October 20th marks the birthday of James Chadwick, the pioneering physicist who discovered the neutron. Here, we pay tribute to the 1935 Nobel Prize winner in Physics, whose groundbreaking work revolutionized our understanding of atoms and paved the way for significant advancements in nuclear science and technology. 

Filling the Gap: The Hunt for a Neutral Particle 

Following the discovery of the nucleus-shell structure, it was initially thought that the nucleus consisted solely of protons. However, discrepancies in mass and charge hinted at another component. While the presence of core electrons was initially proposed, this was ruled out by Heisenberg's Uncertainty Principle.  

Initial Theories and Rutherford’s Hypothesis

Ernest Rutherford, a pioneer in nuclear physics, postulated the existence of a neutral particle with a mass comparable to that of a proton as early as 1920.

The first significant strides toward the discovery of the neutron were made by Walther Bothe and his student Herbert Becker. In 1930, they observed an unusual, high-energy type of radiation when bombarding beryllium with alpha particles, initially mistaking it for gamma rays. Although this “beryllium radiation” exhibited an impressive ability to penetrate matter, its behavior raised questions about whether it was indeed gamma rays.

Joliot-Curie Experiments Reveal New Insights

Further experiments by Irène Joliot-Curie and Frédéric Joliot-Curie a year later showed that the newly discovered "beryllium radiation" did not produce a significant current when passing through an ionization chamber. However, placing a hydrogen-containing material like kerosene in front of the chamber increased the current dramatically. The pair assumed that the radiation was releasing protons from the kerosene, which ionized the gas in the chamber. Their observations of recoil protons in a Wilson cloud chamber confirmed this hypothesis. While initially considering the Compton effect as a possible mechanism, they ultimately realized that the required gamma energy would be unrealistically high to produce the observed proton tracks.

Chadwick Redefines Atomic Theory with Neutron Discovery 

Building on these findings, James Chadwick conducted experiments that conclusively disproved Bothe’s thesis, demonstrating that the "beryllium radiation" did not constitute gamma rays. By measuring the momentum and energy of particles produced when beryllium was bombarded with alpha particles, he was able to show that the radiation was actually a stream of fast-moving, electrically neutral particles with a mass close to that of a proton. This observation aligned with Rutherford’s earlier hypothesis, leading Chadwick to name these particles “neutrons”. 

A Paradigm Shift in Particle Physics 

Chadwick's breakthrough marked a significant change in particle physics. It provided a more complete understanding of atomic structure at the time: The atomic nucleus, composed of protons and neutrons, is encircled by a shell of electrons. In an electrically neutral atom, the number of negatively charged electrons in the electron shell exactly matches the number of positively charged protons in the nucleus, while the number of neutrons in the nucleus can vary. 

His groundbreaking work earned James Chadwick the Nobel Prize in Physics in 1935. 

A Discovery with Far-Reaching Consequences  

Chadwick’s work had several immediate and profound implications. Unlike protons and alpha particles, which face the Coulomb barrier, neutrons can penetrate atomic nuclei even at low speeds, enabling new avenues of research. What followed was a wave of excitement among particle physicists, prompting them to explore the effects of this "new" radiation on various materials. In 1942, Enrico Fermi achieved a groundbreaking milestone by constructing the first man-made self-sustaining nuclear fission chain reaction (Chicago Pile 1). 

Nuclear fission, or the process of splitting atomic nuclei, has enabled a wide range of applications, both civilian and military: 

• The generation of electricity in nuclear power plants is a clean and reliable source of energy that can help reduce greenhouse gas emissions. 

• In medicine nuclear fission can be used to produce isotopes for diagnostic imaging and cancer treatment. 

• Isotopes are also used for industrial applications, such as the sterilization of medical equipment and food, or for research purposes. 

• On the other hand, the devastating consequences of the atomic bombings of Hiroshima and Nagasaki during World War II spurred international efforts to control the proliferation of nuclear weapons and prevent their future use in conflict. 

Neutron physics is a cornerstone of modern physics, providing essential insights into the fundamental building blocks of matter and the forces that govern their interactions. The unique characteristics of neutrons allow scientists to investigate the inner structure of materials in detail, leading to advancements in fields such as condensed matter physics, materials science, and nanotechnology.

Materials science engineer Charlie Briggs from Goodfellow explains:

Neutrons in the Universe:
Neutron Stars and Pulsars  

Neutrons play a crucial role in the life cycles of neutron stars and pulsars: 

When a massive star exhausts its nuclear fuel, it can undergo a supernova explosion, leaving behind a dense core. If this core's mass is between about 1.4 and 3 times that of the Sun, it collapses into a neutron star. In this state, protons and electrons combine to form neutrons, resulting in an incredibly dense object composed almost entirely of neutrons. Neutron stars are incredibly compact, with a radius of about 10 kilometers, yet they contain more mass than the Sun. 

Pulsars are a type of neutron star that emit beams of electromagnetic radiation from their magnetic poles. As the neutron star rotates, these beams sweep across space, and if they point towards Earth, they can be detected as regular pulses of radiation, hence the name "pulsar." The study of neutron stars and pulsars provides valuable insights into the behavior of matter under extreme conditions, the nature of gravitational fields, and the fundamental properties of neutrons themselves.

Sources:  

• Possible Existence of a Neutron | Nature
• James Chadwick and the Neutron | ChemTalk (chemistrytalk.org)
• 80 years since discovery of the neutron - News - University of Liverpool
• The discovery of the neutron and its consequences (1930–1940) - ScienceDirect
• This Month in Physics History | American Physical Society (aps.org)