Half - life is the time during which undergoes radioactive transformations half of the nuclei of the atoms of this isotope. Designated by the letter T. the radioactive decay Law - reduction over time in the number of active atoms - is a common for all types of radioactive transformations and all radioactive isotopes: the number of radioactive atoms of a given isotope decaying per 1 sec., in proportion to their number. The share of atoms, dissolved in 1 sec, for this isotope does not depend on the number of active atoms and time. This value is called the constant of radioactive decay λ. It is measured in units of inverse time: sec^{-1}, min^{-1}, d.^{-1}, year^{-1}. For each type of radioactive nuclei and transformations λ has its value. So, for Na24 λ =1,29·10-6 sec.^{-1}, Ra226 - 1,38·10^{-11} sec.^{-1}. The radioactive decay law only valid for a large number of radioactive atoms.

The value λ characterizes the relative decay rate. The absolute rate of decay (Q); that is, the number of decays per unit time, is called the activity; Q= λ/V, that is, activity increased in proportion to the number of active atoms N. Unit of activity in the absolute system - 1 u/s. Often the activity measured in curp; 1 Curie equals to 3.7 ·10^{10} u/s. This value is still measurements were consistent with the decay rate of 1 gram of radium. Applied also units of 10^{-3} Curie millicurie (mturi) and 10^{-6} Curie microcurie (mccoury). The decrease in the number of active atoms over time subject to exponential law N=N_{0}e^{λt}, where N_{0} - number of atoms at t=0, e is the base of natural logarithms. The essence of this law is that for a certain period of time disintegrates always the same percentage of the active atoms regardless of their amount and timing.

Over time, T, is equal to the half-life is falling apart to half of atoms and twice reduced activity of the drug. Of the excess air, and Tons connected by the relation λТ =0,693, i.e. with the increase in the relative speed of the collapse T decreases and Vice versa. Radioactive substances with small T is called short-lived, with a large T - lived. For Na^{24} T=15,06 hour., for VA^{226} T=1620 years. If m is the mass of the isotope in the city, And atomic weight, activity (Curie) Q=1,13·10^{13} m/A·T, where T is the half-life of 1 sec.