Neutrons are tiny subatomic particles which, together with protons, form the nucleus of a atom.
While the number of protons defines the element of an atom, the number of neutrons in the nucleus can vary, resulting in different isotopes of an element. For example, ordinary hydrogen contains one proton and no neutrons, but the isotopes of hydrogen, deuterium, and tritium have one and two neutrons next to the proton, respectively.
Neutrons are composite particles made up of three smaller elementary particles called quarksheld together by the A mighty force. More precisely, a neutron contains an “up” and two “down” quarks. Particles made up of three quarks are called baryons, and so baryons contribute to all baryonic “visible” matter in the universe.
Related: What is the theory of everything?
Who discovered neutrons?
After Ernest Rutherford (with the help of Ernest Marsden and Hans Geiger) had discovered in 1911 that atoms had a nucleus, then nine years later discovered that atomic nuclei are made, at least in part, of protons, the discovery of the neutron in 1932 by James Chadwick naturally followed.
The idea that there must be something else in the nucleus of an atom came from the fact that the number of protons did not correspond to the atomic weight of an atom. For example, an oxygen atom contains 8 protons, but has an atomic weight of 16, suggesting that it contains 8 other particles. However, these mysterious particles should be electrically neutral, since atoms normally have no overall electrical charge (the negative charge of electrons cancels out the positive charge of protons).
At the time, various scientists were experimenting alpha particles, which are another name for helium nuclei, bombarding material made from the element beryllium with a stream of alpha particles. When the alpha particles collided with beryllium atoms, they produced mysterious particles that appeared to originate from within the beryllium atoms. Chadwick took these experiments a step further and saw that when the mystery particles hit a target made of paraffin wax, they detached high-energy protons. To do this, Chadwick explained, the mystery particles must have more or less the same mass as a proton. Chadwick proclaimed this mysterious particle to be the neutron, and in 1935 he won a Nobel Prize for his discovery.
Neutrons: Mass and Charge
As their name suggests, neutrons are electrically neutral, so they have no charge. Their mass is 1.008 times the mass of the proton, that is, it is about 0.1% heavier.
Neutrons don’t like to exist alone outside the nucleus. The strong force binding energy between them and the protons in the nucleus keeps them stable, but when alone they experience beta decay after about 15 minutes, transforming into a proton, an electron, and an antineutrino.
Albert Einstein, in his famous equation E = mc2, said that mass and energy are equivalent. Although the mass of a neutron and a proton are only slightly different, this slight difference means that a neutron has more mass, and therefore more energy, than a proton and an electron combined. This is why when a neutron decays, it produces a proton and an electron.
Isotopes and radioactivity
An isotope is a variation of an element that has more neutrons. For example, at the top of this article we gave the example of hydrogen isotopes deuterium and tritium, which have 1 and 2 extra neutrons respectively. Some isotopes are stable, for example deuterium. Others are unstable and inevitably undergo radioactive decay. Tritium is unstable – it has a half-life of about 12 years (a half-life is the average time it takes for half of a given amount of an isotope like tritium to decay), but other isotopes decay much faster, within minutes, seconds, or even fractions of a second.
Neutrons are also essential tools in nuclear reactions, especially when inducing a chain reaction. Neutrons absorbed by atomic nuclei create unstable isotopes which then undergo nuclear fission (dividing into two smaller daughter nuclei of other elements). For example, when uranium-235 absorbs an additional neutron, it becomes unstable and falls apart, releasing energy in the process.
Neutrons are also instrumental in the creation of heavy elements in massive stars, through a mechanism known as the r-process, “r” meaning “fast”. This process was first detailed in the famous Nobel Prize-winning B2FH paper by Daisy and Geoffrey Burbidge, William Fowler and Fred Hoyle which describes the origins of the elements through stellar nucleosynthesis – the forging of the elements by the stars.
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Stars As the sun can produce oxygen, nitrogen and carbon elements by nuclear fusion reactions. After massive stars can go on and create shells of heavier and heavier elements up to 56 iron in the star’s core. At this point, the reactions require more energy to fuse elements heavier than iron than is actually produced by these reactions, so these reactions cease, energy production stops, and the nucleus of the star collapses, triggering a supernova. And it’s in the incredibly violent blast of a supernova that conditions can become extreme enough to release lots of free neutrons in a short period of time.
In the supernova explosion, the atomic nuclei are then able to sweep away all those free neutrons before they all decay (which is why it is described as fast), to trigger r-process nucleosynthesis. Once the nuclei are full of neutrons, they become unstable and undergo beta decay, turning those extra neutrons into protons. Adding these protons changes the type of element a nucleus is, so it’s a way of create new heavy elements such as gold, platinum and other precious metals. The gold in your jewelry was made billions of years ago by the rapid capture of neutrons in a supernova!
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As we have seen, only under the most extreme conditions can neutrons survive outside of atomic nuclei, and there are very few places in the universe more extreme than neutron stars. As their name suggests, they are objects made up almost entirely of neutrons.
Neutron stars are what remains of a star’s core after it collapses and explodes into a supernova. The explosion may have blown away the outer layers of the star, but the contracting core remains intact.
In the absence of nuclear reactions to generate energy to counteract gravity, the mass of the nucleus is so great that it undergoes catastrophic gravitational collapse in which the gravitational pressure is great enough for protons and electrons to be able to overcome the electrostatic force between them and mix. , merging to form neutrons in a sort of inverse beta decay. Almost all of the atoms in the nucleus turn into neutrons, which is why we call the result a neutron star. They are small, only 6 to 12 miles (10 to 20 km) in diameter, but they contain all the mass of the dead star’s core.
The most massive neutron star ever discovered has a mass 2.35 times bigger than our sun, all crammed into a very small volume. If you could take the equivalent of a spoonful of material from the surface of a neutron star, that spoonful would weigh as much as a mountain on Earth!
Binary neutron star mergers, which are detectable as kilonovae and via their gravitational waves, are also sites of abundant r-process nucleosynthesis. The kilonova of two merging binary stars that released the burst of gravitational waves GW170817 produces 16,000 times the mass of the Earth in the form of heavy r-process elements, including ten Earth masses of gold and platinumwhich is extraordinary!
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Learn more about neutrons with the US Department of Energy (opens in a new tab). Discover how neutrons are used in experiments that study condensed matter with the British Science and Technology Facilities Council (opens in a new tab). Read it famous B2FH paper (opens in a new tab) on creating elements inside stars using neutron capture.
Particle Physics, by Brian R. Martin (2011, One World Publications) (opens in a new tab)
The Cambridge Encyclopedia of Stars, by James R. Kaler (2006, Cambridge University Press) (opens in a new tab):
Internet-Linked Collins Physics Dictionary (2007, Collins) (opens in a new tab)
This month in the history of physics. American Physical Society Sites, APS News, Volume 16, Number 5. Accessed December 1, 2022, from https://www.aps.org/publications/apsnews/200705/physicshistory.cfm (opens in a new tab)
Neutron decay. Direct sciences. Accessed December 1, 2022, from https://www.sciencedirect.com/topics/physics-and-astronomy/neutron-decay (opens in a new tab)
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