experiment that led to discovery of neutron

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James Chadwick

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• Famous Scientists - Biography of James Chadwick
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James Chadwick (born October 20, 1891, Manchester , England—died July 24, 1974, Cambridge , Cambridgeshire) was an English physicist who received the Nobel Prize for Physics in 1935 for the discovery of the neutron .

Chadwick was educated at the University of Manchester , where he worked under Ernest Rutherford and earned a master’s degree in 1913. He then studied under Hans Geiger at the Technische Hochschule, Berlin. When World War I broke out, he was imprisoned in a camp for civilians at Ruhleben. He spent the entire war there but nevertheless was able to accomplish some scientific work.

After the war ended, Chadwick returned to England to study under Rutherford at the University of Cambridge . He received a doctorate in 1921, and in 1923 he was appointed assistant director of research at the Cavendish Laboratory , Cambridge. There he and Rutherford studied the transmutation of elements by bombarding them with alpha particles and investigated the nature of the atomic nucleus, identifying the proton , the nucleus of the hydrogen atom , as a constituent of the nuclei of other atoms.

After the discovery of the proton, physicists had surmised that there were likely additional particles in the atomic nucleus. Elements heavier than hydrogen had a greater atomic mass than their atomic number (the number of protons). Theories for the additional particles included additional protons whose charge was shielded by electrons in the nucleus or an unknown neutral particle. In 1932 French physicists Frédéric and Irène Joliot-Curie bombarded beryllium with alpha particles and observed that an unknown radiation was released that in turn ejected protons from the nuclei of various substances. The Joliot-Curies hypothesized that this radiation was gamma-rays . Chadwick was convinced that alpha particles did not have enough energy to produce such powerful gamma-rays. He performed the beryllium bombardment experiments himself and interpreted that radiation as being composed of particles of mass approximately equal to that of the proton but without electrical charge—neutrons. That discovery provided a new tool for inducing atomic disintegration , since neutrons, being electrically uncharged, could penetrate undeflected into the atomic nucleus and led to a new model of the atomic nucleus being composed of protons and neutrons.

In 1935 Chadwick was appointed to a chair in physics at the University of Liverpool. In 1940 he was part of the MAUD Committee , which was to assess the feasibility of the atomic bomb . The committee concluded in 1941 that the 1940 memorandum of Otto Frisch and Rudolf Peierls was correct and that a critical mass of only about 10 kilograms (22 pounds) of uranium -235 was needed. Chadwick later said he realized “that a nuclear bomb was not only possible, it was inevitable. I had then to take sleeping pills. It was the only remedy.” The MAUD Committee’s results were influential in giving an impetus to the American atomic bomb program. He became head of the British delegation to the Manhattan Project in Los Alamos , New Mexico , U.S., in 1943 and formed a close rapport with its head, Gen. Leslie Groves .

Chadwick was knighted in 1945. He returned to Britain in 1946 and became the British scientific adviser to the United Nations Atomic Energy Commission . He became master of Gonville and Caius College, Cambridge, in 1946, and he received the Copley Medal of the Royal Society in 1950. He retired in 1958.

James Chadwick: The Man Behind the Neutron

Maya kuppermann may 15, 2018, submitted as coursework for ph241 , stanford university, winter 2018.

James Chadwick was born in Cheshire, England, on 20th October, 1891. He graduated from Manchester University in 1908 and went on to graduate from the Honours School of Physics in 1911. After graduation he spent two years working in Physical Laboratory in Manchester, where he worked on various radioactivity problems, gaining his M.Sc. degree in 1913. After being interned in the Zivilgefangenenlager, Ruhleben during World War I, Chadwick returned to England to continue his research. Chadwick continued to move up the ladder in the world of science when he was elected Fellow of Gonville and Caius College (1921-1935) and became Assistant Director of Research in the Cavendish Laboratory (1923). In 1927 he was elected a Fellow of the Royal Society. [1]

Discovery of the Neutron

In 1932, Chadwick made a fundamental discovery in the domain of nuclear science. Chadwick was fascinated by an experiment done by Frdric and Irne Joliot-Curie that studied the then-unidentified radiation from beryllium as it hit a paraffin wax target. The Curies found that this radiation knocked loose protons from hydrogen atoms in that target, and those protons recoiled with very high velocity. In 1932, Chadwick tried similar experiments himself and hypothesized that the radiation ejected by the beryllium was, in fact, a neutral particle with approximately the same mass as a proton. Fig. 1 depicts a schematic diagram of the experiment done by Chadwick, following on experiments done by the Curies. He later tried other targets including helium, nitrogen, and lithium, which led him to determine that the mass of the new particle was in fact just slightly greater than the mass of the proton. [1] This is reflected in the current understanding of the mass of a neutron as 1.008701 amu or 1.6750 × 10 -24 g and the mass of a proton as 1.007316 amu or 1.6727 × 10 -24 g. [2]

