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 ]

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.

Bibliography: 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).

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.

neutron An elementary particle that is present in the nuclei of all atoms except the lightest isotope of hydrogen. It has a slightly greater mass than the proton and zero charge. Neutrons are stable in the atomic nucleus, but outside it they undergo beta decay to produce a proton, an electron, and an antineutrino. Neutrons are hadrons.

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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