HALF-LIFE of radioactive materials — a time term during which as a result of radioactive decay the quantity of kernels of this radioactive material decreases twice. Respectively twice intensity let out by this radioactive material decreases ionizing radiation (see).
The pct of various radioactive materials makes up to tens of billions of years from million fractions of a second. The it is less than A Pct, the big share of kernels of this radioactive material breaks up in unit of time. The size characterizing a share of the kernels which are breaking up in unit of time is called a constant of disintegration and is measured in units, the return to units of time (sec. - 1 , min. - 1 , hour. - 1 , days. - 1 ). Radioactive materials at which the Pct is small and the constant of disintegration is big, radioactive materials with big Pct and a small constant of disintegration — long-living are called short-lived, and.
Half-life (T 1/2 ) — one of the main characteristics of radioactive materials, to-ruyu consider at their practical application. So, during the use of radioactive materials as sources of external radiation, in particular for a gamma therapy, preference is given to radioactive materials with big half-life, napr, to caesium-137 (T1/2 = 27 years), to cobalt-60 (T1/2 = 5,25 years). At administration of radioactive materials in an organism with the diagnostic purpose aim to minimize an exposure dose of bodies and fabrics therefore use radioactive materials which Pct is small, napr, sodium-24 (T1/2 = 14,9 hours), iodine-132 (T1/2 = 2,3 hours).
Bibliography: Belousova I. M. and Shtukkenberg Yu. M. Natural radioactivity, M., 1961; Margulies U. Ya. Radiation and protection, M., 1974; Moiseyev A. A. and Ivanov V. I. Reference book on dosimetry and physics health, M., 1974.
U. Ya. Margulies.