Physics: Nuclear Physics

Self-sustaining reaction that, once started, continues without further outside influence. Proper conditions for a chain reaction depend not only on various external factors, such as temperature, but also on the quantity and shape of the substance undergoing the reaction.

Few processes of nature have had so great an impact on the course of history in the twentieth century as nuclear fission. Its discovery led almost immediately to bomb projects in several countries.

The fusing together of two lightweight atomic nuclei, typically isotopes of hydrogen or lithium, having a total rest mass which exceeds that of the products. The mass difference is made up by energy released in the process.

Apparatus used in nuclear physics to produce beams of energetic charged particles and to direct them against various targets. Such machines, popularly called atom smashers, are needed to observe objects as small as the atomic nucleus in studies of its structure and of the forces that hold it together.

Radioactive chemical element; symbol Pu; at. no. 94; mass no. of most stable isotope 244; m.p. 641 degrees Celsius; b.p. 3,232 degrees Celsius; sp. gr. 19.84 at 20 degrees Celsius; valence +3, +4, +5, or +6. Plutonium is a silver-gray radioactive metal that has six allotropic forms.

Emission of radiant energy as particles or waves - for example, heat, light, alpha particles, and beta particles (see electromagnetic waves and radioactivity).

Process of disintegration undergone by the nuclei of radioactive elements, such as radium and various isotopes of uranium and the transuranic elements, in order to produce a more stable nucleus. The three most common forms of radioactive decay are alpha, beta, and gamma decay.

Spontaneous change of the nuclei of atoms accompanied by the emission of radiation. Such atoms are called radioactive. It is the property exhibited by the radioactive isotopes of stable elements and all isotopes of radioactive elements, and can be either natural or induced.

Uranium is a hard, dense, malleable, ductile, silver-white, radioactive metal of the actinide series in Group 3 of the periodic table. Uranium has three distinct forms (see allotropy); the orthorhombic crystalline structure occurs at room temperature.

Or A-bomb, weapon deriving its explosive force from the release of atomic energy through the fission (splitting) of heavy nuclei (see nuclear energy). The first atomic bomb was produced at the Los Alamos, N.Mex., laboratory and successfully tested on July 16, 1945.

Or H-bomb, weapon deriving a large portion of its energy from the nuclear fusion of hydrogen isotopes. In an atomic bomb, uranium or plutonium is split into lighter elements that together weigh less than the original atoms, the remainder of the mass appearing as energy.

The wartime effort to design and build the first nuclear weapons (atomic bombs). With the discovery of fission in 1939, it became clear to scientists that certain radioactive materials could be used to make a bomb of unprecedented power.

The energy stored in the nucleus of an atom and released through fission, fusion, or radioactivity. In these processes a small amount of mass is converted to energy according to the relationship E = mc2, where E is energy, m is mass, and c is the speed of light.

From Science in the Contemporary World: An Encyclopedia
Fifteen Nobel laureates worked on the Manhattan Project, directed by the physicist J. Robert Oppenheimer (1904–1967), making it perhaps the single project in human history with the greatest concentration of brainpower. Only in 1938 had the idea of nuclear fission in a chain reaction, making a bomb possible, even been broached.

Device for producing controlled release of nuclear energy. Reactors can be used for research or for power production. A research reactor is designed to produce various beams of radiation for experimental application; the heat produced is a waste product and is dissipated as efficiently as possible.

The radioactive and toxic by-products of the nuclear energy and nuclear weapons industries. Nuclear waste may have an active life of several thousand years.