How Neutron Discovery was a huge Milestone for Physics


In the earlier years of atomic physics, it was believed that atom was indivisible. But as soon as scientists discovered proton and electron, that notion was erased from everybody’s mind. During that time, British Physicist named James Chadwick continued his probe for atomic nucleus. During 1925, the concept of spin had given the physicists an idea to explain the Zeeman Effect in which a spectral line is split into many components under the influence of magnetic field, but it also created unexplained anomalies. During this period, it was believed that nucleus comprised protons and electrons. For many nuclei, it worked as it would give the right mass, but it would also give the wrong spin.

James Chadwick the man who discovered Neutron
James Chadwick By Los Alamos National Laboratory – Los Alamos National Laboratory, Attribution,

Experimentation in which polonium nucleus was bombarded into beryllium, boron and lithium

An experiment in which helium nucleus or the alpha-particle from a polonium nucleus was bombarded into beryllium, boron and lithium, there occurred an unusually penetrating radiation which was not at all influenced by electric field. Many supposed it to be gamma-rays, but the thing didn’t go down well with Rutherford and Chadwick. So, Chadwick performed the experiment and gave the inference that the radiation composed some uncharged particles with the same mass as that of proton. It was called Neutron. Chadwick’s discovery of neutron made it possible to produce elements heavier than uranium in the laboratory by the process of slow neutron capture followed by beta decay. Unlike the alpha-particles which are repelled by electrical forces due to their positive charges, the neutrons don’t need to be prevailed over any coulomb barrier, ultimately making it easy to penetrate the nuclei of even the heaviest elements, e.g., uranium.

Proton-neutron discovery paved the way for more clarification over nuclear spin, which didn’t have a clear picture before the discovery of neutron. Also, the origin of beta radiation could easily be explained by Enrico Fermi. During beta radiation, a neutron gets decayed from a nucleus having more number of neutrons, which creates difficulty in nuclear stability. So, a neutron decays into a proton, also releasing negative beta particle ( e) and anti-neutrino which helps in making the balance of spin on both sides. The reaction is as follows:

                0n1          →      1p1 + e+ υ

This radioactive decay can be possible as mass of neutron is just slightly greater than mass of proton.

Uranium 235 By, Public Domain,

What are Free Neutrons:

  • Free neutron weighs 939,565,413.3 eV/c2 or 1.674927471x 10-27 kg. Free neutrons have a -1/2 spin and they are fermions.
  • They don’t get affected by electric fields as they don’t have any electrical charge, but they get influenced by the presence of magnetic fields as they have their own magnetic moment having a negative value. That’s because of its opposite orientation to the spin of neutrons.
  • Free neutrons are unstable hence they disintegrate to proton, beta particle and anti-neutrino as mentioned before.
  • Neutrons also fall in the category of hadrons (also baryons) as they are composite particles made up of elementary particles called quarks. The zero charge of neutron is because of its being made up of two down quarks (-1/3 e charge) and one up quark (+2/3 e charge). They sum up to give,

+2/3 e + (-1/3 e) + (-1/3 e) = 0 e, which is the net charge of neutrons. These quarks are held together by strong nuclear force.

(neutron as a sum of quarks -picture)

  • Earth’s crust also helps in some way to produce free neutrons by natural radioactive disintegration of spontaneously fissionable elements. Also the cosmic ray bursts from interstellar space cause a small flow of free neutrons that exists on our earth.

Role of Neutrons in Radioactive Decays:

  • Frideric and Irene Joilot-Curie discovered induced radioactivity in 1934 by bombarding materials with neutrons, as a consequence of which new radium-like elements were discovered.

Chain Reaction:

The most important feature of neutron-induced fission is the chain reaction. When a heavy nuclide (e.g. U235) absorbs a neutron during nuclear fission, this absorption makes the nuclide unstable, ultimately breaking into lighter nuclides and additional neutrons.

                    U235 + ­0n1       →          Ba141 + Kr92 + 3 ­0n1

 If the fission event occurs near the surface of bulk uranium material, then there lies a good chance that the produced neutrons will escape from the material without coming across any other U235.

The fraction of the neutrons lost here will get larger provided smaller pieces of uranium are taken. Controlling the size will indeed dominate the rate of fission.

A possible nuclear fission chain reaction.
Uranium Chain reaction By User:Fastfission – Own work, Public Domain,
  1. Another important point is that the neutrons produced in nuclear fission have kinetic energy of almost 2 MeV. These are called fast electrons.
  2. Fast neutrons being captured by U235 is a very low probability, but it has a high probability of capturing slow neutrons which have a kinetic energy of approximately 0.04 eV. The fast neutron won’t get absorbed even if it collides with U235. In each collision, it will lose some part of its kinetic energy and in the end, may slow down to thermal energies of approximately 0.04 eV. Now it can be absorbed by the Uranium-235 nucleus.

Sources of Neutron for Research:

  • Neutrons are used in radioactive disintegration in order to power neutron production.
  • Californium-252 is used as a source of neutron for the process.

Application of Neutrons:

  • Of course, neutrons are used in nuclear reactions for chain reaction.
  • Radioactivity can be induced using neutron capture process.
  • Neutrons perform an important role in enhancing nuclear reactors and also nuclear weapons.
  • Neutrons have a very useful role to play in medical therapies for its penetrating and ionizing nature. So, they can be used for exploiting for medical treatments.
  • Neutrons are utilized in a process called Neutron Tomography in which the three-dimensional information of a given sample is obtained by the detection of absorbance of neutrons created by a neutron source.
Nuclear Reactor

Core of CROCU By Rama – Own work, CC BY-SA 2.0 fr,


  • As neutron has a more penetrating power than other particles like alpha-particle or beta-particle or even gamma-rays, the exposure to its radiation can be really dangerous or even fatal.
  • Its high kinetic energy is also the reason behinds its danger.
  • Another reason is that once a neutron reacts with nuclei of some other atoms, a new isotope will be formed, simultaneously triggering more neutron radiation, as in a chain reaction.

So, it can be opined that the discovery of neutron was somehow a very big milestone for Modern Physics as it gave a whole lot of new options to the particle physicists as well as the nuclear physicists. But it can also be intimidating as it can be misused in creating atom bombs or who knows a neutron bomb!

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Author: Som Abhisek.

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