After only about two weeks of experimentation, Chadwick wrote a paper in which he proposed that the evidence favored the neutron rather than the gamma ray photons as the correct interpretation of the radiation. Only a few months later, in May 1932, Chadwick submitted a paper announcing the discovery of the Neutron. The existence of a neutron as a new fundamental particle was firmly established by 1934. Chadwick was awarded the Nobel Prize in 1935 for its discovery. [3]

Chadwick's discovery of the neutron was the final piece in understanding the atomic puzzle and sparked a revolution leading to the nuclear age and the creation of nuclear weapons. [4]

© Maya Kuppermann. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.

[1] A. Brown, The Neutron and the Bomb: A Biography of Sir James Chadwick (Oxford University Press, 1997).

[2] D. W. Oxtoby and H. P. Gillis, Principles of Modern Chemistry, 5th Ed. (Brooks Cole, 2002).

[3] M. Oliphant, "The Beginning: Chadwick and the Neutron," Bull. Atom. Sci. 38 , 14 (1982).

[4] K. Fischer, A Brief History of Pulsed Neutron Generation ," Physics 241, Stanford University, Winter 2015.

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Sir James Chadwick's Discovery of Neutrons

The September installment of Nuclear Pioneers explored the artificial radioactivity research of Irène and Frédéric Joliot-Curie, for which they were awarded the Nobel Prize in Chemistry on December 10, 1935. A misinterpretation of data perhaps cost the Joliot-Curies an earlier Nobel Prize, but instead led to James Chadwick taking the Nobel podium two days after the Joliot-Curies, on December 12, 1935, to receive the Nobel Prize in physics for discovering the neutron.

Atomic Mass Mystery

When Ernest Rutherford discovered the proton in 1918, scientists at the time might have thought that they had finally figured out atomic structure once and for all. Negatively-charged electrons, orbiting a tiny atomic nucleus composed of positively-charged protons, like a miniature solar system-this model explained atoms being electrically neutral, using only protons and electrons, the two fundamental atomic particles known at the time.

However, it was also well-known that atomic mass is generally twice the atomic number (i.e., the number of protons), and that almost all the mass of an atom is concentrated in the nucleus. What could account for all this additional mysterious mass?

Nuclear Electrons?

The theory at the time was that there were "nuclear electrons" in the atomic nucleus, along with additional protons. The extra protons were thought to provide the extra atomic mass, while the additional electrons would cancel out their positive charge, leaving the atom electrically neutral. Eventually, however, calculations using Heisenberg's uncertainty principle  showed it was not possible for electrons to be contained in the nucleus.

There were other ideas. Ernest Rutherford in 1921 postulated a particle called the "neutron," having a similar mass as a proton but electrically neutral. Rutherford imagined a paired proton and electron somehow joined in one particle. One major problem with Rutherford's "neutron theory"-not much evidence.

Mysterious "Gamma Radiation"

Evidence was difficult to come by. Such a "neutron" would prove difficult to detect with 1920s equipment. Detection methods of that day mainly relied on the electrical charges of particles revealing their presence-but neutrons, having no electrical charge, would leave no trace.

In 1930, the physicists Walther Bothe and Herbert Becker bombarded beryllium with alpha particles (helium nuclei) emitted from the radioactive element polonium, and they found that the beryllium gave off an unusual, electrically neutral radiation. They interpreted this radiation to be high-energy gamma rays (photons).

The Compton Effect

The Joliot-Curie radiation discovery was amazing, because photons have no mass. It was asking quite a lot for a massless particle to eject relatively heavy protons. It was well known that photons could strike a metal surface and eject electrons (as occurs in the then-recently-discovered Compton Effect , proving the particle nature of light) and the Joliot-Curies believed something similar was happening in their experiments.

But protons are 1,836 times heavier than electrons-and that much harder to budge. Nevertheless, the Joliot-Curies stuck to their interpretation that high-energy photons were striking the hydrogen atoms in paraffin to eject protons.

How to Detect a Neutron

James Chadwick was working at the Cavendish laboratory in Cambridge at that time. The lab was directed by Ernest Rutherford, and reportedly when Chadwick relayed the Joliot-Curie results and interpretation to Rutherford, he exclaimed "I do not believe it!"

Chadwick himself was certainly suspicious. He immediately repeated the experiments, using many different elements as radiation targets besides paraffin. By comparing the energies of particles ejected from all these various targets, Chadwick was able to prove that the radiation causing the ejected particles was much more energetic than could be accounted for by photons.

