Neutrons: Facts About the Influential Subatomic Particles
Neutrons are small subatomic particles that, together with protons, form the nucleus of a atom.
While the number of protons defines which element an atom that is, 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, next to the proton.
Neutrons are composite particles made up of three smaller elementary particles called quarksunited by Strong Force. Specifically, a neutron contains one “up” and two “down” quarks. Particles made of three quarks are called baryons, and so baryons contribute to all the baryonic “visible” matter of the universe.
Related: What is the theory of everything?
Who discovered neutrons?
After Ernest Rutherford (with the help of Ernest Marsden i Hans Geigergold leaf experiment) had discovered in 1911 that atoms have a nucleus, and nine years later they discovered that atomic nuclei were made up, 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 arose from the fact that the number of protons did not match 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 mystery particles would have to be electrically neutral, since atoms normally have no overall electrical charge (the negative charge of electrons cancels out the positive charge of protons).
At that time, several scientists were experimenting alpha particles, which are another name for helium nuclei, by bombarding a material made of the element beryllium with a stream of alpha particles. When the alpha particles hit the beryllium atoms, they produced mysterious particles that appeared to originate within the beryllium atoms. Chadwick took these experiments a step further and saw that when the mysterious particles hit a target made of paraffin wax, they would release high-energy loose protons. To do this, Chadwick reasoned, the mystery particles must have roughly the same mass as a proton. Chadwick proclaimed this mysterious particle to be the neutron and in 1935 won a Nobel Prize for its discovery.
Neutrons: mass and charge
As the name suggests, neutrons are electrically neutral, so they have no charge. Its mass is 1.008 times the mass of the proton, that is, it is about 0.1% heavier.
Neutrons do not like to exist alone outside the nucleus. The binding energy of the Strong Force between them and the protons in the nucleus keeps them stable, but when they are alone they undergo beta decay after about 15 minutes, transforming into a proton, an electron and an antineutrium.
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. 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 top of this article, we gave the example of the 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 time it takes on average for half of a given amount of an isotope like tritium to decay), but other isotopes decay very quickly faster, in minutes, seconds or even fractions of a second.
Neutrons are also essential tools in nuclear reactions, particularly when they induce a chain reaction. Neutrons absorbed by atomic nuclei create unstable isotopes which then undergo decomposition nuclear fission (division into two smaller child nuclei of other elements). For example, when uranium-235 absorbs an extra neutron, it becomes unstable and breaks up, releasing energy in the process.
Neutrons are also instrumental in creating heavy elements in massive stars, through a mechanism known as the r-process, with “r” meaning “rapid”. This process was first detailed in the famous Nobel Prize-winning B2FH paper of Margaret i Geoffrey Burbidge, William Fowler i Fred Hoyle who described the origins of elements through stellar nucleosynthesis: the forging of elements by stars.
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stars I like the sun it can produce oxygen, nitrogen and carbon elements nuclear fusion reactions Month massive stars it can go on and create shells of heavier and heavier elements up to iron-56 in the star’s core. At this point, the reactions require more energy to fuse elements heavier than iron than these reactions actually produce, so these reactions cease, energy production stops, and the star’s core s ‘sinks, causing a supernova. And it is in the incredibly violent explosion of a supernova that conditions can become extreme enough to release many free neutrons in a short space of time.
In the supernova explosion, the atomic nuclei are able to sweep up all these free neutrons before they all decay (which is why it is described as fast), to instigate the nucleosynthesis of the r process. Once nuclei are full of neutrons, they become unstable and undergo beta decay, turning those extra neutrons into protons. The addition of these protons changes the type of element that is a nucleus, so it is a way of creating new and heavy elements such as gold, platinum and other precious metals. The gold in your jewelry was made billions of years ago by rapid neutron capture 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 the name suggests, these are objects made almost entirely of neutrons.
Neutron stars are what remains of a star’s core 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 contraction 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 that the protons and electrons are able to overcome the electrostatic force between them and they sink. , fusing to form neutrons in a kind of reverse beta decay. Almost all the atoms in the nucleus become neutrons, so we call the result a neutron star. They are small, only 10-20 km (6-12 miles) in diameter, but they are clustered throughout the mass of the dead star’s core.
The most massive neutron star found so far has a mass 2.35 times bigger than our sun, all packed into a small volume. If you could remove 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 can be detected as kilonovae and through 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 GW 170817 produced 16,000 times the mass of Earth in the form of r-process heavy elements, including ten Earth masses of gold and platinumthat is extraordinary!
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Additional resources
Learn more about neutrons with the US Department of Energy (opens in a new tab). Explore how neutrons are used in experiments that study condensed matter with the UK Science Technology Facilities Council (opens in a new tab). read the famous paper B2FH (opens in a new tab) on the creation of elements inside stars with the help of neutron capture.
Bibliography
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):
Collins Internet-Linked Dictionary of Physics (2007, Collins) (opens in a new tab)
This month in the history of physics. Sites of the American Physical Society, APS News, Volume 16, Number 5. Retrieved December 1, 2022, from https://www.aps.org/publications/apsnews/200705/physicshistory.cfm (opens in a new tab)
Neutron decay. Science Direct. Accessed on December 1, 2022 from https://www.sciencedirect.com/topics/physics-and-astronomy/neutron-decay (opens in a new tab)
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