{"id":11420,"date":"2016-01-04T15:28:30","date_gmt":"2016-01-04T15:28:30","guid":{"rendered":"http:\/\/sitepourvtc.com\/?page_id=11420"},"modified":"2022-10-12T11:01:44","modified_gmt":"2022-10-12T11:01:44","slug":"neutron","status":"publish","type":"page","link":"https:\/\/sitepourvtc.com\/nuclear-power\/reactor-physics\/atomic-nuclear-physics\/fundamental-particles\/neutron\/","title":{"rendered":"Neutron"},"content":{"rendered":"
A neutron<\/strong> is one of the subatomic particles<\/a> that make up matter. The neutron has no electric charge<\/strong> and a rest mass equal to 1.67493 \u00d7 10\u221227<\/sup> kg \u2014 marginally greater than that of the proton but nearly 1839 times greater than that of the electron. The neutron has a mean square radius of about 0.8\u00d710\u221215 m or 0.8 fm, and it is a spin-\u00bd fermion. In the universe, neutrons are abundant, making up more than half<\/strong> of all visible matter.<\/div><\/div>\n
\"The<\/a>
The quark structure of the neutron. The color assignment of individual quarks is arbitrary, but all three colors must be present. Gluons mediate forces between quarks.
Source: wikipedia.org<\/figcaption><\/figure>\n

The neutrons<\/strong> exist in the nuclei of typical atoms, along with their positively charged counterparts, the protons. Neutrons and protons, commonly called nucleons<\/strong>, are bound together in the atomic nucleus, where they account for 99.9 percent of the atom\u2019s mass. Research in high-energy particle physics in the 20th century revealed that neither the neutron nor the proton is not<\/strong> the smallest building block of matter. Protons and neutrons<\/strong> also have their structure. Inside the protons and neutrons, we find true elementary particles called quarks<\/strong>. Within the nucleus, protons and neutrons are bound together through a strong force<\/strong>. This fundamental interaction governs the behavior of the quarks that make up the individual protons and neutrons.<\/p>\n

The competition between two fundamental interactions determines nuclear stability. Protons and neutrons are attracted each other via the strong force. On the other hand, protons repel each other via the electric force due to their positive charge. Therefore neutrons within the nucleus act somewhat like nuclear glue. Neutrons attract each other and protons, which helps offset the electrical repulsion between protons. There are only certain combinations of neutrons and protons, which form stable nuclei. For example, the most common nuclide of the common chemical element lead (Pb) has 82 protons and 126 neutrons.<\/p>\n

\"Nuclear<\/a>
Nuclear binding energy curve.
Source: hyperphysics.phy-astr.gsu.edu<\/figcaption><\/figure>\n

Because of the strength of the nuclear force at short distances<\/strong>, the nuclear binding energy<\/a> (the energy required to disassemble a nucleus of an atom into its component parts) of nucleons is more than seven orders of magnitude larger than the electromagnetic energy binding electrons in atoms. Therefore, nuclear\u00a0reactions<\/a> (such as nuclear fission<\/a> or nuclear fusion<\/a>) have an energy density of more than 10 000 000x that of chemical reactions.
\nKnowledge of the behavior and properties of neutrons<\/strong> is essential to the production of nuclear power<\/a>. The neutron was discovered in 1932<\/strong>. The fact that neutrons might act to form a nuclear chain reaction<\/strong> was realized quickly after that.\u00a0When nuclear fission was discovered in 1938, it became clear that if a fission reaction produced free neutrons<\/strong>, each of these neutrons might cause further fission reaction in a cascade known as a chain reaction<\/strong>. Knowledge of cross-sections<\/a> (the key parameter representing the probability of interaction between a neutron and a nucleus) became crucial for designing reactor cores<\/a> and the first nuclear weapon (Trinity, 1945).<\/p>\n

