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James Chadwick Atomic Theory

Five Types of Atomic Models

Scientists today envision atoms as being composed of tiny, heavy, positively charged nuclei surrounded by clouds of extremely lightweight, negatively charged electrons. This model dates back to the 1920s, but it has its origin in ancient Greece. The philosopher Democritus proposed the existence of atoms around 400 B.C. No one really took up the idea with any fervor until English physicist John Dalton introduced his atomic theory in the early 1800s. Dalton's model was incomplete, but it persisted basically unchanged throughout most of the 19th century.

A flurry of research into the atomic model occurred at the end of the 19th and well into the 20th century, culminating in the Schrodinger model of the atom, which is known as the cloud model. Soon after physicist Erwin Schrodinger introduced it in 1926, James Chadwick – another English physicist – added a crucial piece to the picture. Chadwick is responsible for discovering the existence of the neutron, the neutral particle that shares the nucleus with the positively charged proton.

Chadwick's discovery forced a revision of the cloud model, and scientists sometimes refer to the revised version as the James Chadwick atomic model. The discovery earned Chadwick the 1935 Nobel Prize in physics, and it made possible the development of the atomic bomb. Chadwick participated in the super-secret Manhattan project, which culminated in the deployment of nuclear bombs on Hiroshima and Nagasaki. The bomb contributed to the surrender of Japan (many historians believe Japan would have surrendered anyway) and the end of World War II. Chadwick died in 1974.

How Did Chadwick Discover the Neutron?

J.J. Thompson discovered the electron using cathode ray tubes in the 1890s, and British physicist Ernest Rutherford, the so-called father of nuclear physics, discovered the proton in 1919. Rutherford speculated that electrons and protons could combine to produce a neutral particle with roughly the same mass as a proton, and scientists believed that such a particle existed for several reasons. For example, it was known that the helium nucleus has an atomic number of 2 but a mass number of 4, which meant that it contained some kind of neutral mystery mass. No one had ever observed a neutron or proven that it existed, though.

Chadwick was particularly interested in an experiment conducted by Frédéric and Irène Joliot-Curie, who had bombarded a sample of beryllium with alpha radiation. They noted that the bombardment produced an unknown radiation, and when they allowed it to strike a sample of paraffin wax, they observed high-energy protons being flung from the material.

Unsatisfied with the explanation that the radiation was made of high-energy photons, Chadwick duplicated the experiment and concluded that the radiation had to be composed of heavy particles with no charge. By bombarding other materials, including helium, nitrogen and lithium, Chadwick was able to determine that the mass of each particle was a little more than that of a proton.

Chadwick published his paper “The Existence of a Neutron” in May 1932. By 1934, other researchers had determined that the neutron was in fact an elementary particle and not a combination of protons and electrons.

The Importance of the Chadwick Atomic Theory

The modern conception of the atom retains most of the characteristics of the planetary model established by Rutherford, but with important modifications introduced by Chadwick and Danish physicist Neils Bohr.

It was Bohr who incorporated the concept of discrete orbits to which electrons were confined. He based this on quantum principles that were new at the time but which have become established as scientific realities. According to the Bohr model, electrons occupy discrete orbits, and when they move to another orbit, they emit or absorb not in continuous amounts, but in bundles of energy, called quanta.

Incorporating the work of Bohr and Chadwick, the modern picture of the atom looks like this: Most of the atom is empty space. Negatively charged electrons orbit a small but heavy nucleus composed of protons and neutrons. Because quantum theory, which is based on the uncertainty principle, regards electrons as both waves and particles, they can't be definitively located. You can only talk about the likelihood of an electron being in a particular position, so the electrons form a probability cloud around the nucleus.

The number of neutrons in the nucleus is usually the same as the number of protons, but it can be different. Atoms of an element that have a different number of neutrons are called isotopes of that element. Most elements have one or more isotope, and some have several. Tin, for example, has 10 stable isotopes and at least twice as many unstable ones, giving it an average atomic mass significantly different than twice its atomic number. If James Chadwick's discovery of the neutron had never occurred, it would be impossible to explain the existence of isotopes.

James Chadwick's Contribution to the Atomic Bomb

Chadwick's discovery of the neutron led directly to the development of the atomic bomb. Because neutrons have no charge, they can penetrate more deeply into the nuclei of target atoms than protons. Neutron bombardment of atomic nuclei became an important method to gain information about the characteristics of nuclei.

It didn't take scientists long to discover, however, that bombarding super-heavy Uranium-235 with neutrons was a way to break the nuclei apart and release an enormous amount of energy. The fission of uranium produces more high-energy neutrons that break apart other uranium atoms, and the result is an uncontrollable chain reaction. Once this was known, it was only a matter of developing a way to initiate the fission reaction on demand in a deliverable casing. Fat Man and Little Boy, the bombs that destroyed Hiroshima and Nagasaki, were the result of the secret war effort known as the Manhattan Project that was conducted to do just that.

