LAURITSEN, CHARLES CHRISTIAN

(b. Holstebro, Denmark, 4 April 1892; d. Los Angeles, California, 13 April 1968),

physics, physical and therapeutic properties of high-energy x-rays, nuclear physics, astrophysics, weapons systems.

Lauritsen was an experimental nuclear physicist of remarkable ability and vision. From 1926 until his death he was at the California Institute of Technology (Caltech), first as a graduate student and then as a faculty member. His groundbreaking PhD research on cold field-emission enabled him to design and build x-ray tubes that achieved record voltages. He used these tubes to study properties of x-rays and their physiological effects. Then, in response to British developments in 1932, he converted one of his xray tubes into an accelerator of ions that he then used to probe the interiors of atomic nuclei. He thus became one of America’s earliest pioneers of accelerator-based nuclear research and established at Caltech what became a world famous laboratory for research in nuclear physics. During and after World War II, Lauritsen made significant contributions to the development of important weapons systems and advised the U.S. government on military and defense matters.

Danish Origins . Charles Christian Lauritsen was born in Holstebro, Denmark, in 1892, the son of Thomas Lauritsen and Marie Lauritsen (née Nielsen). The father, a sawmill owner, committed suicide in 1903; the mother, with young Charlie and his older brother Laurits to care for, soon remarried. In 1911 Charlie graduated from Odense Tekniske Skole with a degree in structural engineering and certification to supervise construction work. After completing his required military service in the Danish

infantry (1911–1912), he studied architecture and sculpture at the Royal Danish Academy of Fine Arts. In 1914 and again in 1915 he was called back into service when Denmark mobilized to protect its neutrality during World War I. On 21 May 1915 he married Sigrid Henriksen, a medical student and the daughter of Niels Henriksen, a farmer. In July 1916 Lauritsen emigrated from Denmark to the United States, and Sigrid, with their infant son Thomas, joined him in Florida three months later.

Over the next ten years Lauritsen worked in Florida, Massachusetts, Ohio, California, and Missouri as a draftsman, radio designer, fluids engineer, and chief engineer for the Kennedy Corporation, a manufacturer of radio sets in St. Louis, Missouri. During this time he patented several inventions and taught himself the new technology of radio.

Earliest Work in Physics . In 1926 Lauritsen quit his job in St. Louis and drove his wife and child to Pasadena, California. There at age thirty-four, with little background in formal mathematics or physics, he persuaded doubtful faculty to admit him to Caltech’s graduate physics program. Lauritsen made rapid progress based on his exceptional physical insight coupled with his remarkable talents for designing and building apparatus. These qualities first became apparent after Robert A. Millikan, the Nobel Prize–winning head of Caltech, suggested that for a PhD thesis, Lauritsen examine how electrons are pulled off the surface of a metal by strong electric fields—an effect known as cold field-emission. Millikan had been unable to make progress on this problem, but within a few months Lauritsen found experimental arrangements and created apparatus that yielded the first reliable, reproducible measurements of the electron currents drawn from metals by different strengths of electric field. He showed from his data that cold field-emission currents rise exponentially with the field strength. This behavior was explained by J. Robert Oppenheimer, who visited Caltech in 1928 just after returning from Europe where, working with Max Born, he had mastered the new theory of quantum mechanics. Oppenheimer recognized that Lauritsen’s data were a consequence of quantum tunneling. This occasion began a lifelong friendship between Oppenheimer and Lauritsen.

Lauritsen quickly showed remarkable ability to do physics both in the classroom and in the laboratory. In his first year at Caltech, he completed the research he would use for his PhD. In his second year, working with Ralph Bennett, he used the experience and insights from his cold field-emission work to invent and build a 750,000-volt xray tube, which for a short time was the highest voltage tube in the world. He also became an American citizen. In 1929 he received his PhD and was appointed to the faculty; the next year he became an assistant professor. He built a 1-million-volt x-ray tube that became the principal instrument for a cancer treatment center, the Kellogg Radiation Laboratory, built on the Caltech campus in 1931. That year he was named director of the Kellogg lab and was promoted to associate professor.

