From Big Medical Encyclopedia

ATOMIC NUCLEUS — the central positively charged part of atom consisting of elementary particles of two types — protons and neutrons.

Existence in atom of a kernel was established by E. Rutherford in 1911 D. D. Ivanenko and W. Heisenberg in 1932 proved that I. and. consists of protons and neutrons. The atomic nucleus is about 100 000 times less than the atom; in it almost all mass of atom is concentrated. The proton is a stable elementary particle with the charge in number equal to elementary charge, but having an opposite sign (1,60-10 ~ 19 C). The mass of a proton is equal

1,67*10 "24 g. A neutron — electrically a neutral particle with a weight which is slightly exceeding the mass of a proton. The neutron in a stand-at-ease (out of a kernel) is unstable and spontaneously breaks up (the period of its half-decay is approximately equal to 12 min.), turning into a proton with emission of an electron and antineutrino (see. Elementary particles). For designation of protons and neutrons use the general term «nucleon». Number of protons in I. a. (Z) defines sequence number of a chemical element; total number of protons and neutrons is called mass number (And). For designation Ya. and. use the following record: ^Х, where X — a symbol of a chemical element. Hanp./fC — an atomic nucleus of carbon with mass number 12. This kernel has 6 protons and 6 neutrons. Atoms of chemical elements, kernels to-rykh have different number of neutrons, are called isotopes (see). Isotopes of all chemical elements are known. Many isotopes can be received only in the artificial way. Distinguish stable and unstable isotopes. Stability of a kernel is defined by a ratio of number of protons in it and neutrons. Binding energy of nucleons in a kernel in millions times more energy of chemical bonds therefore at nuclear transformations huge energy (can be emitted see. Nuclear reactions). Kernels of many atoms have magnetic moment.

Properties Ya. and. widely use in medical practice. So, the spontaneous disintegration of unstable isotopes which is followed by ionizing radiation is the cornerstone of radiation therapy, radio isotope diagnosis; besides, from unstable isotopes prepare radiofarma-tsevtichesky drugs. The absorption of electromagnetic waves caused by change of the direction of magnetic moment I. and., apply to studying of structure and the mechanism of effect of biologically active agents, obtaining the image of the studied object, napr, an internal human organ (see Nuclear magnetic resonance).

Bibliograkosta V., Is forged To., and Graham B. Fundamentals of modern physics, the lane with English, M., 1981; Bourne M., Atomic physics, the lane with English, M., 1965; Landau L. D. and Currants-with to and y Ya. A. Lectures on the theory of an atomic nucleus, M., 1955; The Neutron, Background, opening, effects, under red * B. M. Kedrova, M., 1975; About a systematics of particles, Atoms, kernels, elementary particles, M., 1969; Shpolsky В® Atomic physics, t. 1 — 2, M., 1974.