Neutron

The neutron is a subatomic particle with no electric charge and a mass slightly greater than that of the proton. Together with protons, neutrons form the nuclei of atoms and are central to the structure, stability and transformations of matter. Their discovery in the early twentieth century revolutionised nuclear physics, enabling the development of nuclear fission, nuclear reactors and nuclear weapons, while also deepening scientific understanding of atomic structure and stellar processes.

Properties and Structure

Neutrons belong to the family of baryons and are not elementary particles. Each neutron is composed of three quarks bound together by the strong interaction. Although electrically neutral, the neutron possesses a magnetic moment due to the internal arrangement of its charged constituents. A free neutron is unstable and undergoes beta decay with a mean lifetime of about fifteen minutes, transforming into a proton, an electron and an antineutrino.
Inside atomic nuclei, neutrons are bound by the nuclear force. The balance between protons and neutrons determines nuclear stability: atoms of the same element with different numbers of neutrons are known as isotopes. Neutrons also participate in processes such as neutron capture, fission and fusion, playing a major role in the nucleosynthesis of elements in stars.

Discovery

The identification of the neutron in 1932 by James Chadwick marked a major milestone in physics. The concept of a neutrally charged nuclear particle had been suggested earlier, including by Ernest Rutherford in 1920, who proposed a composite of a proton and an electron inside the nucleus. However, developments in quantum theory during the late 1920s made the proposed protonelectron model untenable. The confinement of electrons within the tiny volume of the nucleus violated emerging principles of quantum mechanics, and nuclear spin observations were inconsistent with such a model.
Experimental work progressed during the early 1930s. Walther Bothe and Herbert Becker discovered unusually penetrating radiation when alpha particles struck light elements, leading them to propose gamma radiation. Irène and Frédéric Joliot-Curie later showed that this radiation could eject high-energy protons from hydrogen-rich materials, a result that puzzled researchers. Chadwick reinterpreted the effect as evidence for a neutral particle with a mass similar to the proton. His careful experiments demonstrated that the radiation consisted of neutrons, for which he received the Nobel Prize in Physics in 1935.

Impact on Nuclear Physics

The recognition of the neutron as a fundamental constituent of nuclei enabled the rapid development of modern nuclear theory. Models of nuclear structure based on protons and neutrons were formulated by Werner Heisenberg and others, clarifying nuclear properties such as spin and binding energy. Enrico Fermi explained beta decay in 1934, attributing it to the transformation of a neutron into a proton accompanied by an electron and a neutrino.
Neutron interactions opened new avenues for experimental research. Because neutrons are electrically neutral, they can penetrate atomic electron clouds and strike nuclei directly. Fermi’s experiments using neutron bombardment in the 1930s revealed new radioactive isotopes and nuclear reactions, particularly those involving slow neutrons.
In 1938 Otto Hahn, Lise Meitner and Fritz Strassmann discovered nuclear fission when neutron irradiation caused uranium nuclei to split into lighter elements. This finding laid the groundwork for nuclear power and nuclear weapons. The first self-sustaining nuclear chain reaction was achieved in 1942 with Chicago Pile-1, engineered under Fermi’s direction. The first nuclear weapon was tested in 1945 during the Trinity test.

Neutrons in Nature

Within atomic nuclei, neutrons contribute to binding energy and stability. The lightest hydrogen isotope, protium, contains no neutrons, whereas deuterium and tritium contain one and two neutrons respectively. Heavier elements typically require increasingly larger numbers of neutrons to remain stable. For example, the common isotope of lead, 208Pb^{208}\mathrm{Pb}208Pb, has 82 protons and 126 neutrons.
Beyond Earth, the most dramatic manifestation of neutrons is found in neutron stars. These extraordinary objects are formed from the collapsed cores of massive stars. They consist of matter compressed to densities comparable to atomic nuclei, yet their total mass exceeds that of the Sun. The behaviour of neutrons under such extreme conditions provides insight into fundamental physics.
A small natural background flux of free neutrons exists on Earth due to cosmic rays and spontaneous fission in the crust. Free neutrons are short-lived but are continually generated by these processes.

Applications and Hazards

Neutrons are essential in the operation of nuclear reactors and the production of nuclear energy. Controlled chain reactions rely on the moderation and absorption of neutrons to maintain stable power generation. Artificial neutron sources—including research reactors, neutron generators and spallation sources—produce free neutrons for scientific experiments, industrial applications and medical therapies.
Neutron scattering is a powerful tool for studying materials, revealing structural and magnetic properties that are difficult to observe using photons or charged particles. In medicine, certain forms of neutron radiation can be employed in specialised cancer treatments.
Although neutrons do not directly ionise atoms, they can cause ionising radiation indirectly by interacting with nuclei. Exposure to high neutron doses poses biological risks, including tissue damage and increased cancer probability. For this reason, shielding and radiation monitoring are essential in nuclear science and industry.

Neutrons in Atomic Structure

An atomic nucleus is defined by its number of protons (the atomic number) and its number of neutrons (the neutron number). These numbers determine the isotope and influence the nuclide’s stability, decay pathways and interaction with other particles. Terminology associated with nuclear classification includes:

• Isotopes, which share the same number of protons but differ in neutron number.
• Isotones, which share the same number of neutrons but differ in proton number.
• Isobars, which have the same mass number (total protons plus neutrons) but different compositions.

The mass of a nucleus is slightly less than the total mass of its constituent nucleons due to binding energy, a relationship explained by mass–energy equivalence. Comprehensive data on all known nuclides, including the neutron as a special case, are presented in the table of nuclides.
Neutrons therefore occupy a central place in both the microscopic structure of matter and the macroscopic evolution of stars. Their discovery and subsequent study have shaped modern physics, chemistry and technology, demonstrating the profound role of subatomic particles in the broader understanding of the universe.