Science
NEUTRON
Science > The Atom and Atomic Structure
Current models of atomic structure hold that an atom consists of a small, hard nucleus (diameter of about 10-12 cm) surrounded by a cloud of electrons (10-8 cm). The nucleus consists of protons and neutrons, each with a rest mass of 1.67x10-27 kg, which is 1,800 times the mass of the electron. Unlike protons and electrons, neutrons have no electrical charge. They are contained within the nucleus along with the protons by the strong nuclear force. Unlike the electron, neutrons are not fundamental particles. Rather, they are constituted by smaller, more fundamental particles called quarks. Neutrons are not emitted in any natural radioactive decay process, but they can be ejected from the nucleus of an atom as the result of certain nuclear reactions. Such ejections make a fission chain reaction possible and are central to the construction of nuclear weapons. Neutrons ejected from an atomic nucleus are known to be unstable—meaning that they decay with a half-life of about 12 minutes. Neutrons are also known to disintegrate into a proton through the process of beta decay. 
The discovery of the neutron in 1932 by James Chadwick was one factor that led to the year 1932 being considered the "annus mirabilis"—the extraordinary year—of nuclear physics. The idea of a neutral particle within the nucleus was not unheard of by the time of Chadwick's experiment, but it was often imagined (falsely, by modern models) as being a particle composed of a proton and an electron. In 1930 scientists bombarding beryllium with alpha radiation found what some observers believed was high-energy gamma radiation. Chadwick, however, conducted his own studies and managed instead to show that the "rays" were in fact high-energy neutrons, with a mass that is roughly equal to that of a proton. The discovery of the neutron was tremendously important for nuclear physics. Its importance relied in part on the fact that its neutral charge meant that it would not be repelled by the positively charged protons, a property which potentially made it the ideal projectile with which to probe the nucleus. It was in this capacity that a neutron beam was turned onto uranium nuclei in the late 1930s, setting into motion the discovery of fission. 
During the Manhattan Project, neutrons interacting with fissionable material incited a fission reaction and ejected neutrons sustained the chain reaction. Scientists needed to understand the properties of both slow and fast neutrons. Experimental reactors built during the Manhattan Project, such as CP-1, X-10, and Hanford's plutonium production reactors, required slow neutrons in order to sustain a chain reaction. The slowing of neutrons was accomplished through the use of a moderator (generally graphite). Although it is easier to get the primary fissionable materials, uranium-235 and plutonium, to undergo fission with slow neutrons, the properties of a bomb required that scientists find a way to make them undergo fission with fast ones. Click the "Los Alamos Primer" at right to read Robert Serber's lecture on the need to prevent 'background neutrons' from interfering with the timing of detonation (pages 19-21). |
The text for this page is original to the Department of Energy's Office of History and Heritage Resources. Major sources consulted include the following. For a useful primer on particle physics, funded by the U.S. Department of Energy, see http://www.particleadventure.org/index.html. For current values of particle properties and fundamental constants of nature see, http://www.physics.nist.gov/cuu/Constants/index.html. A useful study of the history of ideas relating to the neutron is Helge Kragh, Quantum Generations: a History of Physics in the Twentieth Century (Princeton: Princeton University Press, 1999), page 184-185. Lawrence Badash, Scientists and the Development of Nuclear Weapons: From Fission to the Limited Test Ban Treaty, 1939-1963 (New York: Humanity Books, 1995), 12-17 also addresses the topic. Also useful for understanding basic atomic science is Henry DeWolf Smyth, Atomic Energy for Military Purposes: The Official Report on the Development of the Atomic Bomb under the Auspices of the United States Government, 1940-1945 (Princeton, NJ: Princeton University Press, 1945), page 5. For information on the post-war discovery of quarks see, http://nobelprize.org/nobel_prizes/physics/laureates/1969/index.html. The fission chain reaction graphic is adapted from a graphic originally produced by the Washington State Department of Health; the modifications are original to the Department of Energy's Office of History and Heritage Resources. The photograph of Chadwick with the pipe is courtesy the Department of Energy. The lecture on background neutrons comes from Serber, Robert, The Los Alamos Primer, Los Alamos, NM: April 1943.