Lauritsen had a gift for using the apparatus that was at hand. For his x-ray research, he took advantage of four 250,000-volt transformers left over from a discontinued testing program of an electrical utility company, and his tubes used glass cylinders from the gasoline pumps typical of that era. His first tube cost less than one hundred dollars.

He studied the physical properties of the very short wavelength x-rays produced with his tubes, which he soon improved to sustain one million volts. Recognizing that energetic x-rays can have useful therapeutic properties, he studied their physiological effects in collaboration with physicians. In 1931, in recognition of his work, the American College of Radiology made him an honorary fellow and awarded him its gold medal.

Lauritsen’s work led Caltech to establish a cancer treatment clinic on its campus. Lauritsen served as its technical director, and he and his students did radiological research, developed improved x-ray tubes, and maintained the equipment of the clinic. Around 1930 he invented an ingenious pocket-sized electroscope for measuring doses of radiation. This Lauritsen dosimeter, or Lauritsen electroscope, which he patented in 1935, became widely used for monitoring levels of exposure to radiation. Lauritsen and others also used it in their nuclear research to detect and identify particles.

Pioneering Nuclear Physics . In 1932 the British physicists John D. Cockcroft and Ernest T. S. Walton reported that they had built an apparatus which accelerated hydrogen ions, that is, protons, to energies sufficient to penetrate lithium nuclei and produce nuclear reactions. On learning of this achievement, Lauritsen converted one of his x-ray tubes into a crude alternating-voltage, positiveion accelerator and began doing nuclear physics. One of the first three Americans to study atomic nuclei by bombarding them with accelerated particles, Lauritsen broke new ground and helped to create the modern field of nuclear physics. For the rest of his career, his principal research was the study of the properties of light nuclei.

For nuclear physicists, the mid-1930s was an exciting time. As Lauritsen’s son Tommy remembered in a 1967 interview by Barry Richman and Charles Weiner, “every day you went to the laboratory you found something new.” From 1933 to 1935, Lauritsen and his students published twenty-six notes and papers. The first of these reported how they produced neutrons by accelerating helium ions into beryllium. It was the first use of an accelerator to produce neutrons (just discovered in 1932 by James Chadwick). They detected the neutrons using an ingenious adaptation of Lauritsen’s electroscope. A few weeks later they became the first to produce neutrons using accelerated ions of deuterium, the heavy isotope of hydrogen just discovered in 1931.

Lauritsen responded quickly when, in January 1934, French physicists Irène Curie and Frédéric Joliot reported the first artificial production of radioactivity. They had bombarded boron with alpha particles from a polonium source and produced nitrogen-13 nuclei that decayed with a half-life of ten minutes into stable carbon-13 nuclei. They noted that nitrogen-13 nuclei could also be produced by bombarding carbon-12 with deuterons. Lauritsen and Horace R. Crane put a carbon target into the beam of deuterons that they were accelerating for their neutron studies, and in late February they reported the first observations of the production of radioactivity with an accelerator.

Lauritsen and Crane were also the first to see evidence of what came to be known as nuclear resonance. They observed that when accelerated to a certain energy,

protons would suddenly be absorbed by lithium-7 nuclei, which then emitted a gamma ray rather than a particle. They saw a similar effect for carbon-12. Ultimately, this behavior was understood to show that a nucleus had well-defined internal energy states, but because of the limits of Lauritsen and Crane’s crude accelerator, their observations were greeted with a skepticism that they themselves shared. The existence of such nuclear resonances was widely recognized only after they had been observed by other physicists working either with very low energy neutrons or with accelerator beams possessing very well-defined energy—in one case relying on a Lauritsen electroscope provided by Lauritsen himself.

Lauritsen and his students built a cloud chamber, a device in which ions, electrons, and gamma rays (very short wave-length electromagnetic radiation) revealed their presence by visible tracks in a supersaturated vapor. By measuring the energies of gamma rays emitted from nuclei, they sought to identify and understand the internal states of light nuclei, often called energy levels. Their first measurements resulted in a mélange of falsely identified levels. Only when they changed to inferring gamma-ray energies from measurements of the energies of electron-positron pairs produced by gamma rays did they get reliable values.

