Neutron Definition

Angel Zamora Ramirez
Degree in physics

Neutrons are subatomic particles without an electrical charge that, together with protons, are part of atomic nuclei. The neutron belongs to the family of baryons, since it is made up of three quarks, it can interact through the strong nuclear force and it obeys the Pauli Exclusion Principle.

In the atomic nucleus, neutrons play a fundamental role. The protons of the atomic nucleus, in addition to interacting through the strong nuclear force, also repel each other electrically because they have charges of the same sign. Neutrons provide an extra strong nuclear force that prevents protons from shooting out of the atomic nucleus and thus provides stability. Neutrons are also key in the production of energy through nuclear fission since they are the initiators of the process and of the chain reaction.

discovery of the neutron

After his experiment with gold foil, Rutherford in 1911 published his conception of the structure of the atom, which in the future would be known as the “Rutherford Model”. This is the classic model that we all have of the atom, a nucleus around which the electrons revolve in a similar way to how the planets orbit the Sun, which is why this model is often also known as the “Planetary Model of the Atom”.

According to Rutherford, positively charged protons were found in the nucleus and negatively charged electrons revolved around them with the help of electrical force. However, this Rutherford model had some problems. First, the protons confined in the atomic nucleus would have to shoot out due to the electrical repulsion between them, otherwise the atom would not even exist. And second, in the case of the hydrogen atom, its mass was the sum of the mass of the proton and the electron, however, as it was scaled up in the periodic table, they realized that the experimental mass of the atoms was greater than the mass of the sum of the number of protons and the number of electrons.

Seeing this problem, Rutherford in 1920 proposed that there should be a particle with a mass similar to that of the proton and that it was equally confined in the atomic nucleus. In addition, this particle had to lack an electric charge so as not to break the neutral electric charge of the atom. Due to this last characteristic, the American physicist William Draper Harkins coined the name “neutron” to refer to this hypothetical particle.

Later, in 1930, German scientists Walther Bothe and Herbert Becker discovered that by bombarding some light elements, especially beryllium, with alpha particles from polonium, they emitted a very penetrating type of radiation. At first it was thought that it was gamma radiation, however, making measurements they realized that this new type of radiation was much more penetrating.

Around 1932, Irene Joliot-Curie and Fréderic Joliot observed that when radiation from beryllium hit a material rich in hydrogen, such as paraffin, it emitted hydrogen nuclei (protons). With this, the hypothesis that the penetrating radiation emitted by the beryllium were gamma rays was ruled out. But in addition to this, when measuring the energy of the ejected protons, they realized that to achieve this, the unknown incident radiation must have an energy 10 times greater than that previously measured. This seemed to indicate a violation of the conservation of energy and linear momentum.

At the end of 1932 the English physicist James Chadwick was based on these results to carry out a series of experiments similar to those of the Joliot-Curie. Chadwick measured the energy of the ejected protons by observing the ionization they produce. Once this was done, Chadwick hypothesized that this unknown radiation was actually corpuscular in nature and the ejected protons were the result of elastic collisions between the corpuscles and the protons, thus ensuring the conservation of energy and linear momentum.

Applying the laws of conservation of energy and linear momentum and knowing the speeds of the ejected protons, Chadwick was able to determine the mass of the corpuscles that made up this type of radiation and realized that they had a value very similar to the mass of the proton. The characteristics of this new particle were practically the same as the neutron proposed by Rutherford. In this way James Chadwick discovered the neutron and his discovery earned him the Nobel Prize in Physics in 1945.

The neutron and nuclear fission

After the discovery of the neutron, many physicists around the world began to carry out experiments with them. In 1938 the German chemists Otto Hahn and Fritz Strassmann were carrying out experiments with uranium, particularly bombarding uranium with neutrons to obtain new elements by capturing neutrons or emitting other particles. Instead they found barium, an element much lighter than uranium. None of the nuclear processes known up to the time could explain this.

A short time later, Lise Meitner and Otto Frisch solved this puzzle and discovered the process of nuclear fission. Some isotopes like Uranium-235 fragment when hit by a neutron, in the case of Uranium-235 Barium, Krypton, neutrons are produced and energy is released. Hahn and Fritz had then observed the fission of Uranium-235.

Later, it was discovered that the neutrons generated in a nuclear fission process could hit other nuclei causing other fission processes and generating more neutrons that would fission other nuclei and so on. This chain reaction is the principle under which nuclear fission reactors work, but it was also the motivation to build the atomic bomb.



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The discovery of the neutron (1): the great dilemma, Scientific Culture