Instead, the range and power of the radiation could be accounted for quite easily if it consisted of particles having the same mass as protons. What really occurred when one bombarded beryllium with alpha particles, Chadwick explained, was the formation of a carbon-12 nucleus and the emission of a neutron. Formation of a carbon-13 nucleus with the emission of a photon, as the Joliot-Curies had postulated, could not provide sufficient energy for the scattering pattern and energies of ejected particles from Chadwick's various targets.

Why Neutrons?

Neutrons are necessary within an atomic nucleus because they bind with protons via the " strong nuclear force "; protons are unable to bind with each other directly because their mutual electromagnetic repulsion is stronger than the "strong force." Neutrons keep the atomic nucleus from flying apart, one of the features that allows for atoms heavier than hydrogen, thus making our universe much more interesting than one would otherwise expect.

Implications

It's hard to imagine a more momentous event than Chadwick's discovery of neutrons. Radiation experiments at that time used helium nuclei, which are electrically charged and therefore repelled by electrical forces. These electrical forces become quite considerable close to the nuclei of heavier atoms, which are loaded with many protons (and neutrons). However, neutrons do not need to overcome any electrical barrier to penetrate (and split) the nucleus of even the heaviest, most-proton-charged atomic nucleus. After Chadwick's discovery, it was soon postulated that neutrons could mediate a nuclear chain reaction , which eventually led to the atomic bomb, and later to nuclear power production.

Paul Bowersox prefers interesting universes with heavy elements and is a regular contributor to ANS Nuclear Cafe.

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James Chadwick: Unveiling the Neutron’s Secrets

February 13, 2024

James Chadwick was a noteworthy figure in the scientific community, remembered most prominently for his groundbreaking work in physics. Born on October 20, 1891, in Manchester, Chadwick’s early education set the stage for his future achievements. At the University of Manchester, he was mentored by Ernest Rutherford, a relationship that would profoundly shape his career. Chadwick’s academic journey led him to the discovery that secured his place in history.

In 1932, Chadwick made a significant scientific breakthrough by discovering the neutron, a particle in the nucleus of an atom that carries no electrical charge. This discovery not only earned him the Nobel Prize in Physics in 1935 but also played a crucial role in the development of atomic research, particularly in the creation of nuclear reactors and weapons. His contribution to science extended far beyond his neutron discovery, influencing atomic theory and nuclear physics for years to come.

Chadwick’s personal life was as remarkable as his professional one. Despite the challenges of his time, including the impact of World Wars, he continued to contribute to important research. His work had a lasting effect not just in academia, but also on the political stage, shaping the path of international nuclear policy. Chadwick passed away on July 24, 1974, leaving behind a legacy of scientific inquiry and achievement.

Key Takeaways

• James Chadwick was an English physicist who discovered the neutron in 1932, revolutionizing the field of nuclear science.
• His discovery led to the Nobel Prize in Physics in 1935 and had significant implications in the development of nuclear reactors and atomic weapons.
• Chadwick’s enduring legacy is marked by his contributions to scientific knowledge, his role in nuclear research during historical events, and his distinguished honors and recognitions.

Early Life and Education

Born into a modest family in a small town near Manchester, England, James Chadwick exhibited a remarkable aptitude for physics from a young age. His tireless curiosity and academic prowess paved the way for a distinguished education that would later define his profound contributions to science.

University Studies and Influences

Chadwick’s journey into the world of physics accelerated when he enrolled at the University of Manchester . There, he was not just a student of the subject; he became part of a lineage of scientists that would change the world. In 1911, he graduated from the Honours School of Physics and two years later, he completed his master’s degree. The time he spent at the university wasn’t just about acquiring knowledge; it was also about establishing connections and absorbing the influences around him, including the tutelage of Ernest Rutherford, a pioneer in the field of nuclear physics.

Key Academic Collaborations

After Manchester, Chadwick’s academic pursuits led him to Gonville and Caius College, Cambridge , arguably one of England’s finest cradles of learning. It was here, surrounded by the intellectual fervor of Cambridge, that Chadwick’s work started to intersect with the significant scientific developments of his time. Collaborating with esteemed physicists, he was at the forefront of exploring the intricacies of atomic structure. However, his academic endeavors were interrupted by World War I. During the conflict, Chadwick found himself at the Ruhleben internment camp for civilians in Germany, where even under difficult circumstances, he continued to engage with physics, demonstrating his resilience and dedication to his field.

Discovery of the Neutron

In 1932, James Chadwick’s groundbreaking work at the Cavendish Laboratory shed light on the atomic structure, revealing the neutron and profoundly influencing the field of particle physics.

