Nu See Nun
Nu, U orig. Thakin Nu (b. May 25, 1907, Wakema, Burma—d. Feb. 14, 1995, Yangon, Myanmar) Burmese independence leader and prime minister of Burma (Myanmar; 1948-58, 1960-62). A prominent nation¬ alist activist since his student days, U Nu became the first prime minister of independent Burma in 1948. Though an able statesman, he was plagued by ethnic-minority insurrections and economic difficulties. He resigned in 1958, was reinstated in 1960, and was overthrown and imprisoned by Ne Win in 1962. After his release he organized resistance to Ne Win. He made an unsuccessful bid for power after Ne Win’s government fell in 1988.
Nu River See Salween River
Nubia Ancient region of the Nile River valley, northeastern Africa. Its borders originally extended north to include Aswan and, before comple¬ tion of the Aswan High Dam, the first cataract of the Nile in Upper Egypt. The region now encompasses part of The Sudan and southern Egypt and contains the Nubian Desert in the northeast. For about 1,800 years in ancient times it was politically dominated by Egypt, although Nubian kings did sit on the Egyptian throne during the 25th dynasty. The king¬ dom of Kush and its culture were centred in southern Nubia. Later Nubia was the centre of a powerful state with Dunqulah as its capital (6th-14th centuries ad), after which it was captured by the Arabs. The region was conquered by Egypt in 1820-22.
Nubia, Lake See Lake Nasser
Nubian Desert Desert, northeastern Sudan. It is separated from the Libyan Desert by the Nile River valley to the west. Rocky and rugged with some dunes, it is essentially a sandstone plateau interspersed with many wadis (seasonal rivers) that die out before reaching the Nile. Precipitation averages less than 5 in (13 cm) annually.
nuclear energy or atomic energy Energy released from atomic nuclei in significant amounts. In 1919 Ernest Rutherford discovered that alpha rays could split the nucleus of an atom. This led ultimately to the discovery of the neutron and the release of huge amounts of energy by the process of nuclear fission. Nuclear energy is also released as a result of nuclear fusion. The release of nuclear energy can be controlled or uncontrolled. Nuclear reactors carefully control the release of energy, whereas the energy release of a nuclear weapon or resulting from a core meltdown in a nuclear reactor is uncontrolled. See also chain reaction, NUCLEAR POWER, RADIOACTIVITY.
nuclear fission Division of a heavy atomic nucleus into two fragments of roughly equal mass, accompanied by the release of a large amount of energy, the binding energy of the subatomic particles. The energy released in the fission of one uranium nucleus is about 50 million times greater than that released when a carbon atom combines with oxygen atoms in the burning of coal. The energy appeal's as kinetic energy of the fragments, which converts to thermal energy as the fragments collide in matter and slow down. Fission also releases two or three free neutrons. The free neu¬ trons can bombard other nuclei, leading to a series of fissions called a chain reaction. The energy released from nuclear fission is used to gen¬ erate electricity, to propel ships and submarines, and is a source of the vast destructive power of nuclear weapons. See illustration opposite.
nuclear fusion Process by which nuclear reactions between light ele¬ ments form heavier ones, releasing huge amounts of energy. In 1939 Hans
Bethe suggested that the energy output of the sun and other stars is a result of fusion reactions among hydrogen nuclei. In the early 1950s American scientists produced the hydrogen bomb by inducing fusion reactions in a mixture of the hydrogen isotopes deuterium and tritium, forming a heavier helium nucleus. Though fusion is common in the sun and other stars, it is difficult to produce artificially and is very difficult to control. If con¬ trolled nuclear fusion is achieved, it might provide an inexpensive energy source because the primary fuel, deuterium, can be extracted from ordi¬ nary water, and eight gallons of water could provide the energy equiva¬ lent to 2,500 gallons of gasoline. See illustration below.
nuclear fusion
Top: Uranium-235 combines with a neutron to form an unstable intermediate, which quickly splits into barium-144 and krypton-89 plus three neutrons in the pro¬ cess of nuclear fission. Bottom: Deuterium and tritium combine by nuclear fusion to form helium plus a neutron.
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nuclear magnetic resonance (NMR) Selective absorption of very high-frequency radio waves by certain atomic nuclei subjected to a strong
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stationary magnetic field. Nuclei that have at least one unpaired proton or neutron act like tiny magnets. When a strong magnetic field acts on such nuclei, it sets them into precession. When the natural frequency of the pre- cessing nuclear magnets corresponds to the frequency of a weak external radio wave striking the material, energy is absorbed by the nuclei at a frequency called the resonant frequency. NMR is used to study the molecular structure of various solids and liquids. Magnetic resonance imaging, or MRI, is a version of NMR used in medicine to view soft tis¬ sues of the human body in a hazard-free, noninvasive way.
nuclear medicine Medical specialty using radioactive elements or iso¬ topes for diagnosis and treatment of disease. A radioisotope is introduced into the body (usually by injection). The radiation it emits, detected by a scanner and recorded, reflects its distribution in different tissues and can reveal the presence, size, and shape of abnormalities in various organs. The isotopes used have short half-lives and decay before radioactivity causes any damage. Different isotopes tend to concentrate in particular organs (e.g., iodine-131 in the thyroid). Radioactive substances are also implanted to treat small, early-stage cancers. This yields a slow, continu¬ ous dose that limits damage to normal cells while destroying tumour cells. See also computed axial tomography; diagnostic imaging; positron emission tomography; radiation therapy; radiology.
Nuclear Non-proliferation Treaty (NPT) officially Treaty on the Non-proliferation of Nuclear Weapons International agreement intended to prevent the spread of nuclear technology. It was signed by the U.S., Britain, the Soviet Union, and 59 other countries in 1968. The three major signatories agreed not to assist states lacking nuclear weapons to obtain or produce them; the nonnuclear signatories agreed not to attempt to obtain nuclear weapons in exchange for assis¬ tance in developing nuclear power for peaceful purposes. France and China, both nuclear powers, declined to ratify the treaty until 1992, and some nuclear powers, including Israel and Pakistan, have never signed it. In 1995, when the treaty was due to expire, it was extended indefinitely by a consensus vote of 174 countries at the United Nations. See also Nuclear Test-Ban Treaty.
nuclear physics Branch of physics dealing with the structure of the atomic nucleus and radiation from unstable nuclei. A principal research tool of nuclear physics is a high-energy beam of particles, such as pro¬ tons or electrons, directed as projectiles against nuclear targets. By ana¬ lyzing the directions and energies of the recoiling particles and any resulting nuclear fragments, nuclear physicists can obtain details of nuclear structure, the strong force that binds nuclear components together, and the release of energy from the nucleus.
nuclear power Energy produced by nuclear fission of heavy atomic nuclei. About one-third of all electric power worldwide now comes from nuclear power plants. The navies of many countries include nuclear- powered warships; almost half of U.S. combat warships are nuclear- powered. Most commercial nuclear reactors are thermal reactors. Two types of light-water reactors in use throughout the world are the boiling- water reactor and the pressurized-water reactor. In the liquid-metal fast- breeder reactor, fuel is utilized 60 times more effectively than in light- water reactors. See also nuclear energy.