NUCLEAR FISSION 



nuclear fission — The division of a heavy nucleus 

 into two approximately equal jDarts. For the 

 heaviest nuclei the reaction is highly exothermic, 

 the release of energy being about 170 million- 

 electron-volts per fission. A well-known exam- 

 ple is the fission of the compound nucleus formed 

 when U^^^ captures a slow neutron. The ap- 

 proximate equality of the fission fragments dis- 

 tinguishes fission from such processes as spalla- 

 tion, in which, relatively small fragments are 

 ejected, leaving only one large residual nucleus. 

 Fission has been induced by neutrons, charged 

 particles, and photons. When induced by pho- 

 tons, it is called photofission. (41) 



nuclear fusion — The act of coalescing two or more 

 atomic nuclei. See thermonuclear reaction. 

 (70) 



nuclear isomer — One of two or more nuclides 

 having the same mass number A and atomic 

 number Z, but existing for measurable times in 

 different quantum states with different energies 

 and radioactive properties. (41) 



nuclear oceanography — The study of the nuclear 

 properties of the marine environment and the 

 nuclear phenomena occurring therein. This is 

 a broad oceanographic discipline which includes 

 radioisotopic oceanography and the applica- 

 tion of nuclear science and technology to ocean- 

 ographic investigations. 



nuclear precession magnetometer — A magne- 

 tometer that utilizes the precessional character- 

 istics of hydrogen nuclei when in an ambient 

 magnetic field. The data output of this instru- 

 ment is in the form of a frequency measurement, 

 which in turn is jDroportional to the magnetic 

 field intensity. 



nuclear reaction — An induced nuclear disintegra- 

 tion, that is, a process occurring when a nucleus 

 comes into contact with a photon, an elementary 

 particle, or another nucleus. In many cases the 

 reaction can be represented by the symbolic 

 equation: X^Oj-^Y+Ty or, in abbreviated form, 

 X(a,5)T', in which X is the target nucleus, a 

 is the incident particle or photon, 5 is an emitted 

 particle or photon, and Y is the product nucleus. 

 (70) 



nuclear reactor — An apparatus in which nuclear 

 fission may be sustained in a self-supporting- 

 chain reaction. It includes fissionable material 

 (fuel) such as uranium or plutonium, and mod- 

 erating material (unless it is a fast reactor) and 

 usually includes a reflector to conserve escaping 

 neutrons, provision for heat removal, and meas- 

 uring and control elements. The terms pile and 

 reactor have been used interchangeably, with 



reactor now becoming more common. They 

 usually are applied only to systems in which the 

 reaction proceeds at a controlled rate, but they 

 also have been applied to bombs. ( 70 ) 



nuclear sediment density meter — An oceano- 

 graphic instrument which employs the back- 

 scattered radiation from a small sealed radioac- 

 tive source for the in situ measurement of sedi- 

 ment densities in the upper 5 meters of the ocean 

 floor. The instrument is a tubelike device, about 

 25 feet in length overall, which is lowered to the 

 ocean floor and projected vertically in the bot- 

 tom sediment for measurement. The radioac- 

 tive source with a G-M detector rides up and 

 down in the tube stopping at 2- foot intervals 

 for measurement of backscattered radiation. 

 Such counts are inversely proportional to sedi- 

 ment density. Recording is accomplished photo- 

 graphically in the upper enlarged barrel of the 

 probe. 



nuclear species — 1. A kind of atom characterized 

 by the charge, mass number, and quantum state 

 of its nucleus ; a nuclide. 



2. A nucleus of a given charge, mass number, 

 and quantum state. Also a collection of such 

 nuclei. 



(41) 



nucleon — A constituent particle of the atomic nu- 

 cleus ; therefore, according to present theory, a 

 proton or a neutron. (41) 



nuclide — A species of atom characterized by the 

 constitution of its nucleus. The nuclear con- 

 stitution is specified by the number of protons, 

 number of neutrons, and energy content, or 

 alternatively, by the atomic number, mass num- 

 ber, and atomic mass. To be regarded as a dis- 

 tinct nuclide, the atom must be capable of 

 existing for a measurable lifetime (generally 

 greater than lO"^" second). Thus nuclear 

 isomers are separate nuclides, but promptly de- 

 caying excited nuclear states and unstable inter- 

 mediates in nuclear reactions are not so con- 

 sidered. (41) 



nudibranch — (or sea slug). One of the order 

 (Nudibranchia) of gastropods in which the 

 shell is entirely absent in the adult. The body 

 bears projections which vary in color and com- 

 plexity among the species. 



nutrient — In the ocean any one of a number of 

 inorganic or organic compounds or ions used 

 primarily in the nutrition of primaiy producers. 

 Nitrogen and phosplioi'us compounds are essen- 

 tial nutrients. Silicates are essential for the 

 growth and development of diatoms. Vitamins 

 such as Bi2 are essential to many algae. 



112 



