  The freeze out of the expanding systems, created in relativistic heavy ion
collisions, is discussed. We combine Bjorken scenario with earlier developed
freeze out equations into a unified model. The important feature of the
proposed model is that physical freeze out is completely finished in a finite
time, which can be varied from 0 (freeze out hypersurface) to infinity. The
dependence of the post freeze out distribution function on this freeze out time
will be studied. As an example model is completely solved and analyzed for the
gas of pions.
