1. Natural N.f. — isotope uranium $^{235}\!{U}$ (uranium$ -235$).
2. Secondary N.f. — obtained artificially in a nuclear reactor plutonium isotope $^{239}\!{P}$ (plutonium $- 239$) and the isotope uranium $^{233}\!{U}$ (uranium $- 233$).

— a branch of physics that studies the structure of atomic nuclei and their conversion (see. Nuclear reactions). Main directions F.: J. structure of nuclei, neutron physics, radioactive transformations, synthesis and study of transuranic elements and others. J. F. and theoretical basis of nuclear technology,nuclear energy and the production of isotopes.

— a set of means and measures related to the use of the technical properties of atomic nuclei of different substances.

— reaction of fission of atomic nuclei of heavy elements under the action of neutrons in each act by increasing the number of neutrons, so it may be a process of dividing self-sustaining. N.ch.r. accompanied by huge amounts of energy (about $200$ MeV per core of uranium or plutonium, which is divided). N.ch.r. underlying nuclear reactor and acts of nuclear (nuclear) weapons.

— the internal energy of the atomic nuclei associated with movement and interaction forming the nucleus of nucleons. There are two ways to get N.e.: implementation of nuclear chain reactions of division of heavy nuclei or thermonuclear reactions fusion of light nuclei.

— an area of modern technology, based on the conversion of nuclear energy into other forms of energy (thermal, mechanical, electrical) and use it for industrial and domestic use.

— fluctuations in the electric current in the system of electronic devices. There is internal E.n., due to thermal motion of charges in conductors and external E.n., which is the source of corpuscular and electromagnetic radiation from the Sun and stars, industrial electrical and radio sets, and so on. E.n. limiting the sensitivity of electronic equipment.

— experiment, which proved the existence of atoms magnetic moment, discrete oriented in an external magnetic field. Put him in $1922$. scholars O. Stern and W. Gerlach (b. $1889$).

— research, which was first directly measured the velocities of thermal motion of molecules. Results of S. e. confirmed the basic provisions of the Maxwell- Boltzmann distribution. Delivered in $1820$ it. physicist A. Stern $(1888 - 1970)$.

— changing energy levels of atoms, molecules and crystals under external electric field, which is manifested in the shift and splitting of spectral lines. Opened in $1913$ it. physicist J. Stark $(1874 - 1957)$.