Their cloud chamber measurements of the energies of positrons emitted from various radioactive nuclei revealed some of the first evidence that the nuclear force between two protons is the same as the nuclear force between two neutrons—that is, nuclear forces exhibit charge symmetry. They were also among the first to determine nuclear masses from measurements of reaction energies; that is, Q-values, a technique of great importance for establishing the relative energies of nuclear ground states and for determining what reactions might be possible in different circumstances—such as deep inside stars, for example.

Lauritsen’s work was known and admired for its reliability and ingenuity. Interpretation of his results benefited from the advice and insights he received from his friends, the brilliant theorist Oppenheimer and fellow Dane and world-famous theoretical physicist and Nobel laureate Niels Bohr. Physicists came from all over the world to work in Lauritsen’s lab. Within a few years he had made Caltech an internationally recognized center of nuclear physics research.

Nuclear Physics Matures . In 1936 Hans Bethe published the first of his three review articles that became the “Bethe’s Bible” of nuclear physicists. This comprehensive account of what had been learned in the preceding four

years marked the end of the pioneering era of nuclear physics. As well as summarizing the basic knowledge of the field, the articles showed experimentalists many promising lines of research. These would require improved precision and accuracy of measurements and higher energy accelerators with better quality beams.

Lauritsen understood this, and aided by his students—one of which was his son, Tommy—and by his former student, William A. Fowler, a Caltech faculty member and future Nobel laureate, he built two Van de Graaff accelerators. The first was a low energy accelerator built in 1937, and the second was a larger machine built in 1938–1939 that could accelerate ions through a potential difference of up to 1.7 million volts. Construction of a planned 5-million-volt machine had to wait until after World War II.

The cancer treatment clinic closed in 1939, due to a study that showed the radiation treatments were either ineffective or did more damage than good. Lauritsen, still director of the Kellogg Radiation Laboratory, took over the building for nuclear physics. He moved his accelerators and research program into Kellogg that summer. However, World War II began in Europe in September, and during the spring of 1940 he moved to Washington, D.C., to work with other scientists on preparations for America’s involvement in the war. From 1940 until 1945 no nuclear physics research was done at Kellogg.

In Washington, Lauritsen helped develop the proximity fuse, but in May 1941 on a visit to Britain, he observed British rocket weapons in use and became convinced that American military forces should also have rocket weapons. He persuaded the federal government’s Office of Scientific Research and Development (OSRD) to set up at Caltech a large program to develop rocket weapons. He returned to Pasadena and, using the Kellogg Laboratory, much of the rest of Caltech, and large parts of Pasadena, he directed the invention, development, production, and testing of a variety of rocket munitions, mostly for the U.S. Navy. Toward the end of the war, Oppenheimer, who was directing the effort to make an atomic bomb, asked Lauritsen to go to Los Alamos, New Mexico, to assist with the project. Lauritsen complied with the request.

Lauritsen’s work on rocketry led to close friendships with key naval officers, and he was influential in the navy’s decision to create the Office of Naval Research (ONR) in 1946. Immediately after the war, ONR became the leading agency through which the federal government began supporting basic research at a level that vastly increased the pace and scale of scientific work in the United States.

Lauritsen used ONR support to reestablish Kellogg’s research program, and he decided to continue its prewar focus on understanding the properties of light nuclei. This was not an obvious decision, because technological advances and generous funding allowed nuclear physicists in the postwar years to work with larger accelerators at higher energies and probe the deep structure of nuclei and their constituent particles.