Neutrons, Radioactivity and Beyond

The Chadwick Atomic Theory also makes it possible to understand radioactivity. Some naturally occurring minerals – as well as manmade ones – spontaneously emit radiation, and the reason has to do with the relative number of protons and neutrons in the nucleus. A nucleus is most stable when it has an equal number, and it becomes unstable when it has more of one than another. In an effort to regain stability, an unstable nucleus throws off energy in the form of alpha, beta or gamma radiation. Alpha radiation is composed of heavy particles, each consisting of two protons and two neutrons. Beta radiation consists of electrons and gamma radiation of photons.

As part of the study of nuclei and radioactivity, scientists have further dissected protons and neutrons to find that they are themselves composed of smaller particles called quarks. The force that holds protons and neutrons together in the nucleus is called the strong force, and the one that holds quarks together is known as the color force. The strong force is a byproduct of the color force, which itself depends on the exchange of gluons, which are yet another type of elementary particle.

The understanding made possible by the James Chadwick atomic model has brought the world into the nuclear age, but the door to a far more mysterious and intricate world is wide open. For example, scientists may one day prove that the entire universe, including atomic nuclei and the quarks from which they are made, is composed of infinitesimal strings of vibrating energy. Whatever they discover, they'll do it standing on the shoulders of pioneers like Chadwick.

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About the Author

Chris Deziel holds a Bachelor's degree in physics and a Master's degree in Humanities, He has taught science, math and English at the university level, both in his native Canada and in Japan. He began writing online in 2010, offering information in scientific, cultural and practical topics. His writing covers science, math and home improvement and design, as well as religion and the oriental healing arts.

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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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Science Struck

James Chadwick’s Atomic Theory and Its Lasting Impact Explained

Today, the concept of neutron and its function is common knowledge. Its existence was first hypothesized by Rutherford in 1920, and later proved by James Chadwick in 1932. This ScienceStruck post explains how the discovery came about and the revolutionary impact it had on the understanding of the atomic structure.

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Missed Opportunity!

In 1932, James Chadwick’s experiment, which led to the discovery of neutrons, was inspired by the work of Irène and Frédéric Joliot-Curie. Had they interpreted their findings accurately, they would have been the first to discover neutrons!

Sir James Chadwick (20 October 1891 – 24 July 1974) was an English physicist, most noted for his discovery of neutrons in 1932. He received a Nobel Prize in the field of physics in 1935 for this significant discovery. In his lifetime, he worked closely with outstanding scientists like Ernest Rutherford and Johannes “Hans” Wilhelm Geiger, both of whom have made various substantial and vital contributions to the field of radiation physics.

With the accidental discovery of radiation and radioactive materials in 1896 by Henri Becquerel, a new path emerged in the study of materials and their compositions. Experimentation led scientists to discover that a radioactive substance when subjected to a magnetic field, emits three types of energy rays (radiations). These rays were identified and named as alpha (α), beta (β), and gamma (γ) rays by Rutherford, based on their charge and mass. Further experiments conducted by Rutherford in collaboration with Ernest Marsden and Hans Geiger, led them to the discovery of the atomic nucleus in 1911. This discovery was instrumental in realizing that the atom was not a solid spherical structure.

Rutherford’s Atomic Model

Based on his findings about the atomic nucleus, from his famous gold-foil experiment, Rutherford put forth an atomic model that postulated:

◈ The atom is a hollow structure with its mass and positive charge concentrated into a tiny and dense core at the center, and the negatively charged lighter electrons orbiting the core like the planetary structure. Hence, this model is also called the planetary model.

◈ The electrons do not affect the pattern and trajectory of alpha particles.

◈ The atomic mass correlates with the charge of the atomic core or the atomic nucleus.

This model, however, had a very obvious limitation. Since the electron was a constantly accelerating particle of a negative charge, it would be attracted to the positive charge of the nucleus, thus causing the atom to become unstable and implode. To overcome this, in 1921, Rutherford with the help of Niels Bohr put forth a theory, hypothesizing the existence of a neutral-charged particle that had the same mass of a proton. The particle would be a composite of an electron and a proton and would be called a “neutron”. This theory, however, was not readily accepted by the scientific community due to lack of proof.

James Chadwick’s Contribution to the Atomic Theory

In 1930, Walther Bothe and Herbert Becker conducted experiments involving bombarding the element Beryllium with alpha particles emitted from radioactive polonium. They observed that the bombardment led to the emission of a neutral radiation from the Polonium, which was highly penetrative. They mistakenly believed this radiation to be a high energy form of gamma radiations. Hence, when Irène and Frédéric Joliot-Curie performed similar experiments in 1932, involving the emission of photons from paraffin and other such hydrogen-containing compounds when bombarded with this neutral radiation, they too believed that the radiation was a high-energy gamma radiation, and published results to that effect.

However, when Chadwick read the paper, he realized that a photon could not possibly be dislodged by a mere alpha particle. He, henceforth, conducted similar experiments of his own utilizing a linear amplifier, a refined polonium source, and an ionization chamber. He treated a number of substances and elements to this radiation and measured the recoil atom’s energy. He finally concluded that the radiation was, in fact, composed of neutral particles that had the same mass as protons. He called these neutral particles as neutrons. A paper to this effect was published by him in 1932, and shortly thereafter, other papers replicating the find were published by scientists like Norman Feather and Philip Dee.

