from any radioactive source. The high energies achieved by Lawrence and his colleagues opened up a vast new field of research for physicists. Bombarding atoms of many elements allowed their nuclei to be split into fragments, some of which were isotopes, often radioactive. Sometimes, accelerated particles "stuck" to target nuclei or induced nuclear reactions, producing new elements that did not exist naturally on Earth. The results obtained showed that with sufficiently high particle energies, almost any nuclear reaction could be achieved using a cyclotron. Cyclotrons were also used to measure the binding energies of many nuclei and to verify Albert Einstein's mass-energy equivalence relationship by comparing mass differences before and after nuclear reactions.

Cyclotrons allowed for the production of radioactive isotopes for medical purposes. Lawrence worked on biomedical applications of nuclear physics with his younger brother, John, a medical doctor and director of the Radiation Biology Laboratory at Berkeley. John Lawrence successfully used isotopes to treat cancer patients, including their own mother, who had an inoperable case of cancer. After the course of treatment, she lived another 20 years.

Lawrence was awarded the Nobel Prize in Physics in 1939 "for the invention and development of the cyclotron and for results obtained with it, especially with regard to artificial radioactive isotopes." The award ceremony was canceled due to the outbreak of World War II. Regarding Lawrence's work, Manne Siegbahn of the Royal Swedish Academy of Sciences stated that the invention of the cyclotron caused "an explosion in the development of nuclear research... In the history of experimental physics... the cyclotron occupies an exceptional place. Without a doubt, the cyclotron is the largest and most complex scientific instrument ever built." Lawrence received the Nobel Prize in 1941 during a ceremony held in Berkeley. He delivered his Nobel lecture in Stockholm in 1951.

Later Career and Legacy

In 1940, Lawrence participated in the establishment of the Radiation Laboratory at the Massachusetts Institute of Technology. Many of his former students joined the laboratory at his urging. Its goal was to improve radar technology, which was first developed in England during World War II for the electronic detection of enemy aircraft. In 1941, Lawrence recruited staff for the Underwater Acoustics Laboratory in San Diego, which focused on developing anti-submarine systems to combat German submarines that threatened convoys transporting military supplies from the United States to Great Britain. Lawrence then, maintaining only informal links with these laboratories, focused on transforming a 37-inch cyclotron in Berkeley into a mass spectrometer for separating split uranium-235 and natural uranium-238. In a mass spectrometer, like in a cyclotron, a combination of electric and magnetic fields is used, but not for particle acceleration, but for spatial separation based on mass and electric charges. Since the masses of isotopes differ slightly, the isotopes move along similar, although not overlapping, trajectories and can be separated, although the efficiency of separation is not very high.

The success achieved by Lawrence was impressive enough for all work on isotope separation to be entrusted to his laboratory. In Oak Ridge, Tennessee, as part of the Manhattan Project (a top-secret plan to develop an American atomic bomb), hundreds of mass spectrometers were built based on the Berkeley cyclotron with a 184-inch magnet. Almost all the uranium in the bomb dropped on Hiroshima in August 1945 was produced by Lawrence and his team in Berkeley. Later, the Oak Ridge isotope separation plant using mass spectrometers was shut down because the gas diffusion method proved to be more efficient.

At the end of the war, Lawrence and his team returned to fundamental research. However, Lawrence continued to be involved in the development of nuclear weapons. He was allocated funds to establish the second scientific research laboratory for the needs of the military industry in Livermore, not far from Berkeley. The laboratory, later named Lawrence Livermore National Laboratory, became the main center for research on the hydrogen bomb.

In Berkeley, Lawrence led the construction of accelerators capable of accelerating particles to energies in the billions of electron volts. On one such accelerator, called the Bevatron, Emilio Segrè and other researchers discovered antiprotons (the antiparticles of protons with a negative charge) while studying mesons (elementary particles with masses intermediate between the masses of electrons and protons).

Lawrence was invited by President Dwight D. Eisenhower to serve as a government consultant to study the feasibility of determining violations of the agreement on the prohibition of nuclear weapons testing, which was under consideration at the 1958 Geneva Conference. Upon his return home, Lawrence underwent surgery for a peptic ulcer aggravation and died at the Palo Alto Hospital in California on August 27, 1958.

In 1932, Lawrence married Mary Kimberly Blumer, the daughter of the dean of the Yale University School of Medicine. The Lawrences had six children.

In addition to his numerous contributions to nuclear physics, Lawrence invented an original design for a television tube called the Lawrence Chromatron, which was produced on an industrial scale in Japan and the United States. Despite spending long hours at work on weekdays and weekends, Lawrence also enjoyed rowing, playing tennis, ice skating, and listening to music. "In his remarkable success, a natural inclination and sound scientific judgment, an enormous reserve of vitality, an exuberant enthusiasm, an extraordinary personality, and a dominating sense of wholeness were all contributing factors," according to Luis W. Alvarez.

Among the numerous awards and honors bestowed upon Lawrence were the Elliott Cresson Medal from the Franklin Institute (1937), the Hughes Medal from the Royal Society of London (1940), and the Holley Medal from the American Society of Mechanical Engineers (1942). He was posthumously awarded the Enrico Fermi Award in 1959. Lawrence's contributions to science and his impact on the field of nuclear physics continue to be recognized and celebrated to this day.