Lauritsen, however, saw that there was still much to learn by studying light nuclei at modest accelerator energies. In 1939 Bethe had proposed that two particular series of nuclear reactions—one called the p-p cycle, and the other called the CNO cycle—could explain energy generation in stars, a dramatic example of how nuclear physics could be used to understand stellar processes. In 1945 Lauritsen, Fowler, and Caltech astronomers held a series of seminars that showed that studies of light nuclei could answer important questions of astrophysics. Lauritsen included this line of research in Kellogg’s postwar plans; he and his colleagues would continue to study light nuclei to discover and understand their properties, but they would also open a new line of research by doing nuclear physics experiments that answered astrophysical questions. This set Kellogg Laboratory researchers on a path along which they created a new field of physics: nuclear astrophysics. It was a field in which Fowler would work with imagination and insight; for his work on how stars synthesize elements, he shared the Nobel Prize in Physics for 1983.

As the Cold War with the Soviet Union intensified in the postwar years, Lauritsen contributed extensively to American military efforts. His low-key, nonconfrontational emphasis on reasoned discourse based on reliable facts made him an influential advisor. After 1945 and for the rest of his life, he typically spent forty-five days a year or more consulting, advising, and evaluating defense

efforts and programs for the army, navy, air force, and Department of Defense.

In 1950 Lauritsen was deeply upset by President Harry Truman’s decision to develop thermonuclear weapons. Lauritsen believed that a weapon a thousand times more powerful than the atomic bombs dropped on Japan had no justifiable military use. He felt that American defense strategy had become dangerously dependent on large bombs. In summer long special study groups such as Project Charles, Project Vista, and the ad hoc Lincoln summer study group, he worked with fellow scientists to identify weapons technologies that would support flexible military strategies offering alternatives to all-out nuclear war. Thus, he strongly advocated the development of tactical nuclear weapons that would permit American forces to fight small wars against numerically superior forces, and he worked strenuously with others to develop a credible system for air defense of the continental United States, believing that adequate warning would reduce the chance of preemptive nuclear war. He was part of a group of scientists that persuaded President Truman and, after him, President Dwight D. Eisenhower, to create a vast air defense system that included a long chain of manned radar stations above the Arctic Circle. The two presidents supported this program over the objections of air force leaders who feared that a diversion of resources from the Strategic Air Command would weaken America’s ability to mount a massive nuclear retaliation in response to any Soviet attack. Partly because of these fears, air force leaders supported efforts to take away Oppenheimer’s security clearance, and they reacted with suspicion and hostility to recommendations made by Vista scientists. Lauritsen testified for Oppenheimer at the hearings in 1954. These ended with the cancellation of Oppenheimer’s clearance and his exclusion from government counsels. Although deeply unhappy with this outcome, Lauritsen continued to be a valuable advisor to government agencies for the rest of his career.

Honors . Lauritsen was honored both in America and in his native Denmark. In 1939 he was elected to the Royal Danish Academy of Sciences and Letters, and in 1953 he was awarded the Commander’s Cross of the Order of Dannebrog by the king of Denmark. In America, in addition to his 1931 gold medal from the American College of Radiology, he was elected to the National Academy of Sciences in 1941; received the President’s Medal for Merit in 1948; served as president of the American Physical Society in 1951; and received the society’s Tom W. Bonner Prize in 1967 for his work in nuclear physics. In 1954 he was elected a member of the American Philosophical Society; in 1958 he was the first recipient of the navy’s Captain Robert Dexter Conrad Award for Scientific Excellence; and in 1965 the University of California at Los Angeles awarded him an honorary degree of doctor of laws. Two libraries, a street, and two buildings are named for him, as well as a crater on the far side of the moon.

BIBLIOGRAPHY

The Charles Christian Lauritsen Papers are in the archives of the California Institute of Technology. The collection contains both personal and professional correspondence, research notes and data, manuscripts, reprints, patents, and photographs.

WORKS BY LAURITSEN

With R. D. Bennett. “A New High Potential X-Ray Tube.” Physical Review 32 (1928): 850–857. In 1935 Lauritsen was issued U.S. patent 1995478 for his “High Potential X-Ray Tube.”

With R. A. Millikan. “Relations of Field-Currents to Thermionic-Currents.” Proceedings of the National Academy of Sciences of the United States of America 14 (1928): 45–49.