Experiments with Beryllium

The path to Chadwick’s discovery began with experiments involving beryllium . When alpha particles, which are helium nuclei, were fired at a beryllium sample, an unknown radiation was produced. Unlike alpha and beta particles , this radiation had no charge and was therefore not deflected by magnetic or electric fields. Chadwick soon realized that these particles must be neutrally charged, and given their ability to penetrate and knock protons out of paraffin wax, he figured they must possess mass. This revelation led him to conclude that the mysterious radiation consisted of particles that were similar to protons in mass but without the charge. He had discovered the neutron .

Impact on Particle Physics

The identification of the neutron was monumental for particle physics. Prior to Chadwick’s work, the atom was thought to be composed of a nucleus containing protons with electrons around it. The discovery of the neutron provided a clearer picture of the atomic nucleus, which now was understood to contain neutrons in addition to protons. This added knowledge was key to furthering the understanding of atomic structure and the strong force holding the nucleus together. It also played a crucial role in the development of nuclear reactors and weapons, reshaping the landscape of modern science and international politics.

Contribution to Nuclear Science

James Chadwick’s work laid the cornerstone for nuclear science as we understand it. He unravelled mysteries of the atomic nucleus, which has ripple effects in both scientific research and practical applications.

The Manhattan Project

In the high-stakes arena of World War II, Chadwick’s expertise was pivotal. After drafting the critical MAUD Report, which galvanized U.S. leaders to invest in nuclear research, he became a leading figure in the Manhattan Project . His insights helped in harnessing nuclear fission , pivotal to the development of the atomic bomb.

Work on Fission

Chadwick not only discovered the neutron but also contributed to the understanding of nuclear fission. Fission is the process where an atomic nucleus splits, releasing a substantial amount of energy. His engagement with this phenomenon was critical in unlocking the potential of nuclear energy , particularly with elements like uranium .

Advancements in Physics

He blazed trails in the study of the atomic nucleus, earning the Nobel Prize in Physics in 1935. Chadwick’s proof of the neutron’s existence was a turning point, it advanced the comprehension of the nucleus profoundly and had major implications for both theoretical and applied physics.

Honors and Legacy

James Chadwick, a notable physicist, earned significant honors throughout his career, making substantial contributions to our understanding of atomic structure. His legacy extends into modern physics, influencing research and technology far beyond his lifetime.

Awards and Recognition

Chadwick’s groundbreaking work on discovering the neutron in 1932 catapulted him to international recognition. This monumental achievement earned him the Nobel Prize in Physics in 1935 , solidifying his reputation as a pioneering scientist.

The honors bestowed upon him include:

• Knighted in 1945, becoming Sir James Chadwick.
• Elected as a member of the Royal Society , a fellowship of many of the world’s most eminent scientists.
• Recipient of the Copley Medal (1950), the Hughes Medal (1932), and the Franklin Medal .

Chadwick’s peers recognized his contributions with several medals and awards, not just in the UK but worldwide, attesting to the impact of his scientific endeavors.

Influence on Modern Physics

James Chadwick’s discovery didn’t just earn him awards; it fundamentally shifted the course of physics. He gave the world a peek at the atom’s inner workings, which led to significant advancements in both physics and chemistry.

He played a vital role in developing atomic energy applications, his discoveries leading to:

• Nuclear power production and its use in medicine and industry.
• Nuclear weapon development, although the moral implications of such applications would remain a point of contention.

Chadwick’s contributions to the war effort through his expertise in nuclear physics were also notable. His neutron discovery was indeed a cornerstone that helped usher in the nuclear age, underscoring his lasting influence on modern science.

Personal Life and Death

James Chadwick led a life that was as remarkable in its personal dimensions as it was in his professional achievements. Born in Cheshire, England, Chadwick’s home life began humbly. He found companionship when he married Aileen Stewart-Brown, a woman who shared his passion for science and supported his research.

Together, they had twin daughters, who brought a new dimension of joy and warmth into Chadwick’s life filled with scientific inquiries.

Chadwick’s final years were spent in the town of Cambridge, where he passed away on July 24, 1974. His biography, which details both his personal life and scientific contributions, paints a picture of a man devoted to his family and his work.

Though Chadwick has long since passed, his legacy lives on—not just in the neutron’s discovery, but also in the memories shared by those who knew him and the family that loved him.

• Place of Death: Cambridge
• Spouse: Aileen Stewart-Brown
• Children: Twin Daughters
• Birthplace: Bollington, Cheshire, England

His story is not just one of scientific endeavor but also a narrative showcasing the balance between personal commitments and professional pursuits.

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