<\/span>Discovery of the Neutron<\/div>
The story of the discovery of the neutron and its properties is central to the extraordinary developments in atomic physics<\/a> that occurred in the first half of the 20th century. The neutron was discovered in 1932 by the English physicist James Chadwick<\/strong>. Still, since the time of Ernest Rutherford, it has been known that the atomic mass number A of nuclei is a bit more than twice the atomic number Z for most atoms. Essentially, all the mass of the atom is concentrated in the relatively tiny nucleus. Rutherford\u2019s model for the atom in 1911 claims that atoms have their mass and positive charge concentrated in a very small nucleus.<\/p>\n
\"Discovery<\/a>
The alpha particles emitted from polonium fell on certain light elements, specifically beryllium, and produced unusually penetrating radiation.
Source: dev.physicslab.org<\/figcaption><\/figure>\n
\"Chadwicks<\/a>
Chadwick\u2019s neutron chamber contains parallel disks of radioactive polonium and beryllium. Radiation is emitted from an aluminum window at the chamber\u2019s end.
Source: imgkid.com<\/figcaption><\/figure>\n

An experimental breakthrough came in 1930 with the observation by Bothe and Becker. They found that unusually penetrating radiation was produced if the very energetic alpha particles<\/strong> emitted from polonium fell on certain light elements<\/strong>, specifically beryllium, boron, or lithium. Since this radiation was not influenced by an electric field (neutrons have no charge), they presumed it was gamma rays (but much more penetrating). It was shown (Curie and Joliot) that when a paraffin target with this radiation is bombarded, it ejected protons with an energy of about 5.3 MeV. Paraffin is high in hydrogen content, hence offers a target dense with protons (since neutrons and protons have almost equal mass, protons scatter energetically from neutrons). These experimental results were difficult to interpret. James Chadwick proved that the neutral particle could not be a photon<\/a> by bombarding targets other than hydrogen, including nitrogen, oxygen, helium, and argon. Not only were these inconsistent with photon emission on energy grounds, but the cross-section for the interactions was also orders of magnitude greater than that for Compton scattering by photons. In Rome, the young physicist Ettore Majorana suggested that the new radiation<\/a> interacted with protons required a new neutral particle.<\/p>\n

The task was that of determining the mass of this neutral particle. James Chadwick chose to bombard boron with alpha particles and analyze the interaction of the neutral particles with nitrogen. These particular targets were partly chosen because the masses of boron and nitrogen were well known. Using kinematics, Chadwick was able to determine the velocity of the protons. Then through conservation of momentum techniques, he was able to determine that the mass of the neutral radiation was almost the same as that of a proton. In 1932, Chadwick proposed that the neutral particle was Rutherford\u2019s neutron. In 1935, he was awarded the Nobel Prize for his discovery.<\/p>\n

See also: Discovery of the Neutron<\/a><\/div><\/div><\/div>\n

\n

Structure of the Neutron<\/h2>\n
\"Quark<\/a>
The quark structure of the neutron. The color assignment of individual quarks is arbitrary, but all three colors must be present. Gluons mediate forces between quarks.<\/figcaption><\/figure>\n

Neutrons<\/strong> and protons are classified as hadrons<\/strong>, subatomic particles<\/a> subject to the strong force,<\/strong> and baryons since they are composed of three quarks<\/strong>. The neutron is a composite particle made of two down quarks with charge \u2212\u2153 \u00a0e and one up quark with charge +\u2154 e. Since the neutron has no net electric charge<\/strong>, it is not affected by electric forces, but the neutron does have a slight distribution of electric charge<\/strong> within it. This results in a non-zero magnetic moment (dipole moment) of the neutron. Therefore the neutron also interacts via electromagnetic interaction but is much weaker than the proton.<\/p>\n

The mass of the neutron is 939.565 MeV\/c2<\/sup><\/strong>, whereas the mass of the three quarks is only about 12 MeV\/c2<\/sup> (only about 1% of the mass-energy of the neutron). Like the proton, most of the mass (energy) of the neutron is in the form of the strong nuclear force energy (gluons). The quarks of the neutron are held together by gluons, the exchange particles for the strong nuclear force. Gluons carry the color charge of the strong nuclear force.<\/p>\n

See also: Structure of the Neutron<\/a><\/p>\n

Properties of the Neutron<\/h2>\n

Key properties of neutrons are summarized below:<\/p>\n