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Neutron

Neutron

A neutron is one of two particles found inside the nucleus (central part) of an atom. The other particle is called a proton. Electrons are particles that move around an atom outside the nucleus.

Discovery of the atom

British physicist Ernest Rutherford discovered the atom in 1911. He constructed a model showing an atom with a nucleus containing protons and electrons. Scientists studying the model knew that something must be missing from it. Rutherford suggested that some sort of neutral particle might exist in the nucleus. He and a graduate student working with him, James Chadwick, could not prove his theory, mainly because neutrons cannot be detected by any standard tools such as cloud chambers or Geiger counters.

Words to Know

Axon: The projection of a neuron that carries an impulse away from the cell body of the neuron.

Central nervous system: The portion of the nervous system in a higher organism that consists of the brain and spinal cord.

Cytoplasm: The fluid inside a cell that surrounds the nucleus and other membrane-enclosed compartments.

Dendrite: A portion of a nerve cell that carries nerve impulses toward the cell body.

Ion: A molecule or atom that has lost one or more electrons and is, therefore, electrically charged.

Myelin sheath: A white, fatty covering on nerve axons.

Neurotransmitter: A chemical used to send information between nerve cells or nerve and muscle cells.

Peripheral nervous system: The portion of the nervous system in an organism that consists of all the neurons outside the central nervous system.

Receptors: Locations on cell surfaces that act as signal receivers and allow communication between cells.

Stimulus: Something that causes a response.

Synapse: The space between two neurons through which neurotransmitters travel.

Finally, Chadwick tried directing a beam of radiation at a piece of paraffin (a waxy mixture used to make candles). He observed that protons were ejected from the paraffin. Chadwick concluded that the radiation must consist of particles with no charge and a mass about equal to that of the proton. That particle was the neutron.

In the early 1960s, the American physicist Robert Hofstadter discovered that both protons and neutrons contain a central core of positively charged matter that is surrounded by two shells. In the neutron, one shell is negatively charged, just balancing the positive charge in the particle's core.

[See also Alzheimer's disease; Nervous system ]

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neutron

neutron, uncharged elementary particle of slightly greater mass than the proton. It was discovered by James Chadwick in 1932. The stable isotopes of all elements except hydrogen and helium contain a number of neutrons equal to or greater than the number of protons. The preponderance of neutrons becomes more marked for very heavy nuclei. A nucleus with an excess of neutrons is radioactive; the extra neutrons convert to protons by beta decay (see radioactivity). In a nucleus the neutron can be stable, but a free neutron decays with a half-life of about 17 min (1,013 sec), into a proton, an electron, and an antineutrino. The fact that the neutron possesses a magnetic moment suggests that it has an internal structure of electric charge, although the net charge is zero. The electron-scattering experiments of Robert Hofstadter indicate that the neutron, like the proton, is surrounded by a cloud of pions; protons and neutrons are bound together in nuclei by the exchange of virtual pions. The neutron and the proton are regarded by physicists as two aspects or states of a single entity, the nucleon. The antineutron, the neutron's antiparticle, was discovered in 1956. The neutron, like other particles, also possesses certain wave properties, as explained by the quantum theory. The field of neutron optics is concerned with such topics as the diffraction and polarization of beams of neutrons. The formation of images using the techniques of neutron optics is known as neutrography.

See D. J. Hughes, Neutron Story (1959); K. H. Beckurts and K. Wirtz, Neutron Physics (tr. 1964); P. Schofield, The Neutron and Its Applications (1983).

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neutron

neutron (symbol n) Uncharged elementary particle that occurs in the atomic nuclei of all chemical elements except the lightest isotope of hydrogen. It is classified as a baryon with spin 1/2. Outside the nucleus, it is unstable, decaying with a half-life of 11.6 minutes into a proton, electron, and antineutrino. Its neutrality allows it to penetrate and be absorbed in nuclei and thus to induce nuclear transmutation and fission.

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neutron

neu·tron / ˈn(y)oōträn/ • n. a subatomic particle of about the same mass as a proton but without an electric charge, present in all atomic nuclei except those of ordinary hydrogen.

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neutron

neutronAgamemnon, Memnon •ninon, xenon •noumenon • Trianon • xoanon •organon • Simenon • Maintenon •crampon, kampong, tampon •Nippon • coupon •Akron, Dacron, macron •electron • natron • Hebron • positron •Heilbronn • micron •boron, moron, oxymoron •neutron • interferon •fleuron, Huron, neuron •Oberon • mellotron • aileron •cyclotron • Percheron • Mitterrand •vigneron • croissant • Maupassant •garçon • Cartier-Bresson • exon •frisson • Oxon • chanson • Tucson •soupçon • Aubusson • Besançon •penchant • torchon • cabochon •Anton, canton, Danton •lepton •piton, Teton •krypton • feuilleton • magneton •chiton •photon, proton •croûton, futon •eschaton • peloton • contretemps •telethon •talkathon, walkathon •Avon • tableau vivant • vol-au-vent

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Neutron

Neutron

Discovery

New radioactive materials

Nuclear energy production

The atoms structure consists of a central, dense nucleus surrounded by a cloud of electrons. Inside the nucleus are two types of particles, neutrons (which are electrically neutral) and protons (which are positively charged). In most atoms, the nucleus is a stable, long-lasting structure; in some, it is unstable, and can break apart (fission) at an unpredictable time. When this happen, energy is released, and usually fast-moving neutrons as well.

