  We present a microscopic formalism that extends the traditional formulation
of Williams, Ericson and Bloch and permits to obtain the transition strengths
(TS) of pre-equilibrium nuclear reactions directly from their quantum
microscopic description. We calculate the TS without resorting to the Laplace
transform approach and the use of the saddle point approximation. We also
analyze some problems that may appear in connection with these mathematical
tools and the Darwin-Fowler approach in this case. We show that, analogously to
the nuclear densities, the strengths for transitions that change the exciton
number by two or leave it unchanged can be estimated microscopically as
convolutions of the functions of simpler states. When using the HO basis for
the Model Space we obtained important departure from the results of the exciton
model (EXM), which can partially invalidate our previous analysis on the
attainment of equilibrium during the PE stage. On the other hand, by using
constant grid of energies for the sp-basis we were able to reproduce the
results of EXM quite well in a large range of excitation energies. A new model
code, TRANSNU, was developed that can be ported to traditional semi-classical
codes like TNG for nuclear data evaluation.
