Neutrons: Facts about the powerful subatomic particles
Neutrons are small subatomic particles that together with protons form the nucleus of atom.
While the number of protons determines what element it is atom well, 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, respectively, along with a proton.
Neutrons are composite particles made up of three smaller, elementary particles called quarksheld together by Strong force. Specifically, a neutron contains one “up” and two “down” quarks. Particles made of three quarks are called baryons, and therefore baryons contribute to all baryonic “visible” matter in universe.
Connected: What is the theory of everything?
Who discovered neutrons?
After Ernest Rutherford (with the help of Ernest Marsden and Hans Geigerthe gold leaf experiment) discovered in 1911 that atoms have 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 comes from the fact that the number of protons does not correspond to the atomic weight of the atom. For example, an oxygen atom contains 8 protons, but has an atomic weight of 16, which suggests that it contains 8 other particles. However, these mysterious particles must be electrically neutral, as atoms usually have no overall electrical charge (the negative charge of electrons cancels out the positive charge of protons).
At the time, various scientists were experimenting with alpha particles, which are another name for helium nuclei bombarding material made of the element beryllium with a stream of alpha particles. When alpha particles collide with beryllium atoms, they produce mysterious particles that appear to originate from 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 would knock out high-energy free protons. For this to happen, Chadwick reasoned, the mystery particles must have more or less the same mass as a proton. Chadwick declared this mysterious particle to be a neutron and in 1935 won the 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 – in other words, it is roughly 0.1% heavier.
Neutrons do not like to exist alone outside the nucleus. The binding energy of the Force between them and the protons in the core keeps them stable, but when out on their own they undergo 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 the neutron and proton differ only slightly, this small difference means that the neutron has more mass and therefore more energy than the proton and electron combined. Therefore, 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 beginning of this article, we gave an example of the hydrogen isotopes deuterium and tritium, which have 1 and 2 extra neutrons, respectively. Some isotopes are stable, deuterium for example. Others are unstable and inevitably undergo radioactive decay. Tritium is unstable – it has a half-life of about 12 years (half-life is the time it takes on average for half of a given amount of an isotope like tritium to decay), but other isotopes decay much faster, in a matter of minutes, seconds or even fractions of a second.
Neutrons are also essential tools in nuclear reactions, especially when causing a chain reaction. Neutrons absorbed by atomic nuclei create unstable isotopes, which then undergo nuclear fission (cleavage into two smaller daughter nuclei of other elements). For example, when uranium-235 absorbs an extra neutron, it becomes unstable and decays, releasing energy in the process.
Neutrons are also instrumental in creating heavy elements in massive stars, through a mechanism known as the r-process, where the “r” stands for “fast.” This process was first described in the famous Nobel Prize-winning paper B2FH Margaret and Geoffrey Burbidge, William Fowler and Fred Hoyle which describes the origin of elements through stellar nucleosynthesis – the forging of elements from stars.
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Stars like the sun can produce elements from oxygen, nitrogen and carbon nuclear fusion reactions. | More ▼ massive stars it can continue and create shells of heavier and heavier elements all the way up to iron-56 in the star’s core. At that point, the reactions require more energy to be put into them to fuse elements heavier than iron than is actually produced by those reactions, so those reactions stop, energy production stops, and the star’s core collapses. causing supernova. And it is in the incredibly violent supernova explosion that conditions can become extreme enough to release many free neutrons in a short time.
In a supernova explosion, the atomic nuclei are then able to absorb all of these free neutrons before they all decay (hence why it is described as rapid) to trigger the r-process nucleosynthesis. Once nuclei are full of neutrons, they become unstable and undergo beta decay, transforming those extra neutrons into protons. Adding these protons changes the type of element that is the nucleus, so it’s a way to creating 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!
Neutron stars
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As we have seen, only under the most extreme conditions can neutrons survive outside atomic nuclei, and there are very few places in the universe more extreme than neutron stars. As their name suggests, these are objects made almost entirely of neutrons.
Neutron stars are what is left of the core of a star after it has undergone core collapse and exploded as a supernova. The explosion may have blown away the outer layers of the star, but the shrinking core remains intact.
Without nuclear reactions to generate energy to counteract gravity, the mass of the nucleus is so great that it undergoes a catastrophic gravitational collapse, in which the gravitational pressure is great enough for the protons and electrons to overcome the electrostatic force between them and collide together , fusing to form neutrons in a type of inverse beta decay. Almost all the atoms in the core become neutrons, which is why we call the result a neutron star. They’re tiny, only 6-12 miles (10-20 km) across, but they pack in 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 small volume. If you could scoop up a spoonful’s worth of material from the surface of a neutron star, that spoonful would weigh as much as a mountain on Earth!
Neutron star binary mergers that are detected as kilobytes and through their gravitational waves are also sites of abundant r-process nucleosynthesis. The kilonova of two merging binary stars that released a burst of gravitational waves GW 170817 produced 16,000 times the mass of Earth in the form of r-process heavy elements, including ten Earth masses worth gold and platinumwhich is extraordinary!
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Additional resources
Learn more about neutrons with US Department of Energy (opens in new tab). Explore how neutrons are used in experiments that study condensed matter with UK Science Technology Facilities Council (opens in new tab). Read on known paper B2FH (opens in new tab) for the creation of elements inside stars using neutron capture.
Bibliography
Particle Physics, by Brian R. Martin (2011, One-World Publications) (opens in new tab)
The Cambridge Encyclopedia of the Stars, by James R. Kahler (2006, Cambridge University Press) (opens in new tab):
Collins Associated Internet Dictionary of Physics (2007, Collins) (opens in 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 new tab)
Neutron decay. ScienceDirect. Accessed December 1, 2022 by https://www.sciencedirect.com/topics/physics-and-astronomy/neutron-decay (opens in new tab)
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