What are Neutrons?

Neutrons are subatomic particles that are one of the primary constituents of atomic nuclei. They are usually denoted by the symbol n or no. Neutrons do not have any net electric charge associated with them. They do, however, have a mass which is slightly greater in magnitude than that of a proton. Neutrons and protons are collectively referred to as nucleons since they behave in a similar manner inside the nuclei of atoms. The mass of a neutron can be roughly approximated to one atomic mass unit (often abbreviated to amu). The branch of science that deals with the study of the properties of neutrons and the interactions of these subatomic particles with other matter and electromagnetic radiation are called nuclear physics.


The overall nuclear and chemical properties of an element are usually determined by the total number of protons in its atomic nucleus (atomic number) and the total number of neutrons in its atomic nucleus (usually referred to as the neutron number). The sum of the total number of protons in an atomic nucleus and the total number of neutrons in the atomic nucleus yields the mass number of that atomic nucleus. It is important to note that different isotopes of the same element share the same atomic number but differ in their mass numbers (which implies that they all contain the same number of protons in their atomic nuclei but vary in the total number of neutrons that are present in their nuclei).

Inside the nucleus of the atom, the protons and the neutrons are bound together via nuclear forces. For the stability of atomic nuclei, the presence of neutrons is a must. The only exception to this rule is the protium (hydrogen-1) nucleus. One of the most important applications of neutrons is in nuclear reactors to facilitate nuclear fission reactions and in some cases, nuclear fusion reactions.

Discovery of Neutrons

  • Neutrons were first theorized by the New Zealand born British physicist Ernest Rutherford in the year 1920.
  • The discovery of neutrons is credited to the British physicist James Chadwick in the year 1932.
  • He was awarded the Nobel prize in physics for this discovery in the year 1935.

During the 1920s, the common assumption on the nature of atoms was that they consisted of protons and also nuclear electrons. However, this failed to comply with the Heisenberg uncertainty relation in quantum mechanics. In the year 1931, two German nuclear physicists observed that when the alpha particle radiation that is emitted by polonium is made incident on beryllium, lithium, or boron, it resulted in the production of an unusually penetrating form of radiation. Later, it was proven by James Chadwick through a series of experiments that these particles that constituted the unusually penetrating radiation were neutrons.

Click here to learn more about the discovery of neutrons and protons.

Charge and Mass of Neutrons

  • The electric charge that is associated with a neutron is 0. Therefore, neutrons are neutrally charged subatomic particles.
  • The mass of a neutron is roughly equal to 1.008 atomic mass units. When converted into kilograms, the mass of the neutron can be approximated to 1.674*10-27 kg.
  • Since neutrons lack electric charge, their mass cannot be directly determined via the analytical technique of mass spectrometry.
  • The mass of the neutron can be calculated by subtracting the mass of a proton from the mass of a deuterium nucleus (deuterium is an isotope of hydrogen containing one proton, one electron, and one neutron in its atomic structure. Since the mass of the electron is negligible when compared to that of the proton and the neutron, the mass of the neutron can be calculated by subtracting the mass of the proton from the mass of the deuterium atom).

Properties of Neutrons

Despite the fact that the neutron is considered to be a neutral particle, the magnetic moment of neutrons is not equal to zero. Even though electric fields have no effects on neutrons, these subatomic particles are affected by the presence of magnetic fields. The magnetic moment associated with the neutron can be considered as an indication of its quark substructure and the distribution of its internal charges. The actual value which can be associated with the neutron’s magnetic moment was directly measured first at Berkeley, California, in the year 1940 by Luis Alvarez and Felix Bloch.

Applications of Neutrons

In several nuclear reactions, the subatomic particle known as the neutron plays a significant role. Neutron capture, for example, often results in the activation of neutrons which, in turn, induces radioactivity. Knowledge of neutrons and their activity has been especially important in the past for the development of many nuclear reactors (and also several nuclear weapons). The nuclear fissioning of such elements as uranium-235 and plutonium-239 is almost always caused by their neutron absorption.

Warm, cold, and hot neutron radiation has a very important application in neutron scattering facilities, where the radiation is also used in condensed matter research with the help of X-rays. When it comes to atomic comparisons, the neutrons are complementary to the latter via different scattering cross sections, their susceptibility to magnetism, their energy spectrum for inelastic neutron spectroscopy, and finally, their deep penetration into matter.

One of the most significant applications of neutrons is in the excitation of delayed and triggered gamma rays from material components. This forms the basis for the study of neutron activation analysis, often abbreviated as NAA. It also forms the basis for the study of the prompt gamma neutron activation analysis (usually abbreviated as PGNAA). NAA is most widely used to analyze small samples of materials in a nuclear reactor while PGNAA is most often used to examine subterranean rocks on conveyor belts around boreholes and industrial bulk materials.

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