Modified Atomic Theory

The existence and discovery of neutrons revolutionized the understanding of the atomic structure. It proved the validity of Rutherford’s atomic model and explained its stability. The postulates added to the atomic theory were:

◈ The nucleus of an atom consists of subatomic particles called nucleons.

◈ These nucleons are of two types: protons and neutrons.

◈ The neutrons are neutrally charged particles with the mass equal to that of a proton.

◈ A neutron is composed of an electron and proton couple.

◈ The collective mass of the protons and neutrons provides the atomic mass of an element.

German physicist Werner Heisenberg later, through his own experiments, proved that the neutrons were, in fact, a new particle and not a electron-proton composite.

This theory along with the discovery of neutrons was later instrumental in the invention of the atomic bomb. The atomic theory was later improved on by Niels Bohr to account for emission and absorption energy spectra observed in atoms, and a new model called the Niels Bohr model was hypothesized. This model is the presently accepted model to explain the atomic structure.

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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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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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National Museum of Nuclear Science & History

James Chadwick

Physicist los alamos, nm, manhattan, ny united kingdom, manhattan project veteran nobel prize winner scientist trinity test eyewitness.

Sir James Chadwick (1891-1974) was an English physicist and winner of the 1935 Nobel Prize in Physics.

James Chadwick began his academic studies at the University of Manchester under the tutelage of Ernest Rutherford. Here, he earned his B.S. in 1911 and his MSc in 1913. Chadwick then decided to take up research under Hans Geiger in Berlin. His intention was to study beta radiation, but shortly into this endeavor, World War I broke out and the German government imprisoned Chadwick in the Ruhleben internment camp for four years.

Aided by sympathetic German soldiers, Chadwick was able construct experiments even while imprisoned during the war. When the war came to a close, Chadwick returned to England and continued research under his previous advisor Ernest Rutherford at the Cavendish Laboratory at Cambridge University. There, he received his doctorate in 1921 and remained with the laboratory until he moved to the University of Liverpool in 1935.

SCIENTIFIC CONTRIBUTIONS

Chadwick is best known for his discovery of the neutron in 1932. A neutron is a particle with no electric charge that, along with positively charged protons, makes up an atom’s nucleus. Bombarding elements with neutrons can succeed in penetrating and splitting nuclei, generating an enormous amount of energy. In this way, Chadwick’s findings were pivotal to the discovery of nuclear fission, and ultimately the development of the atomic bomb. For more on Chadwick’s scientific contributions, please visit the  Nobel Prize website .

WORLD WAR II

Chadwick was a member of the British MAUD Committee, which concluded that the creation of nuclear weapons was possible and even inevitable. This supposition contributed towards President Roosevelt’s decision to build the atomic bomb. Additionally, Chadwick was an integral figure in the Tube Alloy Project—the codename for the British program to devise and develop nuclear weapons. His overtures to government officials in the UK and US were central to UK-US cooperation.

From 1943 to 1946, Chadwick headed the British Mission to the Manhattan Project. He also served as the technical advisor to the US-Canadian-UK Combined Policy Committee, which coordinated control of the project between the three nations involved. In 1944, Chadwick moved his family to the Project’s main research facility in Los Alamos. Finding the housing conditions distasteful, his twin daughters objected to the move, and so the family relocated to Washington D.C. where he continued to contribute to the Project’s efforts.

Chadwick formed a particularly congenial relationship with General Leslie Groves during the war. The two’s friendship aided British efforts to maintain strong support with the United States throughout the Manhattan Project. Chadwick was the only civilian—and non-American—allowed to access the entirety of the Manhattan Project’s research, data, and production plants.

Throughout the war, Chadwick drafted agreements to supply uranium for the Manhattan Project. Additionally, he observed the first atomic explosion, known as the Trinity test. Because of Chadwick’s insistence, British observers were allowed to be present at the bombing of Nagasaki.

Shortly after the war ended, Chadwick became an outspoken advocate for the United Kingdom to acquire a nuclear stockpile of its own. He was appointed to the British Advisory Committee on Atomic Energy (ACEA) and served as the UK delegate to the United Nations Atomic Energy Commission. Patrick Blackett , another Nobel laureate and eminent British scientist, vehemently opposed Chadwick’s argument for British atomic capability. Ultimately, though, Chadwick won out and the United Kingdom pursued its own nuclear arsenal.

Chadwick became the Master of Gonville and Caius College at Cambridge University in 1948. He resided in this administrative position for the remainder of his career. He took up retirement in North Wales beginning in 1958. He eventually moved back to Cambridge, where he passed away in 1974.

In addition to receiving the Nobel Prize in Physics, James Chadwick was knighted in 1945. He was awarded the US Medal of Merit in 1946 and the Copley Prize by the British Royal Society in 1950.

Additional Resources:

Excerpt from Great Lives from History Scientists and Science by Joseph Spradley

Chadwick’s Famous “Possible Existence of a Neutron”

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