The atom is so small that it takes several million of them in a row to span the period at the end of this sentence. The nucleus is much smaller still, about 1/100,000 of an atoms size.

Neutrons and protons were once considered fundamental particles, but it is now known that they are made up of even smaller components called quarks, of which there are three types. Evidence for quarks comes from experiments done with very high energy particle accelerators.

Discovery

The neutron as a distinct, separate particle was discovered in l932 by a British physicist, James Chadwick (18911974). He was using radiation emitted from radioactive radium to bombard various materials. When he irradiated the element beryllium, he found that a very penetrating particle was produced. It could go through an inch thickness of lead. Chadwick reasoned that the new particle must be electrically neutral, because all other radiation known at that time would have been stopped by the lead. Chadwick was awarded the 1935 Nobel Prize in physics for his discovery.

New radioactive materials

Enrico Fermi (19011954), a young Italian scientist, constructed a neutron source according to Chadwicks design, mixing radium and beryllium powder together in a small glass tube. Fermis plan was to use this neutron source for making new radioactive materials. Together with his co-workers, he was successful in producing radioactive sodium, iron, copper, gold, and many other elements. Fermi received the l938 Nobel Prize in physics for his work with neutrons.

Almost all elements found in nature can now be made radioactive. Radioactive potassium and phosphorus are used as tracers to measure how effectively plants take up fertilizer from soil. Radioactive iodine is applied in nuclear medicine to diagnose and treat thyroid problems. Radiation treatment for cancer therapy uses radioactive cobalt, which is made by irradiating ordinary cobalt with neutrons.

Nuclear energy production

When some elements, including uranium, are bombarded by neutrons, nuclear fission takes place. The nucleus breaks into two pieces with a large release of energy. In addition, several extra neutrons are emitted. These can cause more nuclei to split apart, which releases more energy and more neutrons in a chain reaction. In an atomic (i.e., fission) bomb, the chain reaction becomes an uncontrolled explosion. In a nuclear power plant, the chain reaction is maintained in a steady state by materials that absorb extra neutrons. Further, the unstable nuclei in the fuel of a normal fission power plant are too far apart to support an explosive reaction. Only a few special reactors, including certain breeder reactors, have the potential to support an atomic explosion.

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Neutron

Neutron

A subatomic particle with a mass of a proton and no electric charge. It is found in the nuclei of all atoms except hydrogen-1. A free neutron is unstable and decays to form a proton and an electron with a half life of 22 minutes. Because they have no electric charge, neutrons easily penetrate matter, including human tissue. They constitute, therefore, a serious health hazard. When a neutron strikes certain nuclei, such as that of uranium-235, it fissions those nuclei, producing additional neutrons in the process. The chain reaction thus initiated is the basis for nuclear weapons and nuclear power .

See also Nuclear fission; Radioactivity

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Neutron

Neutron

The atom's structure has a nucleus at the center, surrounded by a cloud of electrons. Inside the nucleus are two types of particles, neutrons (electrically neutral) and protons (positively charged). Under special experimental conditions, neutrons can be released from the nucleus. They are useful for creating new radioactive materials or for producing large amounts of nuclear energy .

The atom is so small that it would take several million of them to equal the size of the period at the end of this sentence. The nucleus is much smaller still, about 1/100,000 of an atom's size. Finally, neutrons and protons appear to be made up of three even smaller components called "quarks." Evidence for quarks comes from experiments done with very high energy particle accelerators .


Discovery

The neutron as a distinct, separate particle was discovered in l932 by a British physicist, James Chadwick (1891-1974). He was using radiation emitted from radioactive radium to bombard various materials. When he irradiated the element beryllium, he found that a very penetrating particle was produced. It could go through an inch thickness of lead . Chadwick reasoned that the new particle must be electrically neutral, because all other radiation known at that time would have been stopped by the lead. Chadwick was awarded the 1935 Nobel Prize in physics for his discovery.


New radioactive materials

Enrico Fermi (1901-1954), a young Italian scientist, constructed a neutron source according to Chadwick's design, mixing radium and beryllium powder together in a small glass tube. Fermi's plan was to use this neutron source for making new radioactive materials. Together with his co-workers, he was successful in producing radioactive sodium , iron , copper , gold, and many other elements. Fermi received the l938 Nobel Prize in physics for his work with neutrons.

Almost all elements found in nature can now be made radioactive. Radioactive potassium and phosphorus are used as tracers to measure how effectively plants take up fertilizer from soil . Radioactive iodine is applied in nuclear medicine to diagnose and treat thyroid problems. Radiation treatment for cancer therapy uses radioactive cobalt, which is made by irradiating ordinary cobalt with neutrons.


Nuclear energy production

When the element uranium is bombarded by neutrons, a unique reaction called fission takes place. The uranium nucleus breaks into two pieces, which fly apart with a large release of energy. In addition, several extra neutrons are emitted. These cause more uranium nuclei to split apart, which creates more energy and more neutrons in a so-called "chain reaction" process. In an atomic bomb, the chain reaction becomes an uncontrolled explosion. In a nuclear power plant, the chain reaction is maintained in a steady state by control rods which absorb extra neutrons.

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