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\section{Introduction.}

\hspace{1.0em} At the end of the pre-equilibrium stage, or a
thermalizing process, the residual nucleus is supposed to be left in 
an equilibrium state, in which the excitation energy $E^*$ is shared 
by a large number of nucleons. Such an equilibrated compound nucleus 
is characterized by its mass, charge and excitation energy with no
further memory of the steps which led to its formation. If the
excitation energy is higher than the separation energy, it can still
eject nucleons and light fragments (d, t, $^3$He, $\alpha$).  These
constitute the low energy and most abundant part of the emitted 
particles in the rest system of the residual nucleus. The emission of 
particles by an excited compound nucleus has been successfully described 
by comparing the nucleus with the evaporation of molecules from a fluid
\cite{evap.Frenkel36}. The first statistical theory of compound nuclear
decay is due to Weisskopf and Ewing\cite{evap.Weisskopf40}.