  We apply the ``zone of reactions'' as a tool in studying the interacting
system formed in a collision of relativistic nuclei. With the use of the
intensity of collisions of particles (the number of collisions in unit volume
per unit time), we study the space-time structure of a fireball. In this
approach, three basic regions for the evolution of a system are separated by
the scale of the intensity of collisions: the zone of a hot fireball, the zone
of a cold fireball, and the zone of residual interaction. It is shown that the
conception of a zone of reactions can be used for the determination of the
hypersurfaces of a chemical freeze-out and a sharp kinetic freeze-out.
