James Chadwick

English physicist

• Born: October 20, 1891; Bollington, England
• Died: July 24, 1974; Cambridge, England

Nobel laureate James Chadwick is best known for his discovery of the neutron in 1932. This discovery led to the creation of the atomic bomb, but on a more elemental level, the neutron was the last major piece in the puzzle of atomic structure.

Primary field: Physics

Specialties: Nuclear physics; atomic and molecular physics

Early Life

James Chadwick was born in Cheshire, England, on October 20, 1891, to Anne Mary Knowles and John Joseph Chadwick. The Chadwick family experienced many financial difficulties in a region that relied mostly on the cotton-spinning industry. When James was four years old, his father lost his job and the family moved to Manchester. James was then sent away to live with his grandparents for several years, eventually returning home to attend high school.

One of Chadwick’s teachers, recognizing his talent for physics and math, helped him apply for two college scholarships, both of which he won. Chadwick chose to attend Manchester University, a decision that would catapult him into the world of physics.

At the age of nineteen, Chadwick took it upon himself to research electricity and magnetism, after attending a lecture on the subjects by Manchester professor Ernest Rutherford. One year later, in 1911, Chadwick graduated from Manchester University’s Honours School of Physics. Rutherford saw great potential in Chadwick, eventually taking him on as his protégé. Chadwick worked very closely with Rutherford for much of his career, acting as Rutherford’s assistant in some capacity for more than ten years. Chadwick worked closely with Rutherford on radioactivity while working toward his master’s degree in science, which he received in 1913. During his four years at Manchester, Chadwick published five papers on his research with radioactivity.

After he finished his studies at Manchester, Chadwick was awarded a scholarship to perform research at the Technische Hochschule (Technical University) in Berlin with German physicist Hans Geiger, who had previously worked with Rutherford, beginning in 1914. Chadwick went to Germany, where he and Geiger studied beta particles. Unfortunately, World War I began less than a year into their work. Geiger told Chadwick to leave the country for his own safety, but Chadwick did not believe he was in any danger.

In November 1914, Chadwick was arrested and deemed an enemy alien by the German government; he was imprisoned in a camp in Ruhleben for the rest of the war. Despite being imprisoned for almost five years, Chadwick found himself in the company of a group of intellectuals who aided in his social development and encouraged him to build a makeshift laboratory inside the prison, which he used to continue the research he had begun with Geiger.

Life’s Work

Following Germany’s defeat in 1919, Chadwick was released from prison. He returned to England on another scholarship and resumed his work with Rutherford, who was by then the director of the Cavendish Laboratory at the University of Cambridge. Rutherford had recently discovered the proton, a positively charged particle in an atom’s nucleus, and was trying to figure out whether there were other particles in the nucleus.

Part of Rutherford and Chadwick’s research involved changing elements into other elements, a process known as transmutation, by bombarding elements like nitrogen with radioactive alpha particles. All of their experiments pointed to the existence of another particle in the nucleus, but neither Rutherford nor Chadwick knew what it might be.

In 1920, Rutherford proposed the existence of what he called a neutral doublet in the nucleus of an atom, consisting of a proton and an electron, which would account for discrepancies they had observed between an atom’s atomic number and its atomic weight. Rutherford also proposed the possibility of another type of particle—which he called a neutron—that might also account for these discrepancies.

Chadwick agreed with Rutherford’s assessment that an atom’s nucleus could not exist without a particle fitting the specifications of the neutron. Locating the particles, however, would prove difficult; since the proposed particles held no electrical charge, they would be almost impossible to locate.

Under Rutherford’s tutelage, Chadwick worked on his doctorate while continuing to work on solving the mysteries of the atomic nucleus. After receiving his PhD in 1921, Chadwick became the assistant director of research at Cavendish in 1924. He was also elected a fellow of Gonville and Caius College. In 1927, Chadwick was elected a fellow of the Royal Society. Together with Rutherford and C. D. Ellis, Chadwick wrote Radiations from Radioactive Substances, which was published in 1930.