“Energy Considerations in High Voltage Therapy.” American Journal of Roentgenology and Radium Therapy 30 (1933): 380–387.

“Energy Considerations in Medium and High Voltage Therapy.” American Journal of Roentgenology and Radium Therapy 30 (1933): 529–532. The work reported in this article and the preceding one constituted an important part of the basis for the standard of dose adopted internationally in the late 1940s.

With H. R. Crane and A. Soltan. “Production of Neutrons by High Speed Deutons.” Physical Review 44 (1933): 692–693. The word deuton was one of several proposed names for the nucleus of the mass-2 isotope of hydrogen. By 1935 the name deuteron was coming into general acceptance.

——. “Artificial Production of Neutrons.” Physical Review 44 (1933): 514; 45 (1934): 507–512.

With H. R. Crane and W. W. Harper. “Artificial Production of Radioactive Substances.” Science 79 (9 March 1934): 234–235. The authors published in Science to get into print before their competition at Berkeley and at the Carnegie Institution of Washington.

With W. A. Fowler and L. A. Delsasso. “Radioactive Elements of Low Atomic Number.” Physical Review 49: (1936): 561–574. Fowler says that Oppenheimer and his student, Robert Serber, saw in these results the first evidence for charge symmetry of nuclear forces.

With Thomas Lauritsen. “Simple Quartz Fiber Electrometer.” Review of Scientific Instruments 8 (1937): 438–439. This article describes the device for which Lauritsen had received U.S. patent 2022117 in 1935. It is a more compact version of the earlier device that he developed around 1930.

With Tom Lauritsen and W. A. Fowler. “Application of a Pressure Electrostatic Generator to the Transmutation of Light Elements by Protons.” Physical Review 59 (1941): 241–252.

In the Matter of J. Robert Oppenheimer: Transcript of Hearing before Personnel Security Board, Washington, D.C., April 12, 1954 through May 6, 1954. Washington, DC: U.S. Government Printing Office, 1954. Contains Lauritsen’s verbatim testimony.

OTHER SOURCES

Christman, Albert B. History of the Naval Weapons Center, China Lake, California. Vol. 1, Sailors, Scientists and Rockets. Washington, DC: U.S. Government Printing Office, 1971. Several chapters describe the importance to the navy of Lauritsen’s wartime work. His face is prominent on the cover of the 1992 paperback edition of this book.

Fowler, William A. “Charles Christian Lauritsen: April 4, 1892 to April 13, 1968.” Biographical Memoirs of the National Academy of Sciences 46 (1975): 221–233. This lively, affectionate, and informed account of Lauritsen’s life was written by his former student and colleague of more than thirty years. The article contains a complete bibliography of Lauritsen’s scientific publications.

Goodstein, Judith R. Millikan’s School: A History of the California Institute of Technology. New York: W. W. Norton, 1991. The chapter titled “The Rockets’ Red Glare” gives an account of the wartime rocket project at Caltech.

Holbrow, Charles H. “The Giant Cancer Tube and the Kellogg Radiation Laboratory.” Physics Today 34 (July 1981): 42–49. This article describes the founding of the Kellogg Radiation Laboratory and shows how Millikan promoted Lauritsen’s achievements to obtain funding during the Great Depression.

——. “Charles C. Lauritsen: A Reasonable Man in an Unreasonable World.” Physics in Perspective 5 (2003): 419–472. This is the most carefully researched biography available. It describes Lauritsen’s research, his style of leadership, his active and extensive advising on weapons and defense technologies, and his efforts to foster flexible military strategies offering alternatives to all-out nuclear war.

——. “Scientists, Security, and Lessons from the Cold War.” Physics Today 59 (July 2006): 39–44. Describes how Lauritsen and others made the case for constructing the chain of radar stations above the Arctic Circle.

Lauritsen, Thomas. Interview by Barry Richman and Charles Weiner, 16 February 1967. American Institute of Physics Center for History of Physics, College Park, Maryland.

Charles H. Holbrow