Future Nobel laureates Irène and Frédéric Joliot-Curie were performing similar, though unrelated, experiments with beryllium. Joliot-Curie and her husband discovered that unusual, positively charged radiation was produced during their experiments, which they attributed to the effects of gamma rays. Chadwick recognized that gamma rays were unlikely to cause the reaction Joliot-Curie described, and he postulated a much stronger candidate for the cause of the reaction: neutrons.

To recreate the conditions used by Joliot-Curie and her husband, Chadwick and Rutherford inundated beryllium with alpha particles. Chadwick built a device to capture the particles emitted by the beryllium. After reading Joliot-Curie’s paper, Chadwick worked for three weeks straight, often spending entire nights in his laboratory. Finally, in 1932, Chadwick’s persistence paid off, when he found definitive evidence that proved the existence of neutrons. Though neutrons had clearly been present in prior experiments, Chadwick was the first person to identify them and to report on their importance. Chadwick’s discovery completely changed the conception of the atom and revolutionized physics. What Chadwick did not realize at the time was how much of an effect his discovery would have on the world at large.

When Chadwick discovered the neutron in 1932, the contribution to physics was obvious. The same year he made the discovery, Chadwick was awarded the prestigious Hughes Medal of the Royal Society. Werner Heisenberg, one of the world’s most respected physicists, praised Chadwick’s discovery.

When he began searching for the neutron in 1920, Chadwick inadvertently discovered the key to fission (the process of splitting atoms), which would eventually prove to be pivotal in the development of the atomic bomb. Since neutrons carry no charge, they can be smashed into an atom’s nucleus without being slowed down or stopped by oppositely-charged particles within the atom.

Chadwick was awarded the Nobel Prize in Physics in 1935. Although he had been loyal to Cambridge for many years, Chadwick accepted an offer in 1935 to serve as the chair of the physics department at the University of Liverpool. In recognition of his contributions to atomic theory, Chadwick was invited to work on the Manhattan Project, a collaboration of scientists that eventually developed an atomic bomb in the United States. This large-scale, long-term project saw Chadwick working alongside such influential physicists as Robert Oppenheimer and Enrico Fermi.

Impact

Chadwick was knighted in 1945. He was awarded several other distinctive honors in his later life, including the Copley Medal in 1950 and the Franklin Medal of the Franklin Institute in 1951, as well as an honorary fellowship at the Institute of Physics. Chadwick was also granted membership in prestigious scientific societies such as the Académie Royale de Belgique, the Pontificia Academia Scientiarum, the American Philosophical Society, and the American Physical Society.

In the wake of his revolutionary discovery, Chadwick continued to make important contributions to particle physics, including adding to the understanding of another important elementary particle called a neutrino. With his discovery of neutrons, Chadwick had discredited the hypothesis that the additional weight in atoms was caused by the fusion of protons and electrons in the nucleus. However, Chadwick and American physicist Maurice Goldhaber discovered a similar particle, called a deuteron—a neutron bonded with a proton—in the nucleus of the deuterium atom.

Bibliography

Brown, Andrew. The Neutron and the Bomb: A Biography of Sir James Chadwick. New York: Oxford UP, 1997. Print. A biography of Chadwick that discusses his work in nuclear physics related to the Cavendish Laboratory, World War II, and the Manhattan Project.

Kelly, Cynthia C., ed. The Manhattan Project: The Birth of the Atomic Bomb in the Words of Its Creators, Eyewitnesses, and Historians. New York: Black Dog, 2007. Print. An overview of the Manhattan Project and the creators of the atomic bomb with a collection of primary documents including articles, letters, and research documents. Discusses Chadwick’s discovery of the neutron and his collaboration with other physicists in the project.

Lewin, Walter. For the Love of Physics: From the End of the Rainbow to the Edge of Time: A Journey Through the Wonders of Physics. New York: Free, 2011. Print. An accessible overview of physics that notes Chadwick’s discovery of the neutron and its effect on the